results a%d discussio% - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/3306/12/12...(1980)...

94

Chapter 4

RESULTS A%D DISCUSSIO%

The present study was undertaken to characterize and quantify the

engineering properties and moisture sorption parameters of skim milk powder, dried

acid casein and whey protein concentrate powder produced from buffalo milk, which

could help in developing high protein functional foods and help in appraising the

storage and packaging problems of these products.

The results of investigations on engineering properties of the products are

discussed in the first section. Moisture sorption properties of the products are

characterized with regard to temperature dependence, sorption hysteresis and isotherm

modeling in the second section. The effect of moisture sorption on the selected

engineering properties is finally discussed in the last.

The composition of the products prepared from buffalo skim milk determined

using standard methods are presented in Table 4.1. It may be noted that the maximum

protein content was in dried acid casein (90.50%) followed by whey protein

concentrate powder (76.84%) and skim milk powder (37.04%). Skim milk powder

contained the highest maximum lactose content of 52.3%, where as, the dried acid

casein had the minimum content of 0.33%. The whey protein concentrate powder had

a moderate lactose content of 10.50%. The powders were uniform in colour, the skim

milk powder being creamish white, casein powder having pale cream and the whey

protein concentrate powder greenish white in colour. No scorched particles or free fat

was observed in the powders.

95

Table 4.1 Composition of skim milk powder, dried acid casein and whey protein

concentrated powder prepared from buffalo milk.

Constituent

% (d.b.)

Skim milk powder Dried acid casein WPC powder

Moisture 5.20 5.60 4.90

Total protein 37.04 90.50 76.84

Lactose 52.30 0.33 10.50

Fat 0.65 1.40 1.46

Ash 4.85 2.17 6.30

Hygroscopicity of powders is particularly marked in the milk powders where

lactose and protein contribute towards water absorption. Amorphous lactose is

responsible for the tendency of milk powders to pick up moisture from the

surrounding air. Milk powder becomes sticky during the first phase of moisture

uptake and hard at the end of process due to the crystallization of lactose

(Upadhyay, 2000).

4.1 Engineering Properties

The properties of milk powder vary considerably depending on the type,

composition of the powder as well as and various treatments given to milk during

concentration and drying process (Kelly et al., 2002). The physical nature of spray

dried dairy products are vastly different from those of powders obtained by other

drying methods such as drum drying, tray drying in hot ovens, etc. However, within

the spray dried products, there are considerable variations in their physical properties.

The primary factors for the functional engineering properties of powders are

dependent on powder material density, content of air inside the particle and the

96

amount of air between the particles that may be incorporated into feed during

centrifugal atomization and the drying conditions. The skim milk powder, dried acid

casein and whey protein concentrate powder prepared from buffalo skim milk were

analyzed for their functional engineering properties. The properties, such as bulk

density, dispersibility, wettability and flowability, solubility index play a major role in

acceptability of powders for various food formulations, and thus these properties were

evaluated. The results with respect to these properties for skim milk powder, dried

acid casein and whey protein concentrate powder are summarized in Tables 4.2 to 4.4,

respectively.

Table 4.2 Engineering properties of buffalo skim milk powder

Property Mean* Range

Loose bulk density (g/cm3) 0.35 (0.32-0.37)

Packed bulk density (g/cm3) 0.53 (0.51-0.54)

Particle density (g/cm3) 1.29 (1.29-1.29)

Occluded air content (cm3/100 g) 9.72 (9.66-10.07)

Interstitial air content (cm3/100 g) 111.15 (107.66-118.15)

Porosity (%) 72.86 (71.32-75.19)

Flowability (angle of response, α°) 46.49 (45.06-47.33)

Wettability (s) 36.00 (36.00-36.00)

Insolubility (ml) 1.23 (1.17-1.27)

Dispersibility (%) 89.76 (87.31-91.39)

* Mean of n = 3 replications.

97

Table 4.3 Engineering properties of dried casein

Property Mean* (Range)

Loose bulk density (g/cm3) 0.65 (0.61 – 0.67)

Packed bulk density (g/cm3) 0.87 (0.84 – 0.89)

Particle density (g/cm3) 1.46 (1.40 – 1.51)

Occluded air content (cm3/100 g) 6.72 (6.66 – 6.80)

Interstitial air content (cm3/100 g) 67.10 (66.06 – 71.15)

Porosity (%) 61.80 (61.32 – 64.19)

Flowability (angle of response, α°) 31.04 (30.60 – 32.39)

Wettability (s) 12.00 (12.00 – 12.00)

Insolubility (ml) 6.36 (6.27 – 6.49)

Dispersibility (%) 68.05 (63.30 – 71.35)

* Mean of n = 3 replications

Table 4.4 Engineering properties of whey protein concentrate powder

Property Mean* (Range)

Loose bulk density (g/cm3) 0.33 (0.31 – 0.36)

Packed bulk density (g/cm3) 0.49 (0.48 – 0.49)

Particle density (g/cm3) 1.19 (1.19 – 1.19)

Occluded air content (cm3/100 g) 8.70 (8.36 – 9.05)

Interstitial air content (cm3/100 g) 103.15 (102.68 – 105.15)

Porosity (%) 69.55 (68.15 – 71.10)

Flowability (angle of response, α°) 40.30 (39.05 – 43.70)

Wettability (s) 42.00 (41.00 – 43.00)

Insolubility (ml) 1.63 (1.62 – 1.63)

Dispersibility (%) 75.16 (71.39 – 77.47)

* Mean of n = 3 replications.

98

4.1.1 Bulk density and particle density

Bulk density and particle density were maximum in dried casein followed by

skim milk powder and WPC powder. The loose bulk density of freshly prepared

buffalo skim milk powder was 0.35 g/cm3 and the packed bulk density as 0.53 g/cm3.

Bulk density of cow milk powder has been reported by several researchers. Hols and

VanMill (1991) has reported the loose bulk density of 0.38 g/cm3 and packed bulk

density of 0.41 to 0.43 g/cm3 for the spray dried cow milk powder. Ilari (2002) has

reported the loose and tapped specific weights of dried cow milk as 0.36 and 0.47

g/cm3. and the corresponding values for cow skim milk powder as 0.5 and 0.61 . The

agglomerated powders have bulk density of 0.45 to 0.55 g/cm3 (Masters, 1979).

According to Upadhyay (1989) the packed bulk density of normal spray dried cow

milk powder may vary between 0.5 to 0.8 g/cc. The values of bulk density of buffalo

skim milk powder were found to be close to all the above values and thus there is no

significant difference in the bulk densities of buffalo skim milk powder compared to

cow milk powder.

Boersen (1990) has reported the range of particle density as 1.30 to 1.40

g/cm3 for cow skim milk powder. The particle density obtained for buffalo skim milk

powder 1.29 g/cm3 was thus nearly similar to those for the cow skim milk powder.

The same author has also reported that high particle density contribute to high bulk

density and improved reconstitution quality of powders in water and better shelf-life

of fat-filled products.

The loose and packed bulk densities of dried casein from buffalo milk were

found to be 0.65 and 0.87 g/cm3. The particle density varied between 1.40-1.51 g/cm3.

Bhadania (1985) reported the bulk density of sun dried lactic acid casein from

buffalo milk to vary between 0.444 and 0.541 g/cm3 and particle density as 1.352

99

g/cm3. Buma (1965) reviewed the available literature on densities of cow milk

constituents and found that the particle density of casein varied from 1.25 to 1.46

g/cm3. Munro (1980) studied the densities of various types of casein using

gravimetric method. The reported values of lactic, sulphuric and rennet casein were

1.36, 1.37 to 1.40 and 1.44 to 1.47 g/cm3, respectively. All the above values are

comparable within the limits of experimental error.

The loose and packed bulk densities and the particle density of whey protein

concentrate powder were 0.33 and 0.49 and 1.19 g/cm3 respectively. Boersen (1990)

reported the range of particle density as 1.40 to 1.50 g/cm3 for whey powders. Patil

(1980) has reported the loose and packed bulk density of soy-whey beverage powder

as 0.42 and 0.69 g/cc, respectively. Bulk density of spray dried whey protein

concentrate powder may vary depending upon the protein content in it. Jayaprakasha

(1992) reported a variation in bulk density from 0.63 g/cc for WPC-30 to 0.34 g/cc for

WPC-70. The reason attributed to this is that during the UF process, as the degree of

concentration of whey progresses, more and more minerals and lactose are removed

thus resulting in low bulk density. Since the whey protein concentrate powder

prepared in this study had a protein content of 76.84%, the results obtained are in

close agreement to this.

4.1.2 Occluded and interstitial air contents

The occluded air content of the buffalo skim milk powder immediately after

manufacturing was found to be 9.72 cm3/100 g. The interstitial air content of the

product was evaluated to be 111.15 cm3/100 g. The values of above properties of

dried casein and whey protein concentrate powder were found to be 6.72, 67.10 and

8.70, 103.15 cm3/100 g respectively. The occluded and interstitial air contents were

thus found to be maximum in skim milk powder followed by whey protein

100

concentrate powder and dried casein. Hols and VanMill et al. (1991) have reported

the occluded air content of cow whole milk powder as 125 ml/kg. The low occluded

air content of buffalo skim milk powder could have resulted in slightly high bulk

density compared to cow milk powder as observed above. The occluded air content is

due to the presence of air in the atomized droplets during spray drying process

(Verhey et al., 1973). It has been observed by Pisecky (1985) that if the degree of

agglomeration in powder is more then interstitial air content in the powder shall be

more. Since the preparation of dried casein does not involve the spray drying, its

occluded and interstitial air contents were found to minimum and consequently, the

bulk density was highest amongst the three products.

4.1.3 Porosity

The porosity of the powder is caused by thermal or mechanical stress in the

powder particle during drying resulting in pores in the particle or cracks in the particle

due to mechanical damage in cyclone. The porosity of the buffalo skim milk powder

was found to be 72.86%, whey protein concentrate powder as 69.55% and dried

casein as 61.80%.

Buma (1972) reported that the porosity of spray dried cow whole milk powder

could range from 42 to 77%. The porosity of spray dried skim milk powder and spray

dried whey powder is lesser compared to other powders (Mistry, 1996). The obtained

value of porosity for buffalo skim milk powder is in agreement to this.

4.1.4 Flowability

The flowability was measured in terms of angle of repose (αo) and was found

to be 45.49o for buffalo skim milk powder, 40.30o for whey protein concentrate

powder and 31.04o for dried casein. The flowability of a powder refers to ease with

101

which the powder particles move with respect to each other. The more readily the

powder flowed, the smaller the angle, thus more free flowing it is. The ‘cot α’

therefore is an appropriate measure of free-flowingness. The cotangent of angle of

response has been reported for skim milk powder as 0.97, whole milk powder as 0.45

and that of instant skim milk as 0.93 (Walstra, 1999). Cow skim milk powder has

flowability from 44 to 45° (Ranganadham, 1988) which corresponds to ‘cot α’ value

of 1.0. The cot α value for buffalo skim milk powder was found to be 0.98, which is

in agreement with the above values for skim milk powder from cow milk, indicating

that there is no difference in the flowability of buffalo skim milk powder and cow

skim milk powder.

The cot α value for whey protein concentrate powder is 1.179 and for dried

casein is 1.66. Thus, out of three powdered products the dried casein was most free

flowing. Flowability was highest in dried casein followed by whey protein

concentrate powder and skim milk powder. Jayaprakasha (1992) reported the

flowability of Cheddar cheese whey powder as 32° with a corresponding ‘cot α’ value

of 1.6 indicating that whey protein concentrate powder is less free flowing than

cheddar cheese whey powder.

Kim et al. (2005) observed that the flowability of powders is strongly

influenced by the surface composition of powders and it was found that skim milk

powder flows well compared to other powders because the surface is made of lactose

and protein with a small amount of fat, whereas the high surface fat composition

inhibits the flow of whole milk powders. The similar flowability of the cow and

buffalo skim milk powder thus could be attributed to the absence of free fat on the

powder surface in both the cases.

102

4.1.5 Wettability

The wettability is a measure of the ability of a powder to absorb water on the

surface to be wetted and to penetrate the surface of still water. It is measured as time

required for a given amount of powder to pass through the water surface at ambient

temperature. Wetting time of buffalo skim milk powder was found to be 36 sec. at

20°C. It was found to be 42 s for whey protein concentrate powder and 12 s for dried

casein. Higher is the wetting time lower is the wettability. Wettability was lowest in

whey protein concentrate powder followed by skim milk powder and dried casein.

The wetting time of cow skim milk powder has been reported to vary between

15 to 60 s by Upadhyay (2000). Wettability of powders depends on particle size,

porosity, surface activity of particles and the occluded air content. The lower value of

wettability obtained for buffalo skim milk powder could be ascribed to slightly more

interstitial air content. The wettability of dry powder particles, generally, depends on

the surface charge, particle size, density, porosity and the presence of amphiphilic

substances (Jensen and 8ielsen, 1982). For instant whey powders, wettability was

reported to be 5 to 6 s (Jensen and Oxlund, 1988).

4.1.6 Dispersibility

Dispersibility is the rapid penetration of water in a mass of powder. It is not

related to solubility but due to this penetration of water the powder particles disperse

separately in water, where they can subsequently dissolve. The dispersibility of skim

milk powder was 89.76%. The dispersibility of whey protein concentrate powder was

75.16 and dried casein was 68.05%.

Walstra (1999) discussed that the dispersibility depends on the contact angle

of the system consisting of dried milk, water and air, due to which water is sucked

into the pores between the particles by capillary force. For dried skim milk this angle

103

is 20o and for dried whole milk it is about 50o and the speed of penetration is slower

for a larger angle. Agglomeration increases the dispersibility because the pores

through which water penetrates become wider.

Masters (1979) has reported the dispersibility of whole milk powder and

agglomerated skim milk powder in the range of 95 to 98% and 90 to 98%,

respectively. Sweetsur (1976) has reported the dispersibility of instant skim milk

powder was reported to be in the range of 86 to 99%. The dispersibility of buffalo

skim milk powder has been found to be in close agreement with reported values.

4.1.7 Insolubility index

The ability of a powder to dissolve completely is expressed in the insolubility

index. This index is a measure of volume of insoluble sediments after dissolving and

centrifuging according to prescribed procedure. The insolubility index of fresh dried

casein was 6.36 ml followed by whey protein concentrate powder with 1.63 ml. The

insolubility index of fresh skim milk powder from buffalo milk was observed to be

1.23 ml. The desired value of insolubility index for instant whole milk powder has

been reported to be less than 1.0 ml (Pisecky, 1990). Insolubility index of instant

skim milk powder has been reported to be 0.05 ml (Kudo et al., 1990). The obtained

value of insolubility index for buffalo skim milk powder is higher than what is

normally desired in instant milk powders. However, according to Bureau of Indian

Standard (IS:1165-1967), the maximum insolubility index of spray dried skim milk

powder is 2.0 ml and the value obtained for buffalo skim milk powder was in

agreement to this.

Solubility is an important feature of milk powders. Poorly soluble powders

cause sediments which is undesirable. They can also cause processing difficulties and

can result in poor economy as milk soluble may be lost as insoluble material. The

soluble are the protein, fat and mineral complex, protein being casein and or

denatured whey proteins (Upadhyay, 2000).

104

4.2 Characteristics of Adsorption and Desorption Isotherms

The moisture sorption process is of prime importance since many physical

properties of macromolecular materials are greatly changed by the presence of sorbed

moisture. Experimental evaluation of sorption characteristics and the subsequent

development and use of mathematical models, can aid in the improvement of the

processing quality of foods and appraisal of their packaging and storage requirements.

This section of the present investigation deals with the characterization of moisture

sorption isotherms of skim milk powder, dried acid casein and whey protein

concentrate powder prepared from buffalo skim milk.

4.2.1 Adsorption and desorption isotherms of skim milk powder

The experimental results for the equilibrium moisture content along with their

standard deviations based on triplicate measurements at each water activity and for

three different temperatures 25, 35 and 450C are presented in Table 4.5.

Table 4.5 Equilibrium moisture content (g water/100 g solids) of buffalo skim

milk powder at different temperatures and water activities (aw) for

adsorption

Temperature, °C

25 35 45

aw Mean* σ** aw Mean* σ** aw Mean* σ**

Adsorption

0.113 3.78 0.12 0.113 4.17 0.08 0.112 4.69 0.07

0.328 7.38 0.12 0.321 7.11 0.13 0.311 6.53 0.09

0.529 13.35 0.09 0.499 11.32 0.31 0.469 9.48 0.20

0.689 17.97 0.21 0.670 15.88 0.10 0.653 13.79 0.10

0.810 23.89 0.11 0.802 21.03 0.08 0.792 18.61 0.07

0.900 41.84 0.12 0.881 37.92 0.21 0.867 33.01 0.12

0.973 62.06 0.08 0.967 60.72 0.11 0.961 59.56 0.11

Desorption

0.113 3.81 0.10 0.113 4.77 0.14 0.112 5.28 0.08

0.328 7.79 0.10 0.321 7.40 0.15 0.311 6.89 0.11

0.529 14.06 0.22 0.499 13.32 0.26 0.469 10.12 0.18

0.689 19.76 0.19 0.670 17.51 0.33 0.653 14.71 0.22

0.810 26.07 0.16 0.802 23.91 0.14 0.792 21.78 0.10

0.900 43.90 0.14 0.881 40.92 0.21 0.867 36.05 0.14

0.973 62.25 0.08 0.967 61.03 0.12 0.961 60.22 0.06

* Mean of n = 3 replications; ** Standard deviation based on n = 3 replications.

105

The sorption isotherms relating water activity and moisture content are

presented in Fig. 4.1 for adsorption and Fig. 4.2 for desorption.

Fig. 4.1 Adsorption isotherms of buffalo skim milk powder

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

Fig. 4.2 Desorption isotherms of buffalo skim milk powder

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

106

The moisture adsorption behavior as observed in Fig. 4.1 is manifested in the

form of sigmoid shaped curves reflecting a Type II isotherm (Brunauer, Emmet and

Teller, 1938), which is typical to the most of the foods. No discontinuity was

observed in the graphical data. The isotherms demonstrate an increase in equilibrium

moisture content with increasing water activity. In all isotherms the moisture uptake

was slow up to a water activity of 0.35 aw, it was moderate between 0.35 to 0.8 aw and

the equilibrium moisture content increased sharply beyond 0.8 aw. Similar behaviour

was observed for desorption isotherms in Fig. 4.2. The isotherms showed that the

product adsorbed proportionately more water towards the later part of the curve.

Similar types of observations were found by Stencl (1999) for adsorption and

desorption of cow skim milk powder, Lin et al. (2005) for desorption isotherm of cow

skim milk and whole milk powders at elevated temperatures and Ko et al. (2008) for

infant milk powders. The equilibrium moisture content of buffalo skim milk powder

at all water activities above 0.45 aw observed in this study was slightly higher than the

corresponding values reported by Stencl (1999) indicating that buffalo skim milk

powder is slightly more hygroscopic than the cow skim milk powder. However, below

0.45 aw, the difference between these values was less and was with in the uncertainty

of the moisture measurement.

The experimental data points of isotherm showed clear temperature

dependence. At water activities 0.33 aw and above an increase in temperature resulted

in decrease in equilibrium moisture content of buffalo skim milk powder. The

difference decreased with increasing water activities beyond 0.8 aw and almost

vanished at water activities greater than 0.83. Lin et al. (2005) have attributed this

phenomenon in milk powder to the increase in solubility of lactose caused by an

increase in temperature. The negative temperature effect on equilibrium moisture

content has been observed in many other food systems (Iglesias and Chirife, 1976;

107

Bandyopadhyay et al., 1987), and in khoa, the heat desiccated milk product from

buffalo milk (Sawhney et al., 1991). In nearly all the dairy products, water sorption is

an exothermic process; hence, for given moisture content their water activity increases

with temperature (Van den Berg and Bruin, 1981). This trend may be due to a

reduction in the total number of active sites for water binding as a result of physico-

chemical changes in the product induced by temperature (Al-Muhtaseb et al., 2004).

At increased temperatures, water molecules get activated to higher energy levels,

become less stable and break away from the water binding sites of the material, thus

decreasing the equilibrium moisture content.

The analysis of variance (1-way ANOVA) was performed using SYSTAT-12

to compare moisture content sorbed by the samples at different temperatures during

the adsorption and desorption to study the effect of temperature on moisture content.

The results presented in Table 4.6 revealed that the effect of temperature on moisture

content was statistically not significant (P>0.05) over the temperature range 25– 45oC.

The experimental sorption data for buffalo skim milk powder below 0.33 aw

showed a reverse trend where, an increase in equilibrium moisture content was

observed with the increase in temperature. This phenomenon is known as inversion

point and has been reported for foods containing sugar. Lin et al. (2004) reported an

inversion point in the isotherms for cow skim milk powder that ranged from a water

activity of 0.3 to 0.43 at elevated temperatures of 52.6, 69.4 and 89.6oC. However, in

the present study as the observed difference in equilibrium moisture content at

different temperatures was small and statistically not significant, it could not be

attributed to the inversion point. No clear inversion point was, therefore, observed in

buffalo skim milk powder in the temperature range of 25 to 450C, though it contained

high lactose content.

108

Table 4.6. A8OVA (Analysis of variance) for effect of temperature on equilibrium moisture content in buffalo skim milk powder

Source of Variables Adsorption

SS df MS F P-value F-Crit.

Between groups 43.2635 2 21.6334 0.05304 0.9484 3.554557

Within groups 7340.314 18 407.7952

Total 7383.577 20

Desorption

Between groups 36.484 2 18.24215 0.04407 0.956984 3.554557

Within groups 7449.795 18 413.8775

Total 7486.279 20

Level of significance : P<0.05

109

4.2.2 Adsorption and desorption isotherms of dried acid casein



three different temperatures 25, 35 and 450C are presented in Table 4.7. The sorption

isotherms relating water activity and moisture content are presented in Fig. 4.3 for

adsorption and Fig. 4.4 for desorption.

Table 4.7 Equilibrium moisture content (g water/100 g solids) of dried acid

casein at different temperatures and water activities (aw) for

adsorption and desorption

Temperature, °C

25 35 45


Adsorption

0.113 3.50 0.14 0.113 3.18 0.11 0.112 3.06 0.07

0.328 7.01 0.18 0.321 5.51 0.18 0.311 5.04 0.12

0.529 9.20 0.12 0.499 8.22 0.31 0.469 7.66 0.20

0.689 12.01 0.14 0.670 11.32 0.19 0.653 11.01 0.17

0.810 17.26 0.12 0.802 16.86 0.19 0.792 16.02 0.15

0.900 27.05 0.20 0.881 26.65 0.21 0.867 25.33 0.14

0.973 45.03 0.23 0.967 44.09 0.17 0.961 43.01 0.11

Desorption

0.113 3.91 0.06 0.113 3.81 0.24 0.112 3.03 0.14

0.328 7.91 0.16 0.321 6.55 0.48 0.311 5.93 0.37

0.529 10.81 0.15 0.499 9.55 0.18 0.469 8.79 0.09

0.689 13.75 0.19 0.670 12.67 0.16 0.653 12.09 0.19

0.810 18.93 0.24 0.802 18.01 0.12 0.792 17.13 0.13

0.900 28.22 0.11 0.881 28.06 0.20 0.867 26.53 0.32

0.973 45.08 0.21 0.967 44.35 0.17 0.961 43.35 0.48


110

Fig. 4.3 Adsorption isotherms of dried acid casein

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

Fig. 4.4 Desorption isotherms of dried acid casein

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

111

The moisture sorption behavior for both adsorption and desorption for casein

was found to be of classical sigmoid shaped curves reflecting a Type II isotherm. This

is a particular characteristics of foods with high protein content (Kinsella and fox,

1987). The isotherms demonstrate an increase in equilibrium moisture content with

increasing water activity. In the region-I of the S-shaped curves up to water activities

of 0.3 aw, casein sorbed relatively lower amount of water. The equilibrium moisture

content rose gradually in region-II between aw of 0.3 to 0.8 and larger amount of

water was sorbed at higher water activities above 0.8 aw. Region I is represented by

the monolayer moisture, which is strongly bound to the material. Region II includes

multilayer moisture which is under transition to natural properties of free water and is

available for chemical reactions. The water in Region III is in free state and is held in

voids and capillaries. The isotherms showed that the product adsorbed proportionately

more water towards the later part of the curve. No discontinuity was observed in the

graphical data. The adsorption isotherm of casein prepared from cow’s milk at

equilibrium temperature of 50oC has been reported by Bandyopadhyay et al. (1987).

The equilibrium moisture content of buffalo milk casein at all the water activities

which was when extrapolated to 50oC using GAB correlations has been found to be

lower than the corresponding values reported by above workers.

At all water activities, the equilibrium moisture content was lower at higher

temperature. The difference remained almost same in the water activity range 0.25 to

0.8. It decreased with further increase in water activities and became very small at

water activities above 0.9. Similar temperature effect on equilibrium moisture content

has been observed for foods with high protein content (Okos et al., 1992; Delgando

and Sun, 2002). Loncin (1969) showed that the water binding capacity of casein was

greater at lower temperature as compared to that at high temperature. The negative

112

effect of temperature on the equilibrium moisture content at low water activities can

be related to the fact that the polymers sorb more water than soluble components at

low water activities. A similar trend was observed by Rüegg (1985) for Sbrinz

cheese. The effect of temperature on the sorption isotherm is of great importance

because the foods are exposed to a range of temperatures during storage and

processing and water activity changes with temperature. Temperature affects the

mobility of water molecules and dynamic equilibrium between vapour and adsorbed

phase (Al-Muhtaseb et al., 2004).

The phenomenon of inversion point which was noted in the isotherms of skim

milk powder was not observed in the isotherms of dried casein and it could be

attributed to very low lactose content (0.33%) in the latter.

The analysis of variance (ANOVA) presented in Table 4.8 revealed that the

effect of temperature on moisture content was statistically not significant (P>0.05)

over the temperature range of 25 – 45oC. Foster et al. (2005) has also observed no

obvious temperature dependence in isotherm of high miceller casein powder

measured between 4-37oC but found the isotherm at 50oC to be significantly lower at

water activities above 0.2.

4.2.3 Adsorption and desorption isotherms of whey protein concentrate powder



three different temperatures 25, 35 and 450C are presented in Table 4.9. The sorption

isotherms relating water activity and moisture content are presented in Fig. 4.5 for

adsorption and Fig. 4.6 for desorption.

113

Table 4.8 A8OVA (Analysis of variance) for effect of temperature on equilibrium moisture content in dried acid casein



Between groups 7.0788 2 3.5394 0.01717 0.98399 3.554557

Within groups 3710.897 18 206

Total 3717.976 20

Desorption

Between groups 9.883 2 4.9171 0.02439 3.554557 3.554557

Within groups 3628.55 18 201.586

Total 3638.384 20


114

Table 4.9 Equilibrium moisture content (g water/100 g solids) of whey protein

concentrate powder at different temperatures and water activities (aw)

for adsorption

Temperature, °C

25 35 45


Adsorption

0.113 2.11 0.12 0.113 2.02 0.10 0.112 1.96 0.08

0.328 4.71 0.14 0.321 3.88 0.06 0.311 3.24 0.12

0.529 8.96 0.08 0.499 7.22 0.24 0.469 5.69 0.22

0.689 13.09 0.10 0.670 11.33 0.16 0.653 9.65 0.12

0.810 18.01 0.22 0.802 16.86 0.27 0.792 14.92 0.15

0.900 32.01 0.12 0.881 29.37 0.24 0.867 26.53 0.12

0.973 47.03 0.18 0.967 45.09 0.12 0.961 43.51 0.18

Desorption

0.113 3.49 0.06 0.113 3.18 0.14 0.112 2.80 0.09

0.328 7.29 0.14 0.321 6.90 0.28 0.311 5.73 0.27

0.529 11.81 0.13 0.499 10.16 0.11 0.469 8.36 0.19

0.689 15.75 0.16 0.670 13.97 0.10 0.653 12.09 0.12

0.810 21.02 0.24 0.802 19.89 0.18 0.792 18.71 0.18

0.900 35.56 0.14 0.881 32.81 0.20 0.867 29.25 0.32

0.973 49.68 0.11 0.967 47.35 0.16 0.961 44.93 0.28


115

Fig. 4.5 Adsorption isotherms of whey protein concentrate powder

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

Fig. 4.6 Desorption isotherms of whey protein concentrate powder

-(▲)- 25oC , -(●)- 35

oC , -(■)- 45

oC

116

The moisture sorption behavior for both adsorption and desorption for casein

was of classical sigmoid shaped curves reflecting a Type II isotherm which is a

particular characteristics of proteins.

Whey protein concentrate sorbed relatively lower amount of water in the

region-I of the S-shaped curves was up to water activities of 0.35 aw. The equilibrium

moisture content rose gradually in region-II between aw of 0.3 to 0.8 and larger

amount of water was sorbed at higher water activities above 0.8 aw in region III. At all

water activities between 0.1 and 0.8 aw, the equilibrium moisture content was lower at

higher temperatures in both adsorption and desorption isotherms. The effect of

temperature on equilibrium moisture content was not distinctly marked at water

activities less than 0.1 aw and at water activities higher than 0.85 for adsorption

isotherm and between 0.1 to 0.80 for desorption isotherm. The analysis of variance

(ANOVA) presented in Table 4.10 revealed that the effect of temperature on moisture

contents in adsorption and desorption isotherm was statistically not significant

(P>0.05) over the temperature range 25 – 45oC.

Foster et al (2005) reported the equilibrium moisture content of cow milk

whey protein isolate during adsorption at various water activities. It has been found

that, equilibrium moisture content in whey protein concentrate powder from buffalo

milk in adsorption isotherm was lower than the corresponding reported values for cow

milk whey protein isolate at water activities up to 0.8. However, above 0.8 water

activities the buffalo milk whey protein concentrate powder sorbed more moisture as

compared to cow milk whey protein isolate.

A comparison has been drawn for moisture sorption isotherms of skim milk

powder, dried acid casein and whey protein concentrate powder at 25oC during

adsorption and desorption and is given in Fig. 4.7. It may be noted from the figures

that adsorption isotherms of skim milk powder and dried casein from buffalo skim

milk coincided with each other up to 0.3 water activity.

117

Table 4.10 A8OVA (Analysis of variance) for effect of temperature on equilibrium moisture content in whey protein concentrate

powder



Between groups 29.901 2 14.9505 0.0610 0.9409 3.554557

Within groups 4408.408 18 244.911

Total 4438.309 20

Desorption

Between groups 36.987 2 18.4938 0.07406 0.9289 3.554557

Within groups 4494.796 18 249.711

Total 4531.783 20


118

Fig. 4.7. Moisture sorption isotherms of buffalo skim milk powder, -(●)- ,

dried acid casein -(▲)- and whey protein concentrate powder-(■)- at 25oC.

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 0.2 0.4 0.6 0.8 1

Equilibrium m

oisture content, g water/ 100g solids

Water activity

Desorption

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

Adsorption

119

Beyond water activity of 0.3 the skim milk powder sorbed more moisture than

dried acid casein at all water activities. The adsorption isotherms of dried casein and

whey protein concentrate powder showed almost similar trends. Up to a water activity

of 0.55 the dried casein sorbed slightly more moisture than the whey protein

concentrate powder. At higher water activities the trend is reversed and whey protein

concentrate powder sorbed slightly more moisture. At higher water activities the

adsorption isotherms of both these products were distinctly apart.

All the three products skim milk powder, dried acid casein and whey protein

concentrate powder have similar shape of adsorption isotherm nearly up to 0.3 water

activity. At water activities higher than 0.3 the skim milk power sorbed more water

followed by whey protein concentrate powder and dried acid casein respectively.

Above water activities 0.8 the isotherm of whey protein concentrate powder showed a

trend similar to isotherm of skim milk powder. In the region of lower water activities

milk proteins are known to be the preferred sorption sites in dairy product and at

higher water activities the effect of protein was counteracted by lactose, since its

equilibrium moisture content increases at higher temperatures and higher water

activities (Berlin et al., 1968). Since all the products contained protein, they had the

similar isotherms at the lower equilibrium moisture contents. At higher water

activities the isotherms of whey protein concentrate powder and skim milk powder

showed a similar trend because both these products had lactose content.

The characteristics of desorption isotherm of all the three products skim milk

powder, dried casein and WPC powder were having similar shape and trend as those

of adsorption isotherms.

4.3 Isotherm Modeling

The experimental data on moisture sorption was fitted to seven different

models using Eq. (3.6) to (3.12).

120

4.3.1 Skim milk powder

Estimated parameters and root mean square per cent error (% RMS) for

selected models of isotherm in the water activity and temperature ranges studied are

presented in Table 4.11 for adsorption isotherm and Table 4.12 for desorption

isotherm. The lower is the value of % RMS for predicted and experimental values; the

better would be the goodness of fit. A good description of the isotherm is considered,

on average, to be smaller than % RMS of 7.0 when a model is applied (Palou et al.,

1997).

Table 4.11 Estimated parameters and root mean square percent error for

selected models of adsorption isotherm at different temperatures for

buffalo skim milk powder

Equation Temperature,

°C

Constants % RMS

a b c

Halsey 25 5.9932 1.5684 -- 20.5279

Smith 25 -0.0009 0.1699 -- 16.4100

Modified Mizrahi

25 -0.0019 -0.1910 0.1846 11.5104

Oswin 25 0.1149 0.5133 -- 8.9135

GAB 25 -0.1073 0.0992 2.0734 6.2484

Caurie 25 2.1637 0.5042 -- 8.8135

Halsey 35 6.2544 1.5354 -- 13.2490

Smith 35 -0.0092 0.1744 -- 20.1486

Modified Mizrahi

35 -0.0209 -0.1409 0.1403 14.0103

Oswin 35 0.1131 0.5127 -- 8.4611

GAB 35 -0.1367 0.1339 0.0142 4.9201

Caurie 35 2.1798 0.5127 -- 6.4235

Halsey 45 6.1000 1.5210 -- 7.9670

Smith 45 -0.0180 0.1756 -- 21.5368

Modified Mizrahi

45 -0.0314 -0.0818 0.0885 11.5920

Oswin 45 0.194 0.5052 -- 8.6436

GAB 45 -0.1763 0.1792 0.0069 5.8340

Caurie 45 2.2129 0.5183 -- 8.5874

121


selected models of desorption isotherm at different temperatures for

buffalo skim milk powder


°C

Constants % RMS

a b c

Halsey 25 5.9932 1.5684 -- 16.5279

Smith 25 -0.0009 0.1699 -- 11.4100

Modified Mizrahi

25 -0.0019 -0.1910 0.1846 13.5104

Oswin 25 0.1149 0.5133 -- 8.9135

GAB 25 -0.1073 0.0992 2.0734 6.2408

Caurie 25 2.1637 0.5133 -- 8.8135

Halsey 35 6.2544 1.5354 -- 11.2490

Smith 35 -0.0092 0.1744 -- 13.1486

Modified Mizrahi

35 -0.0209 -0.1409 0.1403 10.0103

Oswin 35 0.1131 0.5127 -- 8.9711

GAB 35 -0.1367 0.1339 0.0142 4.7901

Caurie 35 2.1798 0.5127 -- 6.4235

Halsey 45 6.1000 1.5210 -- 9.9670

Smith 45 -0.0180 0.1756 -- 16.5368

Modified Mizrahi

45 -0.0314 -0.0818 0.0885 9.5920

Oswin 45 0.194 0.5052 -- 7.6936

GAB 45 -0.1763 0.1792 0.0069 5.8340

Caurie 45 2.2129 0.5052 -- 7.6874

122

Examination of the results in above tables indicted that the GAB models best

described the experimental adsorption data for buffalo skim milk powder throughout

the entire range of water activity. In the present study, at 25, 35 and 45oC the GAB

predicted the adsorption isotherm between 4.92 and 6.24% RMS. This was followed

by Caurie model and Oswin model with % RMS between 6.42 and 8.91. The Halsey,

Smith and Modified Mizrahi models were inadequate for representing the adsorption

isotherms of skim milk powder giving % RMS as high as 7.96, 20.14 and 11.51.

Similarly for desorption data, GAB equation predicted the desorption isotherm

between 4.79 and 6.24% RMS, followed by Caurie model between 6.42 and 7.69 and

Oswin model between 7.69 and 8.93% RMS respectively. The Halsey, Smith and

Modified Mizrahi models were inadequate for representing the desorption isotherms

of buffalo skim milk powder giving % RMS above 9.96, 11.41 and 9.59. Stencl

(1999) found the modified Oswin model as a good fit for moisture adsorption and

desorption of cow skimmed milk powder, but the GAB model was not tested by the

author in that study.

The distribution of residuals for the best fit model (GAB model) for moisture

sorption data were calculated by Eq. 3.21. Fig. 4.8 shows the distribution of residuals

with moisture content. Since there was a random distribution of the residuals, we

recommend the GAB model for description of equilibrium isotherms of buffalo skim

milk powder. Fig. 4.9 shows the adsorption and desorption isotherms of skim milk

powder at 25oC and 45oC with experimental data points, bars of standard deviation

and lines the values predicted by GAB equation.

123

- 5- 4- 3- 2- 1012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 5R esid ual s%

M o i s t u r e c o n t e n t ( g / 1 0 0 g s o l i d s )

A d s o r p t i o n

- 5- 4- 3- 2- 1 012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0 5 5 6 0 6 5R esid ual s%


D e s o r p t i o n

Fig.4.8. Distribution of residuals for Guggenheim–Anderson-de Boer (GAB)

model for buffalo skim milk powder

124

Fig. 4.9 Moisture sorption isotherms of skim milk powder at 25oC and 45

oC.

The symbols show the experimental data (■) adsorption, (▲)

desorption, bars the standard deviation and lines the values predicted

by GAB equation.

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m

oisture content, g water / 100 solids

Water activity

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

Adsorption,

Adsorption,

Desorption,

Desorption,

125

The values of monolayer moisture content ‘Wm’ and GAB constants C and k

calculated for buffalo skim milk powder at three temperatures 25, 35 and 45oC are

presented in Table 4.13. The results show that the adsorbed monolayer moisture

content of buffalo skim milk powder decreased from 6.167 at 25oC to 5.209 g/100g

solids at 45oC. Similarly, the desorption monolayer decreased from 6.20 at 25oC to

5.21 at 45oC g/100g solids. The calculated values of monolayer from BET isotherm

equation have been found to be 5.39, 5.21 and 4.4.51 g/100g solids for adsorption and

5.63, 5.37 and 4.76 g/100g solids for desorption for the equilibrium temperatures of

25, 35 and 45oC, respectively, which are lower than the corresponding values found

by using GAB equation. The BET monolayer value obtained for buffalo skim milk

powder are, however, slightly higher than the corresponding values obtained for cow

skim milk powder reported by Ko et al. (2008) which are 5.07 g/100g solids at 15oC

decreasing to 2.96 g/100g solids at 35oC. The temperature dependence of monolayer

moisture has been linked to a reduction in sorption active sites as a result of physico-

chemical changes induced by temperature. This behavior has also been reported for

many other food systems (Iglesias and Chirife, 1976).

Table 4.13 Parameter values for the GAB model to describe the adsorption (A)

and desorption (D) isotherms of skim milk powder different

temperatures

Parameter Temperature, °C

25 35 45

A D A D A D

Wm 6.1670 6.2015 6.0237 5.6831 5.2090 5.2115

k 0.9083 0.9018 0.9290 0.9147 0.9513 0.9320

C 9.2678 10.1820 12.1310 14.5341 16.1521 15.2100

% RMS 6.2480 5.3800 4.9200 5.4710 5.8340 5.2190

126

4.3.2 Dried acid casein

Estimated parameters and root mean square per cent error (% RMS) for

selected models of isotherm for dried acid casein in the water activity and temperature

ranges studied are presented in Table 4.14 for adsorption and Table 4.15 for

desorption.



dried acid casein


°C

Constants

a b c

Halsey 25 1.9339 0.5865 -- 14.7835

Smith 25 1.1617 12.1868 -- 14.6703

Modified Mizrahi

25 -1.9055 -12.7325 13.4432 11.8950

Oswin 25 2.2579 0.4648 -- 5.8135

GAB 25 0.0151 0.1514 -0.1479 5.2392

Caurie 25 -2.2579 0.4648 -- 5.8135

Halsey 35 1.8002 0.6220 -- 13.3095

Smith 35 0.2698 12.1244 -- 20.7486

Modified Mizrahi

35 -1.6501 -10.6367 1.0.8432 15.4331

Oswin 35 2.1452 0.4914 -- 6.4235

GAB 35 0.0175 0.1801 -0.1815 1.9276

Caurie 35 -2.1452 0.4914 -- 6.4235

Halsey 45 1.7919 0.6140 -- 13.2501

Smith 45 0.3215 11.7958 -- 21.3433

Modified Mizrahi

45 -1.6361 -9.9060 9.8763 16.1276

Oswin 45 2.1330 0.4842 -- 8.5874

GAB 45 0.0162 0.1900 -0.1897 5.7759

Caurie 45 -2.1330 0.4842 -- 8.5874

127



dried acid casein


°C

Constants % RMS

a b C

Halsey 25 2.0408 0.5548 -- 15.5008

Smith 25 2.1815 11.9568 -- 9.9704

Modified Mizrahi

25 -2.0934 -14.6327 15.5967 9.4850

Oswin 25 2.3465 0.4412 -- 5.3252

GAB 25 0.0141 0.1301 -0.1244 5.0109

Caurie 25 -2.3465 0.4412 -- 5.3252

Halsey 35 1.9461 0.5789 -- 12.6200

Smith 35 1.3292 12.0296 -- 13.3282

Modified Mizrahi

35 -2.1852 -11.3081 12.1057 14.8413

Oswin 35 2.2670 0.4575 -- 5.8841

GAB 35 0.0127 0.1641 -0.1594 1.4011

Caurie 35 -2.2670 0.4575 -- 5.8841

Halsey 45 1.8818 0.6140 -- 11.7425

Smith 45 0.9303 11.7958 -- 16.3168

Modified Mizrahi

45 -2.0740 -9.9060 11.0832 14.8046

Oswin 45 0.1340 0.4842 -- 6.1765

GAB 45 0.0134 0.1775 -0.1735 4.9506

Caurie 45 -2.1330 0.4842 -- 6.1765

128

The results indicated that the GAB model best described the experimental

adsorption and desorption data for dried casein throughout the entire range of water

activity. The GAB model predicted the adsorption isotherm at 25, 35 and 45oC

between 1.92 and 5.77% RMS, while for desorption isotherm between 1.40 and

5.01% RMS. This was followed by Oswin model and Caurie model with % RMS

between 5.18 and 8.58 for adsorption and between 5.32 and 6.17 for desorption. The

Halsey, Smith and Modified Mizrahi models were inadequate for representing the

sorption isotherms of dried casein giving % RMS above 13.25, 14.67 and 11.89 for

adsorption and 11.74, 7.97 and 9.48 for desorption respectively. The coefficients of

BET equation (3.11) were a = 5.005, b = - 0.01477 and c = 1.0522 for adsorption

(r2=0.99) and a = 6.551, b = - 0.0431 and c = 1.0701 for desorption (r2=0.986). The

calculated values for aw from equation (3.14) for adsorption and desorption have been

found to be 0.494 and 0.491.

The distribution of residuals for the best fit model (GAB model) for adsorption

and desorption data were calculated by Eq. 3.21. Fig. 4.10 shows the distribution of

residuals with moisture content. Since there was a random distribution of the

residuals, we recommend the GAB model for description of equilibrium isotherms of

dried acid casein from buffalo skim milk. Fig. 4.11 shows the adsorption and

desorption isotherms of dried acid casein at 25oC and 45oC with experimental data

points, bars of standard deviation and lines the values predicted by GAB equation.


calculated for dried casein at three temperatures 25, 35 and 45oC are presented in

Table 4.16.

129

- 5- 4- 3- 2- 1012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0R esid ual s%


A d s o r p t i o n

- 5- 4- 3- 2- 1012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0R esid ual s%


D e s o r p t i o n

Fig. 4.10. Distribution of residuals for Guggenheim –Anderson-de Boer (GAB)

model for dried casein

130

25oC

Fig. 4.11. Moisture sorption isotherms of dried acid casein at 25oC and 45

oC.

The symbols show the experimental data (■) adsorption, (▲)

desorption, bars the standard deviation and lines the values predicted

by GAB equation.

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

Adsorption, Desorption,

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

Adsorption, Desorption,

131


and Desorption (D) isotherms of dried casein at different

temperatures


25 35 45

A D A D A D

Wm 5.6029 6.4628 4.7067 5.3432 4.5458 4.9504

k 0.8967 0.8735 0.9247 0.9076 0.9254 0.9143

C 13.1815 12.5632 13.1295 13.1295 14.6739 16.4879

% RMS 5.2392 5.0109 1.9276 1.4011 5.7759 1.9546

These results show that the monolayer moisture content of dried casein

decreases with increase in temperature between 25 and 45oC. Adsorbed monolayer

decreased from 5.6029 at 25oC to 4.5458 at 45oC, whereas, the corresponding

decrease in desorption monolayer was from 6.4628 to 4.9504. The calculated values

of monolayer from BET isotherm equation have been found to be 4.65, 4.07 and 4.35

g / 100g solids for adsorption and 5.49, 5.07 and 4.56 g/100g solids for desorption for

the equilibrium temperatures of 25, 35 and 45oC respectively, which are lower than

the corresponding values found by using GAB equation. However, in both cases the

monolayer moisture was higher in desorption than the adsorption and deceased with

increase in temperature.

4.3.3 Whey protein concentrate powder

Table 4.17 and Table 4.18 present the estimated parameters and root mean

square per cent error (% RMS) for selected isotherm models for adsorption and

desorption respectively in whey protein concentrate powder in the water activity and

temperature ranges studied. As observed in the case of skim milk powder and dried

casein the GAB model best described the experimental adsorption and desorption data

for whey protein concentrate powder throughout the entire range of water activity.

132



whey protein concentrate powder


°C

Constants % RMS

a B C

Halsey 25 1.1339 1.1158 -- 6.3580

Smith 25 1.2853 11.1968 -- 16.6703

Modified Mizrahi

25 -0.9435 -2.7308 -0.6894 7.2850

Oswin 25 1.7820 0.7648 -- 11.3205

GAB 25 0.0651 0.2415 0.3459 5.0380

Caurie 25 -1.7820 0.7248 -- 11.3205

Halsey 35 1.2802 1.0220 -- 9.8395

Smith 35 1.2108 11.2240 -- 18.78.60

Modified Mizrahi

35 -0.6985 -7.2721 4.4320 7.2344

Oswin 35 1.8190 0.7150 -- 6.5235

GAB 35 0.0628 0.1608 -0.2218 5.6276

Caurie 35 -1.8190 0.7150 -- 6.4235

Halsey 45 1.4219 0.6628 -- 7.8851

Smith 45 1.3215 11.0908 -- 15.2134

Modified Mizrahi

45 -1.0360 -7.0906 4.8650 16.1276

Oswin 45 1.8864 0.6256 -- 7.3356

GAB 45 0.0369 0.1988 -0.2305 5.7859

Caurie 45 -1.8864 0.7574 -- 7.3356

133



whey protein concentrate powder


°C

Constants % RMS

a B C

Halsey 25 1.1549 1.0068 -- 10.0350

Smith 25 1.3085 11.0852 -- 14.6703

Modified Mizrahi

25 -1.2660 -7.0073 5.1684 9.0850

Oswin 25 2.0370 0.6563 -- 10.1325

GAB 25 0.0351 0.1678 -0.1978 6.1038

Caurie 25 -2.0370 0.6563 -- 10.1325

Halsey 35 1.8284 0.9029 -- 13.9820

Smith 35 1.3218 11.0240 -- 12.7080

Modified Mizrahi

35 -1.8885 -11.0272 10.0140 11.3340

Oswin 35 2.2088 0.5150 -- 11.9238

GAB 35 0.0202 0.1398 -0.1298 6.6760

Caurie 35 -2.2088 0.5150 -- 11.9238

Halsey 45 1.9421 0.7166 -- 12.0088

Smith 45 1.3258 11.0008 -- 11.1340

Modified Mizrahi

45 -1.8936 -13.2098 13.8065 8.4676

Oswin 45 2.2884 0.4950 -- 9.2335

GAB 45 0.0199 0.1199 -0.1305 5.9859

Caurie 45 -2.2884 0.4950 -- 9.2335

134

The GAB model predicted the adsorption isotherm at 25, 35 and 45oC between

5.03 and 5.78% RMS, while for desorption isotherm between 5.98 and 6.10% RMS.

Halsey equation also showed a good fit for adsorption isotherm at 25oC with % RMS

of 6.35 and Caurie and Oswin models for adsorption isotherms at 35oC with % RMS

of 6.42. All remaining models tested were inadequate for representing the adsorption

and desorption isotherms of whey protein concentrate powder giving % RMS higher

than 7.33. The Smith equation showed the highest % RMS of above 15.21 for

adsorption and above 11.13 for desorption isotherms and was grossly inadequate in

describing sorption characteristics of whey protein concentrate powder.

The distribution of residuals for the best fit model (GAB model) for adsorption

and desorption data were calculated by Eq 3.21. Fig. 4.12 shows the distribution of

residuals with moisture content. Since there was a random distribution of the

residuals, we recommend the GAB model for description of equilibrium isotherms of

whey protein concentrate powder from buffalo skim milk. Fig. 4.13 shows the

adsorption and desorption isotherms of whey protein concentrate powder at 25oC and

45oC with experimental data points, bars of standard deviation and lines the values

predicted by GAB equation.


calculated for whey protein concentrate powder at three temperatures 25, 35 and 45oC

are presented in Table 4.19. These results show that the monolayer moisture content

of whey protein concentrate powder decreases with increase in temperature between

25 and 45oC. Adsorbed monolayer decreased from 5.125 at 25oC to 4.209 at 45oC,

whereas, the corresponding decrease in desorption monolayer was from 5.162 to

4.441.

135

- 5- 4- 3- 2- 1012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0R esid ual s%


A d s o r p t i o n

- 5- 4- 3- 2- 1012345

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0R esid ual s%


D e s o r p t i o n

Fig. 4.12. Distribution of residuals for Guggenheim –Anderson-de Boer (GAB)

model for whey protein concentrate powder

136

Fig. 4.13. Moisture sorption isotherms of whey protein concentrate powder at

25oC and 45

oC. The symbols show the experimental data (■)

adsorption, (▲) desorption, bars the standard deviation and lines the

values predicted by GAB equation.

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

35

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

Adsorption,

Adsorption,

Desorption,

Desorption,

137


and desorption (D) isotherms of whey protein concentrate powder at

different temperatures


25 35 45

A D A D A D

Wm 5.1250 5.1620 5.0237 5.5831 4.2090 4.4411

k 1.2390 0.9678 1.0210 0.9741 1.4028 0.8993

C 11.0268 13.2099 10.2131 11.5092 14.3151 12.0083

% RMS 5.0380 6.1030 5.6276 6.0760 5.7759 5.9850

The calculated values of monolayer from BET isotherm equation have been

found to be 3.96, 3.48 and 3.05 g/100g solids for adsorption and 4.15, 3.81 and 3.49

g/100g solids for desorption for the equilibrium temperatures of 25, 35 and 45oC

respectively, which are lower than the corresponding values found by using GAB

equation. However, in both cases the monolayer moisture was higher in desorption

than the adsorption and deceased with increase in temperature.

4.4 Temperature Dependence of GAB Parameters

The variation in water activity at any given temperature could be predicted by

incorporating temperature term into sorption equations. Equations have been

developed to correlate the GAB constants with temperature employing the isotherm

data at 25, 35 and 45oC. Eq. 3.22 to 3.24 were used to determine W’m, C’ and k’ and

the corresponding exponents in these equations by least square analysis for adsorption

and desorption in skim milk powder, dried acid casein and whey protein concentrate

powder.

138


Adsorption

Wm (T)a = -3.3788 exp (13.311 x 103/RT), r2= 0.985 (4.1)

C (T)a = 11.0473 exp (-21.868 x 103/RT), r

2= 0.972 (4.2)

k (T)a = 0.6383 exp (-1.821 x 103/RT), r

2= 0.991 (4.3)

Desorption

Wm (T)d = -2.8743 exp (13.951 x 103/RT), r

2= 0.975 (4.4)

C (T)d = 13.7116 exp (-19.70 x 103/RT), r

2= 0.989 (4.5)

k (T)d = 0.9461 exp (-1.933 x 103/RT), r

2= 0.971 (4.6)


Adsorption

Wm (T)a = 0.1965 exp (8.246 x 103/RT), r2= 0.969 (4.7)

C (T)a = 842 exp (-10.49 x 103/RT), r

2= 0.988 (4.8)

k (T)a =1.7086 exp (-1.596 x 103/RT), r

2= 0.980 (4.9)

Desorption

Wm (T)d = 0.0765 exp (10.957 x 103 / RT), r

2= 0.990 (4.10)

C (T)d = 303.1 exp (-7.70 x 103/RT), r

2= 0.962 (4.11)

k (T)d = 1.9667 exp (-2.003 x 103/RT), r

2= 0.985 (4.12)

4.4.3 WPC powder

Adsorption

Wm (T)a = 0.3969 exp (6.462 x 103/RT), r2= 0.979 (4.13)

C (T)a = 42 exp (-13.17 x 103/RT), r

2= 0.961 (4.14)

k (T)a =1.1016 exp (-1.059 x 103/RT), r

2= 0.991 (4.15)

139

Desorption

Wm (T)d = 0.1307 exp (9.209 x 103/RT), r

2= 0.970 (4.16)

C (T)d = 303.1 exp (-11.69 x 103/RT), r

2= 0.986 (4.17)

k (T)d = 1.7760 exp (-2.406 x 103/RT), r

2= 0.973 (4.18)

The Equations 4.1 to 4.6 together with equation of GAB model (Eq. 3.12) can

now be used to calculate the equilibrium water content of the skim milk powder at

any given water activity and temperature for adsorption as well as desorption

isotherm. Similarly, Equations 4.7 to 4.12 along with Equation 3.12 could be used for

prediction of equilibrium moisture content of dried acid casein and Equations 4.13 to

4.18 along with Eq. 3.12 could be used for prediction of adsorption and desorption

moisture content of whey protein concentrate powder. These equations could

gainfully be used in shelf life simulation and storage and dehydration of the products.

4.5 Moisture Sorption Hysteresis

The phenomenon of moisture sorption hysteresis is of importance in drying

and rewetting of dried powder. The practical implications of hysteresis are of

significance because of its effect on chemical and microbiological deterioration of the

product. In this study, the hysteresis effect has been observed in all the products

investigated. The hysteresis effect was observed at all the temperatures over the entire

range of water activities in dried acid casein and whey protein concentrate powder.

Whereas, the hysteresis loop in skim milk powder was relatively smaller with respect

to range of water activity. Thus, the hysteresis loop in dried acid casein and whey

protein concentrate powder was of Type ‘C’ according to Everett classification

(Kapsalis, 1981). The extent of hysteresis, however, varied in all the products at

different water activities and temperatures.

140


Hysteresis effect exhibited by skim milk powder at 25, 35 and 45oC are shown

in Fig. 4.14. The lower closure point of hysteresis loop is at 0.2 water activity and

upper closure point is at 0.85 water activity. The distribution of hysteresis loop

relative to water activity showed a marked change at various water activities.

Fig.4.14. Moisture sorption hysteresis in buffalo skim milk powder

Adsorption - (▲) - , Desorption - (■) -

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

� � ��

� � � � � � � � � � � � �

25o

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

� !"#"$ %" !&&'" () !%*+',) *,) -./0) *%1 233 ('#"4 (

5 6 7 8 9 6 : 7 ; < ; 7 =

35o

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

> ?@ABAC DA @EEFA GH @DIJFKH IKH LMNOH IDPQRR GFBAS G

T U V W X U Y V Z [ Z V \

45o

141

There was no hysteresis effect in the monolayer moisture content region. It

occurred predominantly in the water activity range of 0.45 to 0.8 aw. The hysteresis

effect decreased beyond 0.8 aw and vanished completely at 0.85 aw and the adsorption

and desorption isotherms coincided with each other. The maximum hysteresis was

between 0.7 and 0.8 aw. According to Kapsalis (1981) in high protein foods moderate

hysteresis beginning at about 0.85 water activity extends over the rest of isotherm up

to zero water activity while in high sugar foods hysteresis mainly occur in lower water

activities and there is no hysteresis above 0.65 water activity. Skim milk powder

containing both protein and lactose in fairly large amount exhibited a relative

combined effect. It retains sigmoid shape peculiar to protein foods and presence of

lactose as a solution in water of hydration at higher water activities appear to

eliminate hysteresis in this range.

Bell and Labuza (2000) reported that foods with high sugar content

frequently exhibit this phenomenon and explained that when the water moves out

from capillaries of the product, during moisture desorption, the narrow ends of surface

pores trapped and held water internally below the water activity where the water

should have been released, thus there was greater moisture content at a low range of

water activity. During adsorption, the pure water would dissolve solutes, that is

lactose present in skim milk powder and dissolution of solutes increased the surface

tension resulting in lower water activity at given moisture.

A paired t-test and correlation analysis were performed using SYSTAT-12 to

compare moisture content sorbed by the samples at different temperatures during the

adsorption and desorption. Paired t-test revealed that the adsorption and desorption

differ significantly at all the three temperatures with t0.05 = 6.11 at 250C, t0.05 = 6.23 at

350C, and t0.05 = 5.01 at 450C. Further, high correlation (γ = 0.989) between

adsorption and desorption data revealed significant association between them.

142

The hysteresis loop was evaluated in terms of relative hysteresis units. For this

purpose the adsorption values were subtracted from the respective desorption values

at a given water activity intervals of 0.05 and the resulting difference was plotted

versus the corresponding water activity. The graphical integration of area under this

curve gave the hysteresis units. The total hysteresis in skim milk powder was 1.39

units at 25oC. The magnitude of hysteresis was smaller at higher temperatures. Yan et

al. (2008) attributed this phenomenon to the increased elasticity of capillary walls and

greater capability of forming hydrogen bond between protein/carbohydrate and water.

In buffalo skim milk powder, the effect of increasing the isotherm temperature was

found decreasing the total hysteresis from 1.39 units at 25oC to 1.26 at 35oC and 1.08

at 45oC. It also resulted in limiting the span of the loop along the isotherm..

The hysteresis amplitude ratio varied throughout the water activity range of 0.1 to

0.9 at all the temperatures. However, the maximum value for this ratio in skim milk

powder was 0.157 at 25oC. It reduced with increase in temperature to 0.131 at 35oC

and 0.117 at 45oC. In most foods, the sorption capacity decreases with increasing

temperatures because of negative excess heat of sorption (Bizot et al., 1985). The

results obtained on skim milk powder are in agreement to this.

Total hysteresis energy was determined employing the equation 3.26. The

Everett and Whitton plots of sorption data on skim milk powder at 25, 35 and 45oC

are shown in the Figure 4.15. In this figure the x-axis has been expressed, for the

convenience in interpretations, on a moisture content basis, where as for calculating

the total energy, the number of mole of water has been used. The Everett and Whitton

plot indicate that the greater contribution to hysteresis energy is made in the moisture

content range 5 to 13% (d.b.). The energy of hysteresis was not uniform over the

whole water activity range, but varied with the moisture content.

143

Fig.4.15. Everett and Whitton plot for sorption data on skim milk powder

(●) 25oC; (■) 35

oC; (▲) 45

oC

The hysteresis energy increased in the beginning with the moisture content

and there after decreased abruptly and remained almost uniform. The total energy of

hysteresis decreased with increase in temperature; it was 277 J at 25oC, 186.5 J at

35oC and 117.3 J at 45oC. The effect of temperature was thus found to reduce

hysteresis energy.


The adsorption and desorption isotherms of dried acid casein at 25, 35 and

450C are shown in Figures 4.16. The adsorption and desorption isotherms are sigmoid

in shape and are distinctly apart from each other.

0

100

200

300

400

500

600

700

0 5 10 15 20 25

RT ln p (kJ / kg w

ater)

Moisture content (g / 100 g solids)

144

Fig.4.16. Moisture sorption hysteresis in dried acid casein

Adsorption - (▲) - , Desorption - (■) –

The hysteresis effect extends from water activity range from 0.1 to 0.9.

Kapsalis (1981) has reported that in high protein foods moderate hysteresis beginning

at about 0.85 water activity extends over the rest of isotherm up to zero water activity

and both adsorption and desorption isotherms remain sigmoid in shape. High

molecular weight polymers, such as proteins may immobilize large quantities of water

as gels and show marked hysteresis explained in terms of capillary condensation

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

25o

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

0

5

10

15

20

25

30

35

40

45

50

55

60

65

0 0.2 0.4 0.6 0.8 1

Equilibrium m


Water activity

35o

45o

145

theories and steric arrangements (Aguilera and Stanely, 1999). The results on dried

acid casein are in agreement to this. The hysteresis in dried acid casein can be

classified as ‘Type-C’ according to Everett classification.

The distribution of hysteresis loop relative to water activity showed a marked

change at various water activities. There was moderate hysteresis in monolayer

moisture content region, it occurred predominantly between 0.3 and 0.7 aw, it

diminished beyond that and the difference between adsorption and desorption

isotherms became almost negligible above 0.9 aw.

The maximum hysteresis was between aw 0.3 and 0.6 at 250C and between aw

0.3 and 0.5 at 450C. Paired t-test revealed that the adsorption and desorption differ

significantly at all the three temperatures with t.05 = 5.03 at 250C, t.05 = 6.32 at 35

0C

and t.05 = 4.57 at 450C. Further, high correlation (γ = 0.999) between adsorption and

desorption data revealed significant association between them.

As has been observed in skim milk powder the sorption hysteresis in dried

casein was also affected by temperature. The magnitude of hysteresis loop in dried

casein evaluated in terms of relative hysteresis units revealed that the total hysteresis

was 1.65 units at 25oC. By increasing the isotherm temperature from 25 to 450C, the

total hysteresis in casein was reduced from 1.65 to 1.22 units. The effect of increasing

temperature was to decrease the total hysteresis, but it had very little effect on the

span of loop along the isotherm. The hysteresis amplitude ratio varied throughout the

water activity range 0.1 to 0.9 at all the temperatures. The maximum value for this

ratio was 0.192 at 25oC. It reduced with increase in temperature to 0.179 at 35oC and

0.156 at 45oC.

146


Everett and Whitton plots of sorption data on dried acid casein at 25, 35 and 45oC are

shown in the Figure 4.17. The Everett and Whitton plot indicate that the greater

contribution to hysteresis energy is made in the moisture content range 5 to 13% (db).

As in the skim milk powder the energy of hysteresis was not uniform over the whole

water activity range, but varied with the moisture content. The hysteresis energy

increased in the beginning with the moisture content and thereafter decreased abruptly

and remained almost uniform. The increase in the sorption temperature was found to

reduce hysteresis energy. The total energy of hysteresis decreased with increase in

temperature from 307 J at 25oC to 236 J at 35oC at and 192.5 J at 45oC.

Fig.4.17. Everett and Whitton plot for sorption data on dried acid casein

(●) 25oC; (■) 35

oC; (▲) 45

oC

0

100

200

300

400

500

600

700

0 5 10 15 20 25

RT ln p (kJ / kg w

ater)


147

4.5.3 Whey protein concentrate powder

The adsorption and desorption isotherms of whey protein concentrate powder

at 25, 35 and 450C are shown in Figures 4.18.

Fig.4.18. Moisture sorption hysteresis in whey protein concentrate powder

Adsorption - (▲) - , Desorption - (■) -

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

� ��

� � � � � � � � � � � � �

35o

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

� !"#"$ %" !&&'" () !%*+',) *,) -./0) *%1233 ('#"4 (

5 6 7 8 9 6 : 7 ; < ; 7 =

45o

0

10

20

30

40

50

60

0 0.2 0.4 0.6 0.8 1

> ?@ABAC DA @EEFA GH @DIJFKH IKH LMNOH IDPQRR GFBAS G

T U V W X U Y V Z [ Z V \

25o

148

The adsorption and desorption isotherms are sigmoid in shape, distinctly apart

from each other and the hysteresis effect extends over the entire isotherm range, as is

common with the most protein foods. The distribution of hysteresis loop relative to

water activity showed a marked change at various water activities. Hysteresis was

moderate in monolayer moisture content region, it occurred predominantly between

0.2 and 0.85 aw. The hysteresis diminished at higher water activities and the

difference between adsorption and desorption isotherms became almost negligible

above 0.9 aw. At 45oC the adsorption and desorption isotherms coincided with each

other above 0.9 aw. The maximum hysteresis was between aw 0.2 and 0.85 at 25 and

350C, between aw 0.3 and 0.8 at 450C. Paired t-test revealed that the adsorption and

desorption differ significantly at all the three temperatures with t0.05 = 5.96 at 250C,

t0.05 = 5.88 at 350C, and t0.05 = 4.77 at 45

0C. Further, high correlation (γ = 0.980)

between adsorption and desorption data revealed significant association between

them.

The magnitude of hysteresis loop in whey protein concentrate powder

evaluated in terms of relative hysteresis units revealed that the total hysteresis was

2.03 units at 25oC. By increasing the isotherm temperature from 25 to 450C, the total

hysteresis in casein was reduced from 2.03 to 1.67 units. The effect of increasing

temperature was to decrease the total hysteresis, but it had no effect on the span of

loop along the isotherm. The hysteresis amplitude ratio varied throughout the water

activity range 0.1 to 0.9 at all the temperatures. The maximum value for this ratio was

0.227 at 25oC. It reduced with increase in temperature to 0.193 at 35oC and 0.148 at

45oC.

149


Everett and Whitton plots of sorption data on whey protein concentrate powder at 25,

35 and 45oC are shown in the Figure 4.19. The Everett and Whitton plot indicate that

the greater contribution to hysteresis energy is made in the moisture content range 5 to

12% (db). On the same pattern as with skim milk powder and dried casein, the energy

of hysteresis in whey protein concentrates powder was not uniform over the whole

water activity range, but varied with the moisture content. The hysteresis energy

increased in the beginning with the moisture content and there after decreased

abruptly and remained almost uniform.

Fig.4.19. Everett and Whitton plot for sorption data on whey protein

concentrates powder (●) 25oC; (■) 35

oC; (▲) 45

oC

0

100

200

300

400

500

600

700

0 5 10 15 20 25

RT ln p (kJ / kg w

ater)


150

The total energy of hysteresis decreased with increase in temperature; it was

343.7 J at 25oC, 289.3 J at 35oC and 189.6 J at 45oC. The effect of temperature was

thus found to reduce hysteresis energy.

It has been found that at room temperature (25oC) the magnitude of hysteresis

loop in terms of relative hysteresis units was highest in whey protein concentrate

powder (2.03) followed by dried acid casein (1.65) and skim milk powder (1.39). By

increasing the isotherm temperature from 25 to 45oC, the relative hysteresis units

reduced to 1.67, 1.22 and 1.08 units respectively. The maximum value for hysteresis

amplitude ratio for whey protein concentrate powder was 0.227 followed by dried

casein 0.192, and skim milk powder 0.157 at 25oC. Consequently the highest

hysteresis energy was found in the same order of whey protein concentrate powder

followed by dried casein and skim milk powder.

4.6 Thermodynamics of Moisture Sorption

Analysis of sorption isotherm data by application of thermodynamic principles

can provide much important information regarding the dehydration process energy

requirements, food microstructures, physical phenomenon on food surfaces, water

properties and sorption kinetic parameters (Rizvi, 1995). The thermodynamic

functions adopted for analysis of sorption phenomenon included differential enthalpy,

differential entropy and Gibb’s free energy.

4.6.1 Isosteric heat of sorption (Differential enthalpy)

The isosteric heat of adsorption and desorption for skim milk powder, dried

acid casein and whey protein concentrate powder were calculated by applying

Clausius- Clapeyron equation (Eq. 3.27) to the moisture sorption data as expressed by

the GAB model that best described the isotherms. The isosteric heat has a strong

151

dependence on moisture content, with the energy required for sorption (in excess of

latent heat) increasing at low moisture content. The isosteric heat of sorption as a

function of moisture content is presented in Fig. 4.20 for buffalo skim milk powder,

Fig 4.21 for dried acid casein and Fig 4.22 for whey protein concentrate powder.

Fig.4.20. Variation in differential enthalpy (isosteric heat) of adsorption and

desorption of buffalo skim milk powder with moisture content.


desorption of dried casein with moisture content.

152


desorption of whey protein concentrate powder with moisture

content.

Isosteric heat of adsorption was found to decrease with increase in moisture

content and the trend seemed to become asymptotic as the moisture content of above

20 % dry basis was approached. The maximum isosteric heat of adsorption obtained

was 55.86 kJmol-1 for skim milk powder at 25oC, 54.68 for dried acid casein and

51.89 kJmol-1 for whey protein concentrate powder. The maximum enthalpy value

indicates the covering of the strongest binding sites and the greater water solid

interactions in the powder. The covering of less favourable locations and the

formation of multi-layers then follows as shown by the decrease in enthalpy with

increasing moisture content. Therefore, the water binding in all the above products

was found to be week at product moisture content above 20%. Similar trends have

been reported for the isosteric heats of several food such as khoa (Sawhney et al.,

1991), cookies and snacks (Palou et al., 1997), potato (McMinn and Magee, 2003),

tow mints (Kane et al., 2008), chhana podo (Jayaraj et al., 2006), walnut kernels

4 04 24 44 64 85 05 25 45 65 86 0

0 5 1 0 1 5 2 0 2 5 3 0 3 5N eti sost eri ch eat(k J/ mol)

M o i s t u r e c o n t e n t ( % d . b . )

153

(Togrul and Arslan, 2006), kheer (Jayendra et al., 2005). At low moisture levels,

the adsorption is mainly at monomolecular layer where the sorption sites are usually

active (Iglesias and Chirife, 1976).

The isosteric heat of adsorption for skim milk powder ranged from 55.86

kJmol-1 at 3% (d.b.) to 44.6 kJmol-1 at 30 % (d.b.) moisture content. Correspondingly

the heat of desorption ranged from 57.10 to 44.8 kJmol-1. The range of above values

for dried casein were 52.05 to 44.45 kJmol-1 for adsorption and 54.68 to 44.65 kJmol-1

for desorption for the same range of moisture contents. The corresponding values for

isosteric heat of whey protein concentrate powder were 51.89 to 44.40 kJmol-1 for

adsorption and 53.55 to 44.45 kJmol-1 for desorption. The isosteric heat for both

adsorption and desorption were highest in skim milk powder followed by dried casein

and whey protein concentrate powder.

The isosteric heats of adsorption and desorption were higher than the latent

heat of pure water, indicating that the energy of binding between the water molecules

and the sorption sites was higher than the energy which holds the water molecules in

the liquid phase. All the above figures revealed that the heat required for adsorption

process was generally greater than that for the desorption process. The increased

energy requirements of the former process are indicative of the more polar sites on the

surface of material and hence molecules of lower mobility. As the moisture content

increased, the difference between the isosteric heat values for adsorption and

desorption processes was observed to decrease. The difference between heats of

adsorption and desorption was greater at lower moisture contents, converging as the

moisture content increased and practically disappearing above 20% moisture content

(d.b.). Benado and Rizvi (1985) reported a similar observation for the sorption

behaviour of rice. These changes are probably due to changes in molecular structure

154

during sorption which affects the degree of activation of sorption sites. No

relationship exists between the degree of hysteresis and the variation of isosteric heats

of sorption (Al-Muhtaseb et al., 2004).

4.6.2 Gibb’s free energy

The change in Gibb’s free energy of absorbed water was calculated using eq.

3.26. The variations in Gibb’ free energy change (-∆G) of the water adsorption with

moisture content at 25oC in buffalo skim milk powder, dried casein and whey protein

concentrate powder are presented in Figs.4.23, 4.24 and 4.25, respectively. As may be

seen in the figures, the negative value of ∆G decreased with the increase in moisture

content, that is, the Gibb’s free energy of adsorbent increased with increase in

moisture content. The rate of increase, however, was slow at higher moisture levels.

At the moisture content of 3% (d.b.) the maximum value of –∆G for adsorption was

noted dried casein as 5312 J / mol, followed by 4680 J / mol for skim milk powder

and 2378 J / mol in whey protein concentrate powder.

Fig.4.23. Variation in Gibb’s free energy of adsorption and desorption

of buffalo skim milk powder with moisture content at 25oC.

155

Fig.4.24. Variation in Gibb’s free energy of adsorption and desorption of dried

casein with moisture content at 25oC.

Fig.4.25. Variation in Gibb’s free energy of adsorption and desorption of whey

protein concentrate powder with moisture content at 25oC.

156

Similar trend was noticed in desorption process with maximum value of –∆G

in dried casein as 5756 J / mol, followed by 4875 J / mol for skim milk powder and

4618 J / mol in whey protein concentrate powder. The amount of free energy was

more in adsorption process as compared to desorption. In skim milk powder and dried

casein the difference in free energy of both adsorption and desorption processes

remained between 3 to 20% (d.b.) moisture content, but in whey protein concentrate

powder this difference increased with the decrease in moisture content below 15%

(d.b.). However in all the products the difference in Gibb’s free energy in adsorption

and desorption processes coincided with each other above the moisture content of

25% (d.b).

4.6.3 Differential entropy

The differential entropy values for adsorption and desorption, at given

moisture contents, were calculated for buffalo skim milk powder, dried casein and

whey protein concentrate powder by using equation 3.28. The variation in differential

entropy with moisture content is presented in Fig.4.26 for buffalo skim milk powder,

Fig. 4.27 for dried casein and Fig. 4.28 for whey protein concentrate powder.

Fig.4.26. Variation in differential entropy of adsorption and desorption of buffalo skim milk powder with moisture content.

157

Fig.4.27. Variation in differential entropy of adsorption and desorption of dried casein with moisture content.

Fig.4.28. Variation in differential entropy of adsorption and desorption of whey protein concentrate powder with moisture content

158

The entropy data displays a strong dependence on moisture content. The

adsorption data gives lower values than those for desorption. In dried casein and whey

protein powder the entropy decreased smoothly with increase in moisture content to a

minimum value at 20% (d.b.) moisture content. The difference in adsorption and

desorption entropy ended at 20 % (d.b.) moisture content and both adsorption and

desorption curves coincided with each other. In skim milk powder the entropy

decreased with increase in moisture content, but the decrease was not smooth. This

difference in adsorption and desorption entropy also persisted to 30% (d.b.) moisture

content. Similar trends in differential entropy has been reported in tow mints by Kane

et al. (2008) and in potato by McMinn and Magee (2003).

4.7 Effect of moisture sorption on engineering properties

Relative humidity of the air, interstitial as well as head space, in the storage

container affects the bulk materials’ properties. The bulk materials which are

hygroscopic when exposed to humid conditions result in increased moisture content

of the bulk (Ganesan et al., 2008). Physical properties of the materials are highly

dependent on moisture content and each product will behave differently. There are

several parameters which influence the functional engineering properties of milk

powders but some of their characteristics may be directly related to moisture sorption.

In order to understand the effect of moisture sorption on powder properties, the

freshly prepared samples of buffalo skim milk powder, dried casein and whey protein

concentrated powder were equilibrated to four different water activity levels, 0.11,

0.328, 0.529 and 0.689 at 25oC, in the controlled humidity chambers, using saturated

salt solutions. The equilibrated samples were analyzed for their functional engineering

properties using standard procedures as described in section 3.3. All the three powders

were found to pick up moisture from the humid air, by virtue of which, the most

affected engineering properties were bulk density and the flowability of the powders.

159

4.7.1 Effect of water activity on bulk density

The effect of water activity on bulk density of buffalo skim milk powder, dried

casein and whey protein concentrate powder is presented in Fig. 4.29. Bulk density of

the powders increased with moisture pick up at higher water activities. This could be

attributed to the filling of porous spaces in the powder bulk by water molecules. Since

the equilibrium moisture content of three powders at given water activity was

different, the proportionate increase in bulk densities were also found to be different.

Fig.4.29. Variation in bulk density with water activity, -(●)- dried casein, -(■)-

skim milk powder, -(▲)- whey protein concentrate powder

In skim milk powder the bulk density increased from 0.35 g /cm3 at 0.113 water

activity (EMC 3.781% d.b.) to 0.52 g /cm3 at 0.689 water activity (EMC 17.97%

d.b.). The corresponding increase in dried casein for the above water activity range

was 0.65 g /cm3 (EMC 3.50 % d.b.) to 0.79 g /cm3 (EMC 12.01% d.b.) and for whey

protein concentrate powder was 0.33 g /cm3 (EMC 2.11 % d.b.) to 0.54 g /cm3 (EMC

13.01 % d.b.).

160

Bulk density of dried casein was found to be more than that of skim milk

powder and whey protein concentrate powder at all water activity levels. The skim

milk powder had slightly more bulk density than whey protein concentrate powder up

to the water activity of 0.529, and beyond this the whey protein concentrate powder

was found to have more bulk density. Bulk density of powders is important when

determining the volume of transport and storage vessels. Bulk density depends on

particles size as well as moisture content (Johnson 1972). Hollenbach (1983)

reported the increase in bulk density of powders when conditioners are added to them,

which results in modification of density by lowering the inter-particle interactions.

Compressibility and the bulk density of powders are interrelated. Greater the

compressibility, the less flowable the powder is (Peleg, 1977). All the above products

gained moisture when exposed to higher humid conditions. This might increase the

compressibility of the powders and lead to flow problems at increased water

activities.

4.7.2 Effect of water activity on flowability

Flowability is ability of powders to flow. It is a consequence of the

combination of material’s physical properties that influence material flow,

environmental conditions and the equipment used for handling (Prescott and

Barnum, 2000). Some of the factors that affect flowability of powders include

moisture content, humidity, pressure, fat, particles size and flow agents. Angle of

repose corresponds qualitatively to the flow properties of the granular material and is

a direct of potential flowability. Higher angles indicate material with difficult flow,

while lower angle represent a material with relatively easy flow. Angle of repose

gives a reproducible numerical vale, so it has been adopted as a common method to

161

assess flow properties (Craik and Miller, 1958). The effect of water activity on

flowability of buffalo skim milk powder, dried casein and whey protein concentrate

powder is presented in Table 4.20. It has been found that the flowability of all the

products decreased increase in water activity. For buffalo skim milk powder, the angle

of repose increased from 46.49o at 0.113 water activity to 48.64o at 0.689 water

activity. The corresponding increase in angle of repose for dried casein was 31.04o to

33.52o and for whey protein concentrate powder was from 40.3o to 42.97o.

Table 4.20 Variation in angle of repose with water activity

Water activity Angle of repose (αo)

Skim milk

powder

Dried casein Whey protein

concentrate

powder

0.113 46.49 31.04 40.30

0.328 47.03 31.87 41.68

0.529 47.83 32.76 42.04

0.689 48.64 33.52 42.97

Ganesan et al., (2008) reported that flowability of distillers dried grains

decreased with increase in moisture content. Similar effect of higher humidities on

other food products and powders has also been reported by Moreyra and Peleg

(1981). In this study dried acid casein was found to be relatively more free flowing as

compared to skim milk powder and whey protein concentrate powder. This could be

attributed to comparatively larger particle size of dried casein compared to spray dried

skim milk powder and whey protein concentrate powder. Fitzpatrick et al. (2004)

reported that the reduction in particle size often tends to decrease the flowability of

granular materials.

162

The variation in water activity did not showed any effect on dispersibility,

wettability and solubility index of freshly prepared buffalo skim milk powder, dried

casein and whey protein concentrate powder. Wettability and solubility of powders

have been reported decrease with storage (Khamrui, 2000). Long term storage of free

flowing powders can also result in flow problems due to time-consolidation

phenomenon, where powder consolidates or solidifies under its own weight and

flowability is reduced with increasing time (Teunou, 2000). Insolubility of proteins

have also been reported to increase on its long term storage high temperature and

humidity. However, the storage studies on buffalo skim milk powder, dried casein and

whey protein concentrate powder were not carried out as these were not included in

the present investigations.

4.8 Properties of Bound Water

Most of the unit operations employed in food processing and preservation

involve either removal of moisture to stabilize the material as in drying or its

immobilization in structured foods and low and intermediate moisture foods. The

bound water in foods is not available for microbial growth or chemical reactions

(Caurie, 1981). It is therefore very relevant to generate information on various

aspects of bound water in skim milk powder, dried acid casein and whey protein

concentrate powder prepared from buffalo skim milk, so as to assess these parameters

as a function of sorption temperature.

Number of adsorbed monolayers, density of sorbed water, per cent bound

water and surface area of sorption was evaluated by using moisture adsorption data on

all the three products and employing equations 3.9, 3.29 and 3.30 and are presented in

Tables 4.21 to 4.23. The density of sorbed water at 25oC was 1.97 g/cc in skim milk

powder, 1.88 in dried casein and 1.91 in whey protein concentrate powder.

163

Table 4.21 Properties of bound water in skim milk powder

Temperature,

°C

Caurie’s

slope

8o. of

absorbed

monolayer

Density of

sorbed

water

Bound/

unfreezable

water

Surface

area of

sorption

S 8 g/cc % M2/g

25 0.5042 3.97 1.97 27.39 108.17

35 0.5127 3.90 1.81 26.91 106.37

45 0.5183 3.85 1.58 26.26 105.22

Table 4.22 Properties of bound water in dried acid casein

Temperature,

°C

Caurie’s

slope

8o. of

absorbed

monolayer

Density of

sorbed

water

Bound/

unfreezable

water

Surface

area of

sorption

S 8 g/cc % M2/g

25 0.4648 4.30 1.88 24.08 117.34

35 0.4914 4.07 1.79 22.79 110.98

45 0.4842 4.13 1.75 23.12 112.63

Table 4.23 Properties of bound water in whey protein concentrate powder

Temperature,

°C

Caurie’s

slope

8o. of

absorbed

monolayer

Density of

sorbed

water

Bound/

unfreezable

water

Surface

area of

sorption

S 8 g/cc % M2/g

25 0.7248 2.61 1.95 14.08 75.31

35 0.7150 2.79 1.82 14.31 76.27

45 0.7574 2.64 1.79 11.09 72.08

164

The numbers of adsorbed monolayers were 3.97, 4.30 and 2.61 respectively in

skim milk powder, dried casein and whey protein concentrate powder. Per cent bound

water in skim milk powder was 27.39 and that in dried casein was 24.08 and in whey

protein concentrate powder was 13.38. Surface area of sorption was maximum in

dried casein 117.34 m2/g followed by skim milk powder 108.17 and whey protein

concentrate powder had the lowest surface area of sorption as 71.31 m2/g.

Temperature of sorption had profound influence on properties of bound water

and surface area of adsorbent. In skim milk powder during adsorption, the surface

area of adsorbent reduced from 108.17 to 105.22 m2/g as the temperature rose from 25

to 45oC. The corresponding decrease in dried casein was from 117.34 to 112.63 m2/g

and in whey protein concentrate powder from 75.31 to 72.08 m2/g. Similarly, per cent

bound water decreased from 27.39 to 26.56 in skim milk powder, 24.08 to 23.12 in

dried casein and 75.31 to 72.08 in whey protein concentrate powder. The decrease in

density of bound water was from 1.97 to 1.58, 1.88 to 1.753 and 1.95 to 1.79 g /cc for

skim milk powder, dried casein and whey protein concentrate powder respectively as

the temperature was increased from 25 to 45oC.

Water sorption is influenced by surface area, composition, the number of

surface binding sites and the porosity of protein particles. The number and size of

pores in the protein matrix determine total sorption area and the size and surface

p[properties of the pore influence rate and extent of hydration (Kapsalis, 1981).

Schumacher and Cox (1961) reported a relationship between surface area of

globular proteins and the amount of monolayer bound to them. Increased surface area

exposes more charged polar groups and carbonyl functions of peptide bonds

enhancing water sorption. Increase in temperature might influence the surface area of

products under this study by some conformational changes occurring in the proteins.

Caurie (1981) reported the surface area values of egg albumin, beta-lactoglobulin and

serum albumin as 124, 114 and 127 m2/g respectively.

165

It is interesting to note that the density of bound water decreased with the decrease

in amount of bound water and surface area. This shows that the decrease in bound

water is not in proportion to the decrease in surface area. The density of normal free

water is determined by the distance between the water molecules which are

determined by the forces acting between the molecules. The closer the intermolecular

distances, the greater the force acting between the molecules, therefore higher the

density of water. When a monolayer is adsorbed on the surface of adsorbent the

intermolecular distance between water molecules are reduced and at the same time

they are affected by the surface forces of high magnitude which together alter the

density of adsorbed moisture (Caurie, 1981). It can be noticed from the data

presented in the Table 4.18 to 4.20 that an increase in sorption temperature resulted in

considerable decrease in bound water, but did not cause proportionate decrease in

surface area. This resulted in loose packing of water molecules causing corresponding

decrease in density of bound water.

results a%d discussio% - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/3306/12/12...(1980)...

Documents