equilibrium moisture curves · 2019. 8. 18. · equil. moisture content = f ( temperature, humidity...
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• i. Ten grams (10g) of corn kernels were
dried in a convection oven before using
them for volume and specific gravity
measurements. The final weight and the
moisture content of the sample were 8.1 g
and 8% db, respectively. Calculate the
initial moisture content of corn kernels on
wet and dry basis.
10g Final
WB
Final 8.1 g 8% dry basis 0.074074 7.41%
Initial MC
Moisture in
Dry mass final 0.60021 7.49979
db moisture% 0.333371 33.30%
wb 0.250021 25.00%
Free water versus bound water
• Water activity is sometimes defined as
"free", "unbound", or "available water" in
a system. Water activity instruments
measure the amount of free (unbound or
active) water present in the sample.
• A portion of the total water content
present in a product is strongly bound to
specific sites on the chemicals of the
product.
Read More: http://www.fst.ohio-
state.edu/olympiad/Laboratories/Olymp/Lab%201_W
aterActivity.htm
• These sites may include the hydroxyl
groups of polysaccharides, the carbonyl
and amino groups of proteins, and other
polar sites.
Hydrogen bonds, ion-dipole bonds, and
other strong chemical bonds tightly bound
water.
Some water is bound less tightly, but is still
not available (as a solvent for water-
soluble food components).
Many preservation processes attempt to
eliminate spoilage by lowering the
availability of water to microorganisms.
Reducing the amount of free--or unbound--
water also minimizes other undesirable
chemical changes that occur during
storage.
• Eg. concentration, dehydration, freeze
drying and Freezing. Water in frozen foods
is unavailable to microorganisms and for
reactions with food components
Basic concepts
Moisture content = mass of water / unit mass of dry solids
Equil. moisture content = f ( temperature, humidity )
Sorption isotherms : curves relating the m.c. of the material and the humidity of the atmosphere with which it is at equilibrium at different temperatures.
Sorption isotherms of foods are also expressed as moisture content vs. water activity.
Water activity aw = P/Po
(vapor pressure of food moisture/ saturation vapor pressure of pure water)T
Relative humidity of atmosphere = pw / ps
(partial pressure of water vapor/ saturation partial pressure of pure water)T
At equilibrium: pw = P , ps = Po
Sorption behaviour of foods is important in:1. Studying mechanisms of drying and designing dehydration
processes,2. Predicting storage stability of dried foods.
PP0
TºC
RH
%
Water activity (aw)
Eq
uilib
riu
m m
ois
ture
co
nte
nt
(%d
b)
0
10
20
30
40
50
60
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Equilibrium moisture curvesIsotherm
Curve shows the relationship between water
activity (aw) and equilibrium moisture
content (%db).
Typical shape of a sorption isotherm
Relationship between EMC & ERH
ERH
EMC
Water Activity
0 ----------------------------50%--------------------------- 100%
0 --------------------------- 0.5----------------------------- 1
• Different Materials gives different curves
Moisture Sorption Isotherm
aw
Mo
istu
re c
onte
nt
(d.w
.b.)
Zone 3
Zone 2
Zo
ne 1
Zones in Isotherms
• Zone 3: Bulk water
• Zone 2: Loosely bound water
• Zone 1: Tightly bound water.
• EMC depends on
Relative Humidity (RH)
Temperature
Time
MC%
T1 , RH1%
T1 , RH2%
EMC1
EMC2
•Therefore EMC should be given with the environmental conditions
• RH EMC
T EMC
RH1 > RH2
Temperature effect on isotherms
0
5
10
15
20
25
30
35
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Water activity
Eq
uilib
riu
m m
ois
ture
co
nte
nt(
%d
b)
Series1
Series2
Series3
45o C
55o C
35o C
Sorption isothermsfor some selecteddried fruits
Equilibrium moisture curves of different
materials at room temperature (25ºC)
Back to previous
Hysteresis effect
True for
many
foods
Uses of EMC curves
• Can find moisture content in a product in a
given temperature and relative humidity.
• Use in selecting the Storage conditions of
the products.
• Use in selecting the Drying conditions of
the products.
• Use in ingredient selection of foods.
Critical
Source: T.Labuza (1992).
User’s manual,
Water analyzer programs
▪ Preservation aw Limits
• bacteria ~ 0.91 yeast~0.88
• molds ~ 0.7 all microbes ~0.6
Rate of Oxidation of Potato
Chips
0.1
1
10
100
0 0.2 0.4 0.6 0.8
aw
Rela
tive r
ate
consta
nt
Monolayer moisture
0
0.05
0.1
0.15
0 0.5 1
aw
Mois
ture
conte
nt
(dw
b)
The GAB Model
Mm Cka
ka kaw Cka
w
w w
0
1 1( )( )
mo -monolayer value
k- multilayer parameter
C -temperature dependency parameter
db
Texture ChangesM
ois
ture
conte
nt
(d.w
.b.)
Soft
Crispy/crunchy
0.2-0.5
Powder ChangesM
ois
ture
conte
nt
(d.w
.b.)
Agglomerated
Free flowing
~0.4
▪ Conditions provided
• Temperature - Using incubators
• Relative humidity -Using salts
• Duration - 3 weeks
Method of Developing EMC curves
Two methods:
1. Static method (Wink’s weight equilibrium method) -
slow (3-4 weeks) –probs. of fungi attacks
2. Dynamic method – fast – conditioned air is
agitated or passed over the product
▪ Creation of constant humidity environments
Name of the
chemical
Relative Humidity at
35o C 45o C 55o C
KF
MgCl2.6H2O
K2CO3
NaBr
NaNO2
NaCl
KCl
K2SO4
24.59
32.05
43.64
54.55
62.84
74.87
82.95
96.71
21.46
31.10
42.99
51.95
60.11
74.52
81.74
96.12
20.60
29.93
42.40
50.15
57.64
74.41
80.70
95.53
Table 01- Greenspan values for saturated salt solutions by FDA.
Usually used for low RH trials but safe use is important
▪ Model fitting
• Guggenheim, Anderson,deVreis (GAB) equation -
m = (mo*Kb*C*aw) / (1- Kb*aw) (1- Kb*aw+ C*Kb*aw)
• Modified Handerson equation - Good for grains
m = (ln(1- aw) / (-A*(T+C)))^(1/B)
• Chung and Pfost - Accurate between 20-90%
m = (-1/B) ln ( - (T+C) / A ln (aw))
Method using ‘statistica’ software
• Determination of EMC isotherms and appropriate mathematical
models for canola
• Ali Zomorodian, a, , Zahra Kavoosia and Leila Momenzadeha
• a Agricultural Engineering Department, Shiraz University, Shiraz,
Iran
• Received 29 April 2010;
• revised 10 October 2010;
• accepted 18 October 2010.
• Available online 4 November 2010
http://www.sciencedirect.co
• Equilibrium Moisture Content of Thai Red
Chillies
•
• Sukruedee Nathakaranakule*,
Wattanapong Rakwichian*, Rak
Dandamrongrak**, Sirichai Thepa
• - Posted to MIS
Evaluation of goodness of fit of a model
• Coefficient of determination (R2),
• Mean Relative Error (MRE),
• the Standard Error of Estimate (SEM)
• and residual plots (Mohamed et al., 2004).
N
i i
preii
M
MM
NMRE
1 exp,
,exp,100
f
N
i
preii
d
MM
SEM
1
2,exp,
Where Mi, exp is the ith experimental moisture content, Mi,
pre is the ith predicted moisture content, N is number of
observations and df is the number of degrees of freedom
of the regression model.
▪ Evaluation of goodness of fit –Contd.
• Mean Relative Deviation Modulus (P)
(P < 5%)
P = (100/n)* (Mi – Mpi) / Mi
Mi - Moisture content observation i
Mpi- Predicted moisture content
n - Number of observations