36

TEXTURE OPTIMIZATION OF IDLI

3.1 INTRODUCTION

Indigenous or native fermented foods have been prepared and consumed for thousands of

years, and are strongly linked to culture and tradition. The fermented foods are better in

terms of nutrition and easy for digestion than the normal cooked foods. The fermentation

process causes enrichment and improvement of food through flavour, aroma, and change

in texture, preservation by providing organic acids, nutritional enrichment, reduction of

exogenous toxins and reduction in the duration of cooking. During traditional

fermentation process, locally available ingredients, which may be of plant or animal

origin, are converted into edible products by the physiological activities of

microorganisms and have distinct odour (Steinkraus 1996, Reddy and Salunkhe 1980)

namely Lactobacillus sp. and Pediococcus sp. which produce organic acids such as lactic

acid and acetic acid, alcohol and carbon dioxide (Caplice and Fitzgerald 1999) and

reduce the pH, thereby inhibiting the growth of food spoiling microorganisms. These

fermented foods can be preserved for several days (Tamang, 1998) and also have

therapeutic properties (Sekar and Mariappan, 2007). There are different types of

fermented foods in which a range of different substrates are metabolized by a variety of

microorganisms to yield products with unique and appealing characteristics (Caplice and

Fitzgerald 1999). Among all traditional fermented foods in India, idli is a white,

fermented (acid leavened), steamed, soft and spongy texture product, widely popular and

consumed in the entire South India. Idli is the resultant product from the naturally

fermented batter made from washed and soaked rice (Oryza sativa L.) and dehusked

black gram dhal (Phaseolus mungo L.). Apart from its unique texture properties, idli

makes an important contribution to the diet as a source of protein, calories and vitamins,

especially B-complex vitamins, compared to the raw unfermented ingredients (Reddy et

al., 1982).

Traditionally, rice and black gram in various proportions are soaked and ground adding

water in mortar and pestle to yield a batter with the desired consistency. Parboiled rice is

preferred over raw rice for idli and dosa with rice: black gram usually fermented at 3:1

37

(Steinkraus et al, 1967, Jama and Varadaraj, 1999) weight ratio for making soft and

spongy textured idli (Nazni and Shalini 2010). Black gram, the leguminous component of

idli batter, serves not only as effective substrate but also provides the maximum number

of micro-organisms for fermentation (Balasubramanian and Viswanathan, 2007a). As a

result of fermentation, (Padhye and Salunkhe, 1978) observed a significant increase in

predicted biological value. Fermentation also improves the protein efficiency ratio (PER)

of idli over the unfermented mixture (Van Veen et al, 1967).

Idli preparation in the conventional manner takes at least 18 h. The available instant idli

pre-mixes do not provide the desired textural characteristic and also lack the typical

fermented aroma and on the other hand, idli prepared in different households do not have

consistent quality (Nisha et al, 2005). Fermented foods in general have immense scope

for commercialization as foods with improved nutritional value as well as functional

foods. Fermented foods with scientifically developed starter cultures can aid the

commercialization of these products. However scientific optimization of the process is

the basic necessity for commercialization of any product including the fermented foods.

Several researchers have used RSM successfully to optimize the conditions for making

products like boondi (Ravi and Susheelamma, 2005), tandoori roti, puri and parotta

(Saxsena and Haridasrao, 1996 and Vatsala 2001) .The current study is undertaken to set

an optimized condition for the preparation of idli which will help the manufacturers at

industrial level to produce idli with the desired textural property. This would also help to

make proprietary products using proper starter culture. The main objectives of this study

were to explore the effect of rice and black gram dhal and fermentation time on the

texture of idli, analyzing the instrumental texture profile (TPA) parameters as a function

of raw material composition and fermentation time and to find the optimum levels to

maximize the desirable textural properties of idli using RSM.

38

3.2 MATERIALS AND METHODS

3.2.1 Materials

Different rice varieties namely IR 20 idli rice, raw rice, broken rice, ration rice and red

rice were procured from local market and black gram variety Aduthurai 3 (ADT3) which

has 24.16 per cent protein content was procured from Tamil Nadu Rice Research

Institute (TRRI), Aduthurai, Tamil Nadu, India. They were cleaned and stored at

refrigerated conditions until use.

3.2.2 Preparation of idli

Before framing the design using CCRD, preliminary trails were conducted to choose the

ratios of rice to black gram dhal. The trails were done using the rice to black gram dhal

ratios as 3:0.5, 3:1, 3:1.5, 3:2, 3:2.5 and 3:3 respectively where rice ratios were kept

constant and the dhal ratios varied. The fermentation time varied between 10 to 14h. In

the trial, idli made from the ratio 3:1 and 3:1.5 with a fermentation time between 11 to 12

h gave better results. Based on this, the maximum and minimum values for the

independent variables were chosen to frame the model. The rice and black gram dhal

were mixed at different ratios as per the CCRD (Table 2.1). The rice and dhal were

soaked for 4 h and ground separately to a coarse consistency and mixed together with

salt. The batter was left overnight (time based on the developed design) for fermentation.

The fermented batter was mixed thoroughly to expel the gas formed due to the release of

carbon-dioxide .The batter was poured in idli mould, and steamed in the idli steamer for

15 minutes. The idli were brought to room temperature and then used for instrumental

texture profile.

3.2.3 Experimental design

3.2.3.1 Response surface Methodology

RSM is a collection of statistical and mathematical techniques useful for developing,

improving, and optimizing processes in which a response of interest is influenced by

several variables and the objective is to optimize this response. RSM has important

39

application in the development and formulation of new products, as well as in the

improvement of existing product. It helps to study the effect of the independent variables,

alone or in combination, on the responses. In addition to analyzing the effects of the

independent variables, it provides a mathematical model, which describes the

relationships between the independent and dependent variables (Myers and Montgomery,

1995). RSM has been very popular for optimization studies in recent years. RSM reduces

the number of experiment trials needed to evaluate multiple parameters and their

interactions. The graphical perspective of the mathematical model has led to the term

Response Surface Methodology. Generally an optimization study involving RSM has

three stages. The first stage is the preliminary experimental trials, in which the

determination of the independent variables and their limits are carried out. The second

stage involves the selection of appropriate experimental design followed by prediction

and verification of the model equation. The last stage is the generation of response

surface plots as well as contour plots of the responses as a function of the independent

parameters and determination of optimum conditions. The model used in RSM is

generally a full quadratic equation or the diminished form of the equation. The second

order model can be written as Eqn.1.

where Y is the predicted response, β0, β j, β jj and β ij are regression coefficients for

intercept, linear, quadratic and interaction coefficients respectively, k is the number of

independent variables and Xi and Xj are coded independent variables.

Response surface methodology has been widely applied in the food industry optimizing

complex processes and products (Wong et al, 2003, Lee et al, 2006 and Sin 2006). In the

present study RSM was used to determine the optimum conditions of two independent

variables (rice to black gram dhal ratio and fermentation time) on the TPA and colour

attributes of idli. A CCRD was constructed using software package Statistica (1999) from

StatSoft, OK, USA. Five levels of each predictor variable were incorporated into the

developed design. Table 1 shows levels of predictor variables.

…………….Eqn. 1

40

3.2.3.2 Optimization of idli

The procedure was based on the hypothesis that quality attributes of idli were

functionally related to rice to black gram dhal ratio and fermentation time, and attempts

were made to fit multiple regression equations describing the responses. Two coded

independent variables in the process were rice to black gram dhal ratio (X1) and

fermentation time (X2). Five levels of each of the independent variable were chosen for

the study (Table 3.1); thus, there were 13 combinations, including the replicates of the

center point that were performed in random order, based on an experimental CCRD for

two factors. The dependent variables were hardness, adhesiveness, springiness,

cohesiveness, chewiness and resilience and colour attributes.

Table 3.1

Central composite rotatable design: Coded and actual values of independent

variables

Experimental

design points

Rice : black gram Ratio

(w / w)

Fermentation time

(h)

Actual Coded

Actual Coded

1 3 : 0.72 -1.000 10.58 -1.000

2 3 : 0.72 -1.000 13.42 1.000

3 3 : 1.78 1.000 10.58 -1.000

4 3 : 1.78 1.000 13.42 1.000

5 3 : 0.50 -1.414 12.00 0.000

6 3 : 2.00 1.414 12.00 0.000

7 3 : 1.25 0.000 10.00 -1.414

8 3 : 1.25 0.000 14.00 1.414

9 3 : 1.25 0.000 12.00 0.000

10* 3 : 1.25 0.000 12.00 0.000

*Centre point repeated 3 times

3.2.3.3 Instrumental Colour Measurement

The colour parameters of idli were measured using a Hunter Lab colour flex model A60-

1012-312 (Hunter Associates laboratory, Reston, VA). The equipment was standardized

each time with white and black standards. Samples were scanned to determine lightness

(L*), red-green (a*) and yellow-blue (b*) colour components (Olajide, 2010). As in the

41

work done by (Ronald and Daniel, 1998) the hue angles were derived as the arctangent of

b*/a* expressed as degrees and the chroma values were also calculated as the square root

of the sum of the squared values of both CIE a* and CIE b*.

The chroma and Hue angle were calculated by the formula Eqn.2 and Eqn. 3,

respectively.

Where a* indicated Red-Green colour components, while b* indicates yellow to blue

colour components (Ali, 2008).Plate 3.1 shows the picture of colour flex.

Plate 3.1 Color flex

3.2.3.4 Texture profile analysis (TPA)

The TPA test consists of compressing a bite-size piece of idli two times in a reciprocating

motion that imitates the action of the jaw. The idli was cooled to room temperature and

was cut into an inch cube (Plate 3.3) using an inch cubic mould (Plate 3.2.a). The texture

of each idli was analyzed using SMS/75mm (Plate 3.2.b) compression platen in Texture

…………….Eqn. 2

…………….Eqn. 3

42

Rice (IR20 idli rice) and Black

gram dhal (ADT3) (ratio based

on the experimental design)

Soak (4h) and grind

Pour batter in idli mould and steam

for 15 minutes

Ferment the ground batter (Based

on the experimental design)

Cool idli to room temperature and

cut the centre using one inch cubic

mould

TPA of cut idli using SMS/75mm

compression platen

Statistical Analysis

(RSM; Regression)

43

Fig.3.1 Flow chart showing work design for TPA of idli

Analyzer (Stable Micro Systems, Surrey,UK). The extra top and bottom layers were

sliced off to make the idli fit to the mould. The cut piece was placed on the heavy duty

platform and the test speed was set to 5mm/sec and the probe compressed 50% of the idli

to get the TPA of the idli. Based on the force deformation curves, several parameters like

adhesiveness, springiness, cohesiveness, chewiness and resilience can be calculated.

Plate 3.2 Cutting idli with the designed mould

44

Plate 3.3 One inch cubic mould and SMS/75mm compression probe

Plate 3.4 Texture analyzer

a)

b)

45

3.2.4 Statistical Analysis

The independent variables and dependent variables (responses) were fit to the second-

order polynomial function and examined for the goodness of fit. The R2 or coefficient of

determination is defined as the ratio of explained variation to the total variation and is a

measure of the degree of fit (Haber and Runyon, 1997). All experimental designs and

statistical data were analyzed and response surfaces, ANOVA, regression analysis were

reported using Statistica (StatSoft, OK, USA) statistical software.

3.3 RESULTS AND DISCUSSION

The results of chapter 3 are discussed under the flowing heads:

3.3.1Effect of rice varieties on rice batter volume

3.3.2 Effect of black gram on batter volume

3.3.3 Effect of ratios of rice to black gram dhal on batter volume

3.3.4 Response surfaces

3.3.5 Instrumental Colour measurement of idli

3.3.6 Texture parameters

3.3.7 Simultaneous optimization

3.3.1 Effect of rice varieties on rice batter volume

In the present study five varieties of rice namely ration rice, raw rice; broken rice, red rice

and parboiled rice were used for idli making. The rise in CO2 production can be

correlated with the increase in batter volume (Sridevi et al., 2010).The percentage of

increase in batter volume was significant (p< 0.05) in the batter prepared with ration rice,

followed by parboiled rice, raw rice, broken rice, and red rice. Though there is high

increase in batter volume, after expulsion of gas the volume of batter gets significantly

decreased in ration rice, whereas the batter volume did not show significant (p< 0.05)

decrease in parboiled rice. Table 3.2 and Fig 3.2.a shows the effect of rice varieties on

batter volume. The sensory score of idli showed variation with the variety of rice used.

As the idli prepared from parboiled rice is very soft when compared with idli made with

other varieties. Parboiled rice may be best suited for idli making which is in par with the

46

result reported by Juliano and Sakurai (1985) that parboiled rice is better suited than raw

rice for producing idli , i.e., it is soft without becoming sticky. The idli prepared using

very light coloured parboiled rice are preferred by consumers traditionally accustomed

to eating raw rice. Sowbhagya et al., (1991) studied the effect of variety, parboiling and

ageing of rice on the quality of idli and reported that the normal parboiled rice is best

suited for making idli as shown by its higher scores for softness. In the present study the

idli made of parboiled rice is soft and it may also be due to fact proved by Sharma et al.,

(2008) that the greater starch damage in parboiled rice during wet grinding, attribute to its

greater susceptibility to undergo damage owing to its softness after soaking as well as to

the longer duration of grinding favouring parboiled rice to be suited for idli making. Roy

et al., (2010) noted that the hardness and adhesion of cooked rice were dependent not

only on the moisture content but also on the forms and variety of rice. Roy et al., (2004),

Roy et al., (2008), Islam et al., (2001) and Shimizu et al., (1997) reported that the

hardness of the cooked rice depend on the moisture content of cooked parboiled and

untreated rice. In case of idli, steaming increases the moisture content of idli and it is a

major factor that makes idli made with parboiled rice softer and for the same reason that

red rice has acquired more moisture which affected its texture losing firmness.

3.3.2 Effect of black gram on batter volume

The percentage of increase in batter volume was significant (p< 0.05) at five per cent

level (Table 3.3) for the batter made from parboiled rice and black gram used with husk,

and thou the idli made from the same batter were spongy, the colour was unappealing to

the panel members. The difference in batter volume was not significantly higher with the

batter made from the black gram with husk removed. On the other hand, though, the

percentage of increase in batter volume was low (38.9%) in the batter made from

parboiled rice and black gram dhal with husk removed after soaking,

47

Table 3.2

Effect of rice varieties on the batter volume after fermentation

mean values in a row with different letters differ significantly p<0.05 by LSD (n=3)

the texture was very spongy and the colour was also appealing making this variation a

better choice in terms of colour and texture on sensory basis. Fig 3.2.b shows the effect of

variation in black gram dhal on batter volume.

3.3.3 Effect of ratios of rice to black gram dhal on batter volume

The percentage of increase in batter volume was high for the ratio 3:3.5 (w/w) of rice to

black gram dhal respectively with 5% significance followed by other ratios such as 3:3,

3:2.5 and so on. When the texture of idli was compared on sensory basis, the idli made

of ratio 3:1 was very spongy compared to idli made of other ratios of rice and black gram

dhal showing that the proportions of compositions of the substrate also have an important

role in the outcome of the product. Table 3.4 and Fig 3.2.c shows the effect of ratios of

ingredients on batter volume. Hence for the further study parboiled rice namely IR 20,

black gram variety namely ADT 3 with husk removed after soaking was used to find the

effect of ingredients and descriptive sensory profile of idli.

Batter characteristics

Varieties of rice

Parboiled

rice

Ration

rice

Raw

rice

Broken

rice

Red

rice

Initial volume of the batter (cm3)

221.6 ±2.05 d

211.1±1.55 b 218.6±0.98 c 200.5±0.14 a 238.2±0.14 e

Final volume of the batter (cm3)

306.1± 3.74 e

299.7±1.41 d 275.8±2.61 b 248.8±2.61 a 293.7±3.53 c

Batter volume increased after

fermentation (%) 38.1±1.27 c 42.0±0.70 d 26.2±0.35 b 24.1±0.21 a 23.3±0.28 a

Volume of the batter after

expulsion of gas (cm3)

202.0±0.56 e 150.8±0.84 a 181.0±0.70 c 193.0±1.83 d 158.0±1.41 b

Batter volume decreased after

expulsion of expulsion of gas (%) 34.0±1.41 b 49.7±0.00 d 34.4±0.63 b 22.4±1.41 a 46.1±0.0 c

Sensory Rank I IV II III V

48

Table 3.3

Effect of black gram (var. ADT 3) on the batter volume after fermentation

Mean values in a row with different letters differ significantly p<0.05 by LSD (n=3)

BHR-black gram husk removed; BHRAS-black gram husk removed after soaking; BWH -black gram with husk

Table 3.4

Idli batter volume characteristics as affected by parboiled rice

and black gram dhal (without husk)

Mean values in a row with different letters differ significantly p<0.05 by LSD (n=3)

Batter characteristics

Black gram

BHR

BHRAS BWH

Initial volume of the batter (cm3)

263.8±0.14 b 271.4 ±0.14 c 226.1 ±0.84 a

Final volume of the batter (cm3)

339.2±0.131 b 376.9 ±0.07 c 324.1 ±0.00a

Batter volume increased after

fermentation (%)

28.6 ±0.00 a

38.9 ±0.07 b

043.3 ±0.07 c

Volume of the batter after

expulsion (cm3)

248.7 ±0.35 b

256.3 ±0.42 c

211.1 ±0.07 a

Batter volume decreased after expulsion

of gas (%)

26.7 ±0.00 a

32.0 ±0.00 b

34.9±0.14 c

Sensory Rank II I III

Batter

characteristics

Rice and black gram ratio (w/w)

3 : 1

3 : 1.5 3 : 2 3 : 2.5 3 : 3 3 : 3.5

Initial volume of the

batter (cm3)

150.7 ±0.07a 241.2±0.28c 301.5±0.21d 324.1 ±0.14e 339.2 ±1.13f 414.6 ±0.07g

Final volume of the

batter (cm3)

248.7±1.13a 316.6 ±0.84c 422.7 ±0.07d 452.3 ±0.07e 467.4 ±0.00f 603.1 ±0.07g

Batter volume

increase after

fermentation (%)

65.0 ±0.0a

31.3 ±0.07c

40.2 ±0.07d

39.6 ±0.28e

37.8 ±0.35f

45.5 ±3.0.07g

Volume of the batter

after expulsion (cm3)

158.3 ±0.21a

173.4 ±0.0b

233.7 ±0.14d

248.7 ±0.28e

256.3 ±0.14f

301.5 ±0.0g

Batter volume

decrease after

expulsion of gas (%)

36.3 ±0.28b

45.2 ±0.28d

44.7 ±0.28c

45.0 ±1.41d

44.2 ± 1.13c

50.0±0.14e

Sensory Rank I III II IV V VI

49

Fig.3.2.c Effect of ratios of rice to black gram dhal on batter volume after

fermentation

Fig.3.2.a Effect of rice varieties on batter volume after fermentation

Fig.3.2.b Effect of type of dhal on batter volume after fermentation

50

3.3.4 Response surfaces

Several parameters namely raw material variety, quality, their proximate composition,

raw material composition, particle size, temperature etc., affect the texture of idli but still,

the texture of idli is very unique from the consumer point of view. Among all the

parameters mentioned, fermentation time is one of the key factors which can affect the

texture due to its air production and leavening action. The texture of the cooked idli is a

subject of interest, to judge and optimize the production process of good textured idli

with the selection of the ingredients and the process. The fermentation periods are

slightly different for idli making owing to the difference in raw materials, composition,

process and region (Balasubramanian and Viswanathan, 2007b).

3.3.5 Instrumental Colour measurement of idli

Colour of the idli is one of the most important parameter for the acceptability of the

product. The colour of the idli showed variation based on the ratio of rice and black gram

dhal used. The L*, a*, b* values and graph are shown in Table 3.5 and Fig.3.3.a, b, c

respectively. The L* value which correspond to lightness ranged from 73.40 to 75.99

indicating the difference in the proportion of black gram dhal used. The positive values of

b* indicates yellowness in the idli, which may be due to the use of black gram with husk

for soaking. The chroma (Fig.3.3.d) values are closer to the b* values. The hue angle

value corresponds to whether the object is red, orange, yellow, green, blue, or violet (Ali

et al, 2008). The negative values in the hue angle shows that the product deviates from

the colour adding positive factor to the current study because lightness in the colour of

the idli is an important factor in the view of customer perception. The intensity of chroma

is low for the idli made with the ratio of 3:0.5 and is higher for the idli made from the

ratio 3:2 showing that the ratio of rice and dhal used for idli making has an impact on the

intense of chroma of the idli.

51

Table 3.5

Experimental design: CCRD with actual levels of independent variables for colour

parameters

*Centre point replicated 3 times

Experimental

design points

Instrumental colour parameters

L* a* b* Chroma Hue angle

(°)

1 74.03 + 0.07 -0.44+0.021 11.52+ 0.064 11.56 -87.72

2 74.13 + 0.07 -0.57+0.007 10.60+0.035 10.59 -86.92

3 75.76 + 0.11 -0.25+0.028 12.21+0.085 12.15 -88.92

4 73.99 + 0.06 -0.24+0.021 13.57+0.007 13.56 -89.03

5 75.57 + 0.07 -0.76+0.035 10.01+0.360 9.936 -85.79

6 75.78 + 0.03 -0.02+0.070 15.97+0.085 16.03 -89.89

7 73.40 + 0.11 -0.43+0.014 13.09+0.177 12.96 -88.14

8 74.32 + 0.51 -0.13+0.014 11.88+0.205 11.74 -89.14

9 74.35 + 0.11 -0.40+0.007 10.56+0.163 10.44 -87.81

10* 74.36 + 0.05

-0.43+0.028 10.61+0.361 10.35 -87.73

52

Fig. 3.3.a. Response surface graph showing relation between independent parameters on L*

Fig. 3.3.b Response surface graph showing relation between independent parameters on a*

53

Fig.3.3.c Response surface graph showing relation between independent parameters on b*

Fig.3.3.d Response surface graph showing relation between independent parameters on Chroma

54

3.3.6 Texture parameters

The experimental values for the response variables of texture analysis are shown in Table

3.6. Figure 3.4.a and Figure 3.4.b shows the typical TPA graph. Hardness of idli is

indicated by the maximum force required to compress the idli and usually represented by

the first peak in the graph. The hardness of the idli (Fig.3.5.a) varies between a minimum

force of 20.58 N to a maximum force of 44.19 N i.e., the minimum force was required to

compress idli of ratio 3:0.72 at 13.42 h fermentation time and the maximum force for the

ratio 3:1.78 at 10.58 h of fermentation time. This variation in the force is due to the

variation in the ratio of the ingredients and fermentation time of the batter. Higher the

force shows that harder is the idli. ANOVA results indicated that the ratio of rice and

black gram dhal used for idli making (in the linear effect) is significant (P< 0.05) to the

hardness of the idli. The co-efficient of regression is given in Table 3.7. The goodness of

fit was high with R2 value =0.942.

Adhesiveness of idli can be defined as the negative force area for the first bite and

represents the work required to overcome the attractive forces between the surface of the

cut piece of idli and the surface of the probe with which the idli comes into contact, i.e.

the total force necessary to pull the compression plunger away from the food. The

negative area in the graph is taken as the adhesiveness. The adhesiveness of the idli varies

between -0.00051N s to -0.05127 N s. If the product is sticky, the adhesiveness will be

higher. Ghasemi et al, (2009) reported that the adhesiveness may be due to the

gelatinization and more fluidity of rice starch structure in the cooked samples. As idli is

adhesive in nature, to optimize the product minimum adhesiveness can be considered. In

the current study since the batter was coarse ground and cooking time was constant the

adhesiveness must be due to the ratio of rice and dhal and the quality of the ingredient.

The minimum adhesiveness is obtained for the idli made of ratio 3:0.5 at 12 h

fermentation time and the maximum adhesiveness is obtained for the ratio 3:0.72 at

10.58h fermentation time. Fig. 3.5.b shows the response surface graph for adhesiveness.

55

Fig.3.4.a Texture profile of idli made of ratio 3:1.25 at 12 h fermentation time

Fig.3.4.b Texture profile of idli made of ratio 3:2 at 12 h fermentation time

Force (N)

Force (N)

Time (sec)

Time (sec)

56

Table 3.6

Experimental design: CCRD with coded and actual levels of independent variables

for TPA

Experi

mental

design

point

Dependent variables

Hardness

(N)

Adhesiveness

(N s)

Springiness Cohesiveness Chewiness Resilience

1 23.73±2.01 -0.0512±0.0045 0.926±0.33 0.876±0.12 1963.61± 16.26 0.595 ± 0.12

2 20.58±1.42 -0.0337±0.0038 0.960±0.28 0.819±0.04 1650.89±14.05 0.562 ± 0.52

3 44.19±2.02 -0.0284±0.0042 0.809±0.41 0.643±0.09 2344.08±21.01 0.340 ± 0.41

4 36.57±2.24 -0.0005±0.0037 0.847±0.20 0.674±0.07 2127.97±16.42 0.404 ± 0.24

5 20.66±3.52 -0.0051±0.0069 0.854±0.32 0.912±0.17 1845.66±18.01 0.654 ± 0.42

6 32.47±4.13 -0.0290±0.0053 0.965±0.48 0.825±0.02 2333.37±14.01 0.511 ± 0.54

7 35.36±1.41 -0.0085±0.0075 0.733±0.24 0.526±0.04 1389.17±13.32 0.285 ± 0.10

8 24.12±2.14 -0.0008±0.0061 0.916±0.42 0.755±0.04 1701.18±12.42 0.483 ± 0.27

9 30.85±0.05 -0.0062±0.0047 0.928±0.31 0.876±0.02 2557.13±11.14 0.579 ± 0.13

10* 30.72±1.28 -0.0057±0.0039 0.913±0.31 0.885±0.06 2532.79±15.05 0.574 ± 0.41

*Centre point replicated 3 times

Springiness is the height that the idli recovers during the time that elapses between the

end of the first bite and the start of the second bite, usually in TPA the first compression

and second compression. The difference between the first peak and the second peak in

the graph is taken as springiness. The springiness of idli depends on the quantity of the

dhal used because the soft spongy texture observed in the leavened steamed idli made

out of black gram is due to presence of two components, namely surface active

protein (globulin) and a polysaccharide (arabinogalactan) in black gram (Susheelamma

and Rao 1974, 1979a, 1979b, 1980). The specialty of black gram in idli preparation is due

to the mucilaginous property which helps in the retention of carbon-dioxide evolved

during fermentation (Nazni and Shalini, 2010). In the current study the springiness

varied from 0.733 to 0.965. The maximum springiness is obtained for the ratio 3:2 at 12 h

57

fermentation time. Hence the result reveals that the quantity of black gram dhal used has

a major role in the springiness of the idli. The response surface graph in 3D is depicted in

Fig.3.5.c showing the relation between rice to black gram dhal ratio and fermentation

time on springiness. From the ANOVA table it is clear that the independent variables in

the linear effect showed a significant influence on the springiness of the idli and the

model showed high goodness of fit (R2 = 0.909) .

Cohesiveness is defined as the ratio of the positive force area during the second compression to

that during the first compression. Cohesiveness may be measured as the rate at which the material

disintegrates under mechanical action. The cohesiveness is minimum (0.526) for the ratio

3:1.25 at 10 h fermentation time and maximum (0.912) for the ratio 3:0.5 at 12 h

fermentation time. Both the independent variables namely rice to black gram dhal ratio

in linear effect and fermentation time in quadratic effect is significant at 5 % level on the

cohesiveness of the idli. The graph in Fig.3.5.d shows an initial increase in the

cohesiveness as the fermentation time increases, but gradually decreases with further

increase in fermentation time.

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Fig.3.5.a Response surface graph showing relation between independent parameters on hardness

Fig.3.5.b Response surface graph showing relation between independent parameters on adhesiveness

59

Fig.3.5.c Response surface graph showing relation between independent parameters on springiness

Fig.3.5.d Response surface graph showing relation between independent parameters on cohesiveness

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Fig.3.5.e Response surface graph showing relation between independent parameters on Chewiness

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Fig.3.5.f Response surface graph showing relation between independent parameters on resilience

62

Table 3.7

Regression co-efficient for dependent TPA parameters

L - Linear effect; Q - Quadratic effect; *=P < 0.05

Table 3.8

Analysis of Variance (ANOVA) for dependent TPA parameters: F values

L - Linear effect; Q - Quadratic effect; *=P < 0.05

Chewiness is defined as the product of hardness x cohesiveness x springiness and is

therefore influenced by the change of any one of these parameters. Lower the chewiness

softer is the idli. The chewiness of the idli varied between 1389.172 for the ratio 3:1.25

at10 h fermentation time to 2557.135 for the ratio 3:1.25 at 12 h fermentation time. It is

proved by the ANOVA table (Table 3.8) that the ratio of rice to black gram dhal in linear

effect and fermentation time in quadratic effect also have significant impact (P < 0.05)

Independent

variables

Regression Co-efficient

Hardness Springiness

Cohesiveness

Chewiness

Resilience

Mean/Interaction 34.390 -2.254 -5.873 00.00 -4.602

1. Rice : Dhal ratio (L) 31.132 -0.182 -0.529 661.94 -0.603

Rice : Dhal ratio (Q) 1.981 -0.001 -0.049 -378.64 -0.003

2. Fermentation time (L) -3.517 0.525* 1.161* 4241.93* 0.906*

Fermentation time (Q) 0.147 -0.021 -0.049* -178.51* -0.038*

1L by 2L -1.645 0.008 0.042 63.76 0.036

R2

0.942 0.908 0.886 0.85 0.931

Independent

variables

Dependent parameters

Hardness Springiness Cohesiveness Chewiness Resilience

1. Rice : Dhal ratio (L) 000.000 15.644* 12.755* 11.161* 0.524*

Rice : Dhal ratio (Q) 241.174* 0.001 0.228 1.134 96.244

2. Fermentation time (L) 000.000 15.404* 5.074 0.487 3.823*

Fermentation time (Q) 063.752 7.401 11.447* 12.628* 2.724*

1L by 2L 1050.770* 0.138 1.027 0.198 31.967

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on the chewiness of the idli. As hardness, springiness and cohesiveness show significant

influence because of the independent variable hence the chewiness of the idli will also be

affected by the both independent and dependent variables. The chewiness (Fig.3.5.e) of

the idli varied for the same ratio of idli with difference in fermentation time which relates

the decrease in cohesiveness with further increase in fermentation time.

Resilience is a measurement of how the sample recovers from deformation both in terms

of speed and forces derived. It is taken as the ratio of areas from the first probe reversal

point to the crossing of the x-axis and the area produced from the first compression cycle.

The resilience varies between 0.285 for the ratio 3:1.25 at 10 h fermentation time to 0.654

for the ratio3:0.50 at 12 h fermentation time. Lower resilience value shows that the

product can recover faster from deformation proving the firmness of the product. The

response surface graph in 3D is depicted in Fig.3.5.f showing the relation between rice to

black gram dhal ratio to fermentation time on resilience of the idli. From the ANOVA

table it is evident that the resilience of the idli is influenced significantly by rice to black

gram dhal ratio in linear effect and by fermentation time both linear and quadratic effect.

The closer the value of R2 approaches unity, the better the empirical model fit the actual

data (Nuraliaa et al., 2010). As the R2 value for resilience (0.932) was closer to unity and

the result of resilience fit to the actual data.

3.3.7 Simultaneous optimization

Simultaneous optimization was performed on the TPA parameters like hardness,

adhesiveness, springiness, cohesiveness, chewiness and resilience by imposing

desirability constraints. In case of springiness, the softer idli shows high springiness.

Hence the software take into account of the values of independent and dependent values

and finally gives a maximum desirable score and the condition at which the maximum

score can be obtained with some constraints by assigning maximal desirability score as

one and minimal desirability score as zero. Table 3.9 shows the constraints imposed for

good textured idli with the desirable value for both independent and dependant variables.

The maximum desirable score that can be achieved with the desirable value will be

0.8279. On the basis of these calculations good textured idli could be made when 3:1.575

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(mass) ratio of rice to black gram dhal respectively is fermented for 14 h. The optimum

results were validated by performing the experiment at the optimized ratio and

fermentation time by comparing the observed and the predicted values. The predicted

values are shown in Table 3.9. The predicted values were insignificant with observed

values indicating the appropriateness of the model developed.

Table 3.9 Simultaneous optimization of process parameters by desirability approach

3.4 CONCLUSION

The optimization results indicated that the optimum ratio of rice to black gram dhal is

3:1.575 (w/w), with 14 h of fermentation time will provide the product with maximum

score for desirable textural parameters.

Independent parameters Dependent variables

Overall

Desirability

score

Rice : dhal

ratio (w/w)

Fermentation

time (h)

TPA

parameters

and L* values

Constraints

imposed

Predicted

values

Observed

values

3 : 1.575

14.00

Hardness Minimum 19.340 019.92 ± 01.03

0.8279

Adhesiveness Minimum -0.030 -0.032 ± 00.01

Springiness Maximum 0.947 0.930 ± 00.14

Cohesiveness Minimum 0.773 000.78 ± 00.02

Chewiness Minimum 1299.7 1286.8 ± 32.20

Resilience Maximum 0.555 0. 547 ± 00.030

L* (lightness) Maximum 75.16 075.21 ± 00.58