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 36
(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. 37
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 38
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..Eqn. 1 where Y is the predicted response, β 0, 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 X i and X j 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. 39
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 (X 1 ) and fermentation time (X 2 ). 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 40
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..eqn. 2.Eqn. 3 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 41
Rice (IR20 idli rice) and Black gram dhal (ADT3) (ratio based on the experimental design) Soak (4h) and grind Ferment the ground batter (Based on the experimental design) Pour batter in idli mould and steam for 15 minutes 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) 42
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 43
a) b) Plate 3.3 One inch cubic mould and SMS/75mm compression probe Plate 3.4 Texture analyzer 44
3.2.4 Statistical Analysis The independent variables and dependent variables (responses) were fit to the secondorder polynomial function and examined for the goodness of fit. The R 2 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 CO 2 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 45
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, 46
Table 3.2 Effect of rice varieties on the batter volume after fermentation Varieties of rice Batter characteristics Initial volume of the batter (cm 3 ) Final volume of the batter (cm 3 ) Batter volume increased after fermentation (%) Volume of the batter after expulsion of gas (cm 3 ) Batter volume decreased after expulsion of expulsion of gas (%) Parboiled rice 221.6 ±2.05 d 306.1± 3.74 e Ration rice Raw rice Broken rice Red rice 211.1±1.55 b 218.6±0.98 c 200.5±0.14 a 238.2±0.14 e 299.7±1.41 d 275.8±2.61 b 248.8±2.61 a 293.7±3.53 c 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 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 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 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. 47
Table 3.3 Effect of black gram (var. ADT 3) on the batter volume after fermentation Batter characteristics Initial volume of the batter (cm 3 ) Final volume of the batter (cm 3 ) BHR 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) Black gram BHRAS BWH 263.8±0.14 b 271.4 ±0.14 c 226.1 ±0.84 a 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 (cm 3 ) 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 3 : 1 Rice and black gram ratio (w/w) 3 : 1.5 3 : 2 3 : 2.5 3 : 3 3 : 3.5 Initial volume of the batter (cm 3 ) 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 (cm 3 ) 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 (cm 3 ) 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 48
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 Fig.3.2.c Effect of ratios of rice to black gram dhal on batter volume after fermentation 49
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. 50
Table 3.5 Experimental design: CCRD with actual levels of independent variables for colour parameters 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 *Centre point replicated 3 times 51
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* 52
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 53
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 R 2 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. 54
Force (N) Time (sec) Fig.3.4.a Texture profile of idli made of ratio 3:1.25 at 12 h fermentation time Force (N) Time (sec) Fig.3.4.b Texture profile of idli made of ratio 3:2 at 12 h fermentation time 55
Table 3.6 Experimental design: CCRD with coded and actual levels of independent variables Experi mental design point Hardness (N) Adhesiveness (N s) for TPA Dependent variables 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 56
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 (R 2 = 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. 57
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 58
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 59
Fig.3.5.e Response surface graph showing relation between independent parameters on Chewiness 60
Fig.3.5.f Response surface graph showing relation between independent parameters on resilience 61
Table 3.7 Regression co-efficient for dependent TPA parameters Independent variables Hardness Regression Co-efficient 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 R 2 0.942 0.908 0.886 0.85 0.931 L - Linear effect; Q - Quadratic effect; *=P < 0.05 Table 3.8 Analysis of Variance (ANOVA) for dependent TPA parameters: F values Independent variables Hardness 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) 62 Dependent parameters 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
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 R 2 approaches unity, the better the empirical model fit the actual data (Nuraliaa et al., 2010). As the R 2 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 63
(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 Independent parameters Rice : dhal Fermentation ratio (w/w) time (h) 3 : 1.575 14.00 Dependent variables TPA Constraints Predicted Observed parameters imposed values values and L* values Hardness Minimum 19.340 019.92 ± 01.03 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 Overall Desirability score 0.8279 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. 64