THE EFFECT OF EXTRUDED FLOUR AND FERMENTATION TIME ON SOME QUALITY PARAMETERS OF IDLI

KULWINDER KAUR, NARPINDER SMGH' and HARDEEP SINGH

Department of Food Science and Technologv Gum Nanak Dev University

Amritsar-143005. India

Accepted for Publication December 17, 1998

ABSTRACT

The effects on idli characteristics offermentation time and substituting rice with differing proportions of rice flour extrusion-cooked under different conditions were studied. The specific gravity, pH and viscosity of batter and idli characteristics such as expansion, compression force and sensory attributes were measured and described using regression to fit a response sur$ace anaiysis. Fermentation time showed the most pronounced effect on specific gravity, pH and aqueous dispersion viscosity of batter and taste scores of idli. Substituting rice flour with extruded rice flour at diferent levels were observed to have significant effect on expansion and appearance scores of idlis. The textural scores and compression force was significantly affected by extrusion temperature. The formulation containing rice substituted by 30% extruded rice JGur extruded at I75C and fermented for 24 h produced idlis with highest expansion and overall acceptability.

IN'TRODUCTION

Idli is a traditional fermented rice-black gram (Phaseolus mungo L.) based breakfast food of South India. It is very popular because of its textural and sensory attributes (Stemkraus et al. 1967; Reddy et al. 1981). Several workers have tried to use mfferent proportions of black gram to rice for idli making.(Khandwala et al. 1962; Joseph et al. 1961; Padhye and Salunkhe 1978; Reddy and Salunkhe 1980a). Black gram has been reported to play a major role in idli fermentation as a source of microorganisms and as a fermenting substrate (Radhalaishnamurthy et al. 1961).

'To whom correspondence should be addressed

Journal of Food Quality 23 (2000) 15-25. ANRights Resewed. OCopyright 2000 by Food & Nutrition Press, Inc., Trumbull, Connecticut. 15

16 K. KAUR, N. SINGH and H. SINGH

During the fermentation of rice-black gram batter different physico-chemical changes occur. Total acidity of the batter increases during the fermentation of the batter. A number of studies have investigated the effect of processing variables such as soaking time, grinding conditions, temperature and adjuncts on idli quality (Desikachar et al. 1960; Radhahshnamurthy et al. 1961).

Parboiled rice is better suited than raw rice for producing idli, i.e., it is soft without becoming sticky (Juliano and Sakurai 1985). The idlis prepared using very light colored 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. They reported that the normal parboiled rice is best suited for making idli as shown by its higher scores for softness, firmness and elastic recovery. Parboiling of rice caused gelatinization and solubilization of starch along with some discoloration of grains. The gelatinization and solubilization of starch can be achieved more conveniently and economically without any discoloration during extrusion cooking of rice. Extrusion cooking is a versatile high temperature-short time (HTST) process and an excellent process for the manufacture of new and traditional products (Smith and Singh 1996). In the present study, we have used regression to fit a response surface analysis to study the effects on idli characteristics of fermentation time and substituting rice with differing proportions of rice flour extrusion-cooked under different conditions.

MATERIALS AND METHODS

Preparation of Samples

Samples of clean PR- 106 variety of paddy were obtained from the 1996 harvest, while the black gram sample was procured from a local market. Paddy samples were dehusked in a laboratory McGill sheller (Rapsco Brookshire, Houston, TX). The brown rice obtained was polished in the McGill Miller No 2 (Rapsco Brookshire, Houston, TX) to a uniform 6% degree of polish (Singh et al. 1990). The broken rice was separated using a laboratory rice sizing device (Burrows Equipment Co, Evanston, IL) and ground in a laboratory grinder to pass through a 20 mesh sieve. The moisture content of rice grits was adjusted to 25% by spraying calculated amount of distilled water and mixing continuously in Hobart mixer (Model N-50). The rice samples were packed in polyethylene bags and kept in the refrigerator overnight to equilibrate the moisture before extrusion.

Extrusion Cooking

Extrusion cookmg was carried out in a single screw extruder (Model-2003, C. W. Brabender, Hackensack, NJ) with 1.9 cm barrel diameter and 20: 1 barrel length

QUALITY OF IDLl 17

and diameter ratio using 4 mm diameter die. The feed and compression zone temperature was maintained at 70C and 1292, respectively, while die zone temperature were held at 125, 150 or 175C. The extruder screw and feeding screw was run at a constant speed of 100 rpm.

Preparation of Batter and Idlis

Weighed quantities of rice flour (100 g) substituted with extruded rice flour at 20%, 30% and 40% and dehulled gram (50 g) were mixed with 300 mL of water. The resultant batter was mixed with 2% salt, transferred to a bowl and fermented at 32C for 6, 12 and 24 h. The specific gravity of idli batter was determined by dividing the weight of a glass beaker (50 mL) filled with batter to the weight of the same beaker filled with water. Aqueous dispersion viscosity of idli batter was measured with a Brookfield viscometer (Brookfield Engineering Inc., Stoughton, MA) using T bar spindle No. C. All measurements were taken at 20 rpm and 25C. The idlis cooking stand was placed in a pressure cooker containing a small quantity of water. The cooker was closed without pressure and the idlis were steamed for 10 min (Singh et al. 1995). The expansion (thickness) of idlis was measured at the center with a vernier calliper (Singh et al. 1995). The firmness of idlis was measured with an Instron Universal Testing Machine (Model-4464, Instron Corp., Canton, MA) at a cross head speed of 10 d m i n using a probe of 3.5 cm diameter. The compression force required for 5 mm compression of idlis at the center was measured in “k”’. The IX series software supplied along with Instron was used to trigger the force data collection. The data collection was started when the probe sensed the product.

Sensory Analysis

Nine samples were evaluated by a team of thirty semi-trained panelists per day who were familiar with the product and frequent consumers of the same. Preference testing was camed out for appearance, taste, texture and overall acceptability. The average scores for each attribute was calculated and statistically analyzed.

Statistical Analysis

The second order polynomials were computed using 33 experimental design by regression to fit a response using Minitab Statistical Software (Minitab Inc., State College, PA). Extrusion temperature, substitution levels and fermentation time were used as independent variables. These variables had values of X, (extrusion temperature), 125, 150, 175C, X2, (substitution levels) 20, 30, 40% and X, (fermentation time), 6, 12, 24 h. All the observations of 27 experiments were included in the design. The data for all the parameters except sensory attributes

18 K. KAUR, N. SINGH and H. SINGH

were average of three replications. The polynomials were fitted to measure dependent variables such as specific gravity, pH and aqueous dispersion viscosity (measured at 20 rpm) of batter and expansion, compression force and sensory attributes of idlis. The polynomials equation fitted to measure dependent variables (y,) was as follow (Singh and Smith 1997):

y, = B, + 2 Bi Xi +z 2 Bij X i Xj I - 1 i l l j ~ ,

Where yi is the response X , are the independent variables, B,, B, and B, are the regression coefficients. The equations obtained for different parameters were tested for adequacy and fitness by analysis of variance.

RESULTS AND DISCUSSION

The second-order polynomials were computed for the batter and idlis characteristics using actual variables and the equations tabulated are reported in Table 1 and 2. All the models correlate well with the measured data, possessing nonsignificant F-values (pr;O.OOl) for lack of fit and were statistically significant with R2 values in the range of 93.8-98.2.

Specific Gravity of Idli Batter

Specific gravity (SG) is used as an index to evaluate foam formation of idli batter that occur during fermentation due to microbial activity. Formation of foam during fermentation is essential in the preparation of idli. The foam consisted of gas (air) droplets encapsulated by a liquid film containing soluble proteins. The coefficients obtained using regression for SG of idli batter are summarized in Table 1. The SG of idli batter decreases with the increase in fermentation time; extrusion temperature and proportion of extruded flour substituted did not significantly effect SG. The SG of idli batter after 24 h of fermentation decreased from 0.93 to 0.90, 0.86 and 0.84, respectively, with substitution of flour extruded at 125, 150 and 175C at 40% level. Similar effects of fermentation time on batter volume has been reported earlier (Steinkraus et al. 1967; Padhye and Salunkhe 1978). The decrease in SG with the increase in fermentation time, extrusion temperature and substitution levels of extruded flour corroborate well the decrease in pH. With the increase in fermentation time the foam stability improved as evidenced by decreased SG of idli batter. This effect on foam stability may be attributed to the effect of pH on net charge and conformation of proteins (Kinsella 1981).

TABLE 1. COEFFICIENTS OF REGRESSION MODELS FOR BATTER CHARACTERISTICS

Term Specific Gravity PH Aqueous Dispersion Viscosity

Constant 1.99961 14.710 11466.6

Extrusion temperature -0.00828 -0.0694' -91.3

Substitution levels -0.00486 -0.0353 65.7 Fermentation time -0.02455*** -0.3726*** -69.1

Extrusion temperature x Extrusion temperature 0.00002 0.0002 0.4 Substitution levels x Substitution levels 0.00003 0.0001 4.3** Fermentation time x Fermentation time 0.00052*** 0.0097*** 7.9*** Extrusion temperature x Substitution levels 0.00000 0.0002 -1.1*

Extrusion temperature x Fermentation time -0.0000 -0.0001 -0.7 Substitution levels x Fermentation time 0.00002 -0.0005 -4.6 R2 95.3 98.2 94.3 ***p s 0,001, ** p s 0.01, *p s 0.05, ns = nonsignificant

td 0

TABLE 2. COEFFICIENTS OF REGRESSION MODELS FOR IDLI CHARACTERISTIC ~

Term Expansion Compression Appearance Taste Texture Overall Force Acceptability

?F i2

1.07"' -6.9.' 0.488**' 0.051 0.34*** 0.30*** -s Constant - 10.66 679.01 -5.658 -4.763 -20.01 - 10.8

Extrusion temperature 0.14 -5.976'** 0.06 0.087* 0.25*** 0.14"'

Substitution levels ? Fermentation time -0.15 -3.452 -0.065 0.452*** 0.09 0.15*

Substitution levels x Substitution levels -0.02*** 0.158*** -0.009*** 0.OOO -0.01*** -0.01 *** z m Fermentation time x Fermentation time 0 0.035 0.001 -0.012*** -0.00 -o.oo*** a

3: Extrusion temperature x Substitution levels 0 -0.016 -0.000 -0.000 0.00 -0.00

Extrusion temperature x Fermentation time 0 0.003 0.001* 0.000 0.00 0.00

Substitution levels x Fermentation time O* 0.01 8 0.000 0.001 -0.00 0.00 z R' 97.1 93.8 97.5 98.3 94.1 97.4

***p < 0.001, ** p I 0.01, *p < 0.05, ns = nonsignificant

Extrusion temperature x Extrusion temperature 0 0.019*** -0.OOO -0.000 -o.oo*** -0.00*** g

8

QUALITY OF IDLI 21

pH of Idli Batter

The change in pH reflects the activity of lactic acid bacteria in idli batter during fermentation. Table 1 summarizes the statistical analysis and coefficients of regression model for pH. The regression analysis showed fermentation time had most pronounced effect on pH of idli batter. Extrusion temperature also showed significant effect, however, at higher p value. The pH of idli batter decreased with the increase in fermentation time and extrusion temperature. The pH of batter containing rice flow substituted at 40% level with rice extruded at 175C after 6 h fermentation was 6.0 which decreased to 4.0 after 24 h of fermentation. A drop in pH from 6.0 to 4.3-5.5 during 20-24 h fermentation has also been observed earlier (Reddy and Salunkhe 1980b; Steinkraus et al. 1967). The decrease in pH as a function of fermentation time and extrusion temperature is consistent with the decrease in SG. The effect of extrusion temperature may be attributed to induced fermentation of the batter as a result of increased quantities of soluble carbohydrates, enzyme susceptible denatured proteins and gelatinized starch. Similar observations have been made earlier (Singh et al. 1995). An increase in soluble starch during extrusion of wheat has been reported earlier (Singh and Smith 1997).

Aqueous Dispersion Viscosity of Batter

The aqueous dispersion viscosity has been reported to be a useful index of protein structural changes and is influenced by pH, processing treatments and temperature (Kinsella 1976). Aqueous dispersion viscosity decreased with the increase in shear rate indicating apparent pseudoplastic-shear thinning behavior of idli batter. The coefficients of regression models in Table 1 revealed fermentation time had the most pronounced effect on the aqueous dispersion viscosity of the batter followed by substitution levels. Fermentation time interacted significantly with substitution levels of extruded flour in affecting the aqueous dispersion viscosity of batter. Aqueous dispersion viscosity of batter decreased with the increase in fermentation time and extrusion temperature and increased with the increase in substituting level of extruded flour. A decrease in aqueous dispersion viscosity of batter with the increase in fermentation time may be attributed to enhanced aeration of batter and increase in activity of microbial enzymes such as amylases and proteases (Reddy and Salunkhe 1980a; Soni and Sandhu 1989).

Physical and Textural Attributes of Idli

The coefficients of regression models for various physical and textural attributes are reported in Table 2. The substitution of rice flour with extruded flour showed the most pronounced effect on the expansion of idlis. Expansion of idlis improved with the substitution of rice flour up to 30% level with extruded rice flour

22 K. KAUR, N. SINGH and H. SINGH

while substitution beyond this level showed an adverse effect on expansion. The extrusion temperature and fermentation time did not show significant effect on expansion of idlis. Idlis with maximum expansion were produced using an extrusion temperature of 175C, a substitution level of 30% and a fermentation time of 24 h. Table 2 summarizes the coefficients of regression models for compression force of idlis. Extrusion temperature showed hlghly significant effect on the force required to compress the idlis up to 5 mm thickness. The substitution of rice flour with extruded rice flour also had significant effect on compression force of idlis, however at a higher probability (p) value. A decrease in compression force of idlis with the increase in extrusion temperature may be attributed to an increase in formation of airvoids due to enhanced fermentation. A decrease in compression force corresponds to an increase in expansion of idlis and decrease in pH and SG of idli batter as a function of fermentation time and extrusion temperature.

Sensory Attributes

The coefficients obtained for various sensory attributes are summarized in Table 2. The fermentation time showed a highly significant effect on taste scores of idlis whereas substitution of rice flour with extruded rice flour showed significant effects on appearance scores. The substitution levels of extruded flour also showed significant effects on taste scores, however at higher p value. The improvement in taste with the increase in fermentation time and substitution levels may be due to improvement in acidity as indicated by a reduction in pH. Acidification occurring during fermentation due to microbial activity is considered desirable. The major microorganism responsible for this fermentation was identified as Leuconostoc mesenteroides (Stemkraus et al. 1967). Texture scores were significantly effected by extrusion temperature and substituting rice flour with extruded rice flour. Texture scores increased with the increase in extrusion temperature and substitution levels which corroborate the decrease in compression force data measured using the Instron universal testing machine. Idlis with higher appearance and texture scores can be produced using extrusion temperature of 175C, substitution levels of 30% and fermentation time of 24 h. The batter fermented for 24 h resulted into idlis which had higher sensory scores as compared with those produced from the batter fermented for 6 and 12 h with similar extrusion temperature and substitution levels. Therefore, 24 h fermentation time was chosen for preddon of optimal conditions for idlis with respect to extrusion temperature and substitution levels. The contour plots for different sensory attributes and expansion of idlis as a function of extrusion temperature and substitution levels set to 24 h fermentation time were superimposed to find the area of best performance of idlis. According to Fig. 1, idlis with highest expansion and overall acceptability could be achieved using extrusion temperature, substitution levels and fermentation time of 175C, 30% and 24 h, respectively. These conditions resulted in batters having SG and pH of less

QUALITY OF IDLI 23

than 0.87 and 4.2, respectively and produced idlis with the highest expansion of 19 111111. Thus, using extrusion cooked rice in idli formulation produced idlis of more acceptable quality.

Substitutions levels ( " / a )

FIG. 1. THE OPTIMUM CONDITIONS TO OBTAIN BEST QUALITY IDLIS (Expansion, - . . . - , . Taste,-; Texture, ----; Appearance, - . - . - and

Overall acceptability, . . . . .)

CONCLUSION

Batter and idli characteristics were modeled using regression analysis. The correlation of the models with batter and idli characteristics showed that second order polynomials can be used as a tool for predicting the effect of substituting extruded flour, extrusion temperature and fermentation time on batter and idli characteristics. The formulation containing rice substituted by 30% extruded rice flour extruded at 17% and fermented for 24 h produced idlis with improved quality attribute. Extruded rice can economically replace parboiled rice in idli formulation as extrusion, high temperature short time process, causes gelatinization and solubilization of starch without any discoloration.

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24 K. KAUR, N. SINGH and H. SINGH

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RADHAKRISHNAMURTHY, R., DESIKACHAR, H.S.R., SRINIVASAN, M. and SUBRAMANYAN, V. 1961. Studies on idli fermentation. 11. Relative participation of black gram flour and rice semolina in the fermentation. J. Sci. Ind. Res. 20C, 242-345.

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QUALITY OF IDLI 25

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