Effect of Boiling Treatment of Soymilk on the Composition, Yield, Texture and Sensory Properties of Tofu

Con Ins!. Food Sci. Techno!. J. Vol. 19. No.2. pp. 53-56. 1986

RESEARCH

Effect of Boiling Treatment of Soymilk on the Composition, Yield, Texture and Sensory Properties of Tofu Elias E. Escueta 1, Malcolm e. Bourne and Lamartine F. Hood Institute of Food Science Cornell University Geneva, New York 14456

a small part exists as free sulfhydryl groups. The optimum heating time for the preparation of tofu was reported by Saio (1979) to correspond to the maximum range of sulfhydryl groups. On the other hand, a soybean protein coagulated without heating (no unfolding of the polypeptide chains) exhibits a globular structure rather than a three dimensional network and produces a softer texture (Saio and Watanabe, 1968). Aoki and Sakurai (1969) stated that boiling the milk for 15 min gave a harder curd than the curd from the milk just brought to boil prior to coagulation. eatsimpoolas and Meyer (1970) reported that complete dissociation of the protein into subunits, by cleavage of disulfide bonds, is one of the conditions favoring aggregation. This is brought about mainly by the cleavage of intramolecular disulfide bridges which facilitates disruption of quaternary structure and exposure of reactive groups due to dissociation and unfolding of subunits resulting in enhancement of gel strength. Gel formation during heating of a protein paste is prevented by addition of reducing agents having S-S bond cleaving ability (Aoki and Sakurai, 1968). Hashizume et al. (1975) heated the water extract from defatted soybean flakes at 60, 70, 80, 90 and 100°C and then subjected the samples to disc electrophoresis. The 7S protein decreased at 60°C and disappeared at 70°C. The lIS protein did not show changes up to 80°C, decreased at 90°C and disappeared at 100°C. The decrease in the lIS protein content was accompanied by an increase of new, fast moving components thought to be protein subunits, although they were not yet found when the 7S protein had disappeared. In the earlier studies, Watanabe and Nakayama (1962) and Saio et al. (1968), reported that heating a solution of water extracted proteins of soybean flakes at 80°C for 10 min caused the main components to disappear and new ones to form with higher sedimentation velocities. The 7S protein fraction was found to be more sensitive to heating than the lIS protein fraction. King (1977) exposed the 7S fraction to temperatures in the range of 70 to 100°C and reported

Abstract Soymilk was boiled for 0, 12, 30 and 60 min before coagulation with CaS04 to make tofu, and a texture profile analysis was performed on the product obtained. Hardness and gumminess increased slightly up to 12 min of boiling but decreased significantly after 30 min and 60 min of boiling the soymilk. Chewiness decreased significantly after 30 min and 60 min of boiling. Springiness was not affected by boiling. Cohesiveness increased slightly after 12 min boiling and then decreased slightly but not significantly after 30 min and 60 min of boiling. Composition and sensory qualities of tofu were not significantly affected by the length of time soymilk was boiled prior to coagulation.

Resume L'influence du temps d'ebullition du lait de soja avant sa coagulation au C0 4Ca dans la fabrication du tofu a ete etudiee il 0, 12, 30 et 60 min en rapport avec les qualites texturales des produits obtenus. La fermete et la tendance il coller augmenterent legerement jusqu'il 12 min d'ebullition, mais diminurent significativement apres 30 min et 60 min d'ebullition du lait de soja. La spongiosite ne fut pas afffectee par ('ebullition. La cohesion augmenta legerement apres 12 min d'ebullition pour diminuer ensuite un peu, non significativement apres 30 min et 60 min d'ebullition. La composition et les qualites sensorielles du tofu ne furent pas significativement affectees par Ie temps d'ebullition du lait de soja avant la coagulation.

Introduction Heat unfolds the polypeptide chains of soybean proteins dispersed in water (Wolf, 1972; Huang and Rha, 1974). This unfolding of the globular proteins before aggregation is a requirement and an initiating step for the formation of the three dimensional network in the protein curd (Saio and Watanabe, 1968; Aoki and Sakurai, 1969; Matsumoto, 1975; Furukawa et al., 1979). Furthermore, heating increases the sulfhydryl groups in soybean protein (II S) and results in harder tofu (Saio et al., 1971). It is suggested that most part of the sulfhydryl groups exists as disulfide bonds and I Present

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1986 Canadian Institute of Food Science and Technology

53

that sedimentation coefficients of the 7S aggregate increased with incubation temperature. Shemer and Perkins (1975), however, reported partial degradation of soy protein polypeptide chains in boiling water. All of these studies cited showed that heat produces changes in soybean proteins that affect the quality of the resulting tofu. There is a need for further studies on the effects of heat on soybean proteins and soybean protein products. This study was conducted to determine the effect of boiling soymilk prior to tofu coagulation on the yield, composition, textural properties and sensory qualities of the resulting tofu.

Materials and Methods

Materials Philippine grown soybeans of the L-I 14 variety were used in this. study.

Preparation of Soymilk Two kg of soybeans were soaked in tap water (three times the amount of soybeans) at room temperature for 8 h, then drained and washed with fresh tap water. The soaked beans were then ground in batches using a one gallon capacity stainless steel Waring Blender for 5 min at high speed. The ratio of the beans to water was maintained at I: 10 (w/ w). The slurry was filtered through cheesecloth to obtain the soymilk. All extractions were done at room temperature. The term "soymilk" as used in this paper, means the water extract from whole soybeans with most or all of the insolubles removed. This definition is important because most studies on water extractable soybean proteins have used hexane-defatted flakes, grits or flour as the starting material. These materials had been exposed to varying degrees of heat treatment before the extraction began.

Preparation of Tofu The soymilk was first pre-heated to about 85°C in a stainless steel bucket immersed in vigorously boiling water in a steam jacketed kettle to minimize scorching of the soymilk during boiling. The boiling water was discarded, the pre-heated soymilk was poured into the kettle and covered with aluminum foil except when the soymilk was being stirred. At time intervals of 0, 12,30 and 60 min, 1.5 kg of soymilk was withdrawn and placed in a stainless steel beaker immersed in cold tap water to bring the temperature to about 80°C. A water suspension of calcium sulfate was added at a level of 0.25% (w/w) of CaS0 4 • The mixture was allowed to coagulate for I h after a short but vigorous stirring to mix the calcium sulfate with the soymilk. The curd was poured into a wooden box (approximately 13 x 15 x 25 cm.) lined with cheesecloth and the supernatant was allowed to drain through the 1.3 cm dia. holes in the sides and bottom of the box for I h. The curd was allowed to set overnight at IO-15°C under a weight of approximately 6.65 g/cm 2 , before analyses and sensory evaluations were performed.

Textural Analysis Texture Profile Analysis using an Instron Universal Testing Machine and following the method of 54 / Escueta et al.

Bourne (1968) was used to determine the textural qualities of tofu. The Instron crosshead was set to cycle with a vertical reciprocating movement at a constant speed of 50 mm per min and with a stroke length of 7.5 mm. The maximum clearance between the moving horizontal plate and the stationary horizontal bedplate of the machine was 10 mm and the minimum clearance was 2.5 mm giving a 751Jfo compression. Each piece of tofu was compressed two times to give a first and second bite. The chart speed was set at 50 cm/min and the full scale load was adjusted to hardness of the sample. Cylinders of tofu (2 cm dia. by 1 cm height) used in the analyses were prepared using a cork borer. Each test was replicated eight to ten times at room temperature after equilibrating the samples for two h. Hardness was measured as the peak of the first bite and springiness was the distance of recovery between the two bites. Areas under the curves were measured by a planimeter. Based on the method of Bourne (1968), cohesiveness, gumminess and chewiness were calculated as representative textural parameters.

Chemical Analyses. Analyses of samples from each treatment were made in duplicate. Total nitrogen was determined by the micro Kjeldahl method (A.O.A.C., 1975, method no. 47.021) on solid samples and the protein content was calculated using a factor of 6.25 x N. Crude protein content of liquid samples was determined by the method of Lowry et al. (1951). Fat was determined by petroleum ether extraction in the Goldfisch fat extractor (A.O.A.C. 1975, method no. 14.080) and moisture and total solids data were obtained in a forced draft hot air oven at 100-I02°C for 24 h.

Recovery Calculations Percent recovery of tofu was calculated by dividing the weight of tofu by the original weight of soymilk and multiplying the value by 100. The recoveries of the total solids, protein and fat components of tofu were calculated by dividing the weight of each of the above components by its respective weight in the original soymilk.

Sensory Evaluation A trained panel evaluated flavor, mouthfeel, moistness and overall acceptability using 7 point scales; their ranges are indicated in the Results section. The tofu was fried and cooled to room temperature (28-30°C) before presenting a 40-50 g sample to the panelists for evaluation. All samples were coded with randomly Table I. Texture profile analysis of tofu from pre-boiled soymilk l Boiling time (min)

o 12 30 60 Hardness (N) 4.04 c 4.07 c 3.45 b 2.77 a Springiness(cm) 0.26a 0.24 a O.27 a 0.27 a Chewiness (N-cm) 0.51 b 0.51 b 0.38 a 0.28 a Cohesiveness (A 21AI) 0.48 ab 0.52 b O.4l a 0.39 a Gumminess (N) I. 96ab 2.IO b 1.42ab 1.09a 1Row means with the same letters are not significantly different (P = 0.05). LSD values: Hardness, 0.34; Springiness - F value not significant; chewiness, 0.12; Cohesiveness, 0.09; Gumminess, 1.01. J. InSl. Can. Sci. Techno/. Alimem. Vol. 19. No.2, 1986

Table 2. Composition (070 "as is") of soymilk, tofu and whey as affected by boiling. I Component

Boiling Time (min)

o

60 12 30 A. Soymilk Total Solids 5.3" 5.5" Fat 1.0" 1.2 b Protein 2.4" 2.8" B. Tofu Moisture 79.2" 80.9" 79.5" 79.8" Fat 5.4" 5.4" 5.4" 5.3" Protein 11.9" 11.7" 11.2" 11.3" C. Whey Total Solids 1.4" 1.5" 1.6"b 1.8 b Fat 0.2" 0.2" 0.2" 0.2" Protein 0.6" 0.6" 0.6" 0.6" IRow means with the same letter are not significantly different (P = .05) LSD values: A) Soymilk, total solids, 0.68; Fat, 0.19; Protein, 0.42; B) Tofu, Moisture, Fat and Protein F-values not significant; C) Whey, total solids, 0.26; Fat and Protein F-values were not significant.

the increase in hardness of tofu, can occur between 12 min and 30 min boiling.' Saio et al. (1971) observed the maximum amount of sulfhydryl groups after heating for 1 min at 100°C possibly because their soymilk was prepared from a defatted soybean meal. Soybean meal, unlike fresh whole soybeans, receives heat treatment that may affect its protein as a result of the solvent extraction process. Springiness was not affected significantly by the boiling treatment as indicated by the fairly constant values throughout the 60 min boiling period (Table 1). Chewiness decreased with boiling time with the decrease becoming significant after 30 min of boiling. Between 30 min and 60 min boiling, however, the chewiness values did not differ significantly. Cohesiveness and gumminess behaved similarly to hardness. Both did not change significantly after 12 min boiling but decreased significantly after 30 min boiling.

Chemical Composition assigned numbers and presented to the panelist in a randomized arrangement to equalize the sample sequence effect on food preference (Eindhoven et al., 1964).

Statistical Design The experiments were performed using the randomized complete block design (RCB). Each experiment was done independently with 3 replications (3 blocks) prepared as separate batches of soymilk. Analyses and measurements were made in duplicate, giving a total of six values per observation. Analysis of variance was performed using the RCB design, followed by a multiple range test using the least significant difference (LSD) when the F value was significant (Cochran and Cox, 1957).

Results and Discussion

Texture Profile Analysis The data on textural qualities of tofu as affected by the length of time of boiling soymilk prior to coagulation are summarized in Table 1. The hardness was not affected by boiling for the first 12 min, but decreased significantly after boiling for 30 min and further decreased after 60 min boiling. An increase in the hardness of tofu might have been observed if boiling exposures between 12 min and 30 min had been explored as it is possible that the maximum increase in sulfhydryl groups, which will cause Table 3. Percent yield of tofu, fat, protein and total solids from pre-boiled soymilk I. Boiling Time (min) Component o 12 30 60 Tofu 2 20.8" 2I.l" 21.2" 23.2 b 84.9 b Solids 3 79.6" 81.7" 88.3 c Fat 3 88.5" 94.7 b 88.9" 93.6 b Protein 3 88.5" 88.6" 95.4c n.5 b I Row means with the same letter are not significantly different. 2Percent of original weight of soymilk. 3Percent of original amount in soymilk. LSD values: Tofu, 0.71; Solids, 2.27; Fat, 1.56; Protein, 1.71. Can. lnst. Food Sci. Technol. J. Vol. 19. No.2, 1986

The soybean used had the following chemical composition (average of 3 analyses, "as is" basis): moisture, 10.5070; protein, 38.0%; fat, 16.2%; crude fiber, 4.8070; ash, 5.1 %; carbohydrates (by difference), 25.42%. Boiling soymilk before coagulation caused no significant effect on the composition of tofu (Table 2). The composition of soymilk was affected significantly by boiling. Sixty min of boiling showed a highly significant increase in the total solids content of soymilk over that of 0 min, 12 min and 30 min boiling. The protein content of soymilk also increased significantly after 30 min and 60 min of boiling. The fat content increased significantly after 12 min of boiling with further, statistically non-significant increase after 60 min of boiling. These changes in the components of soymilk during boiling were due primarily to the loss of water through evaporation which occurred during the boiling treatment despite covering the kettle. It was also shown (Table 2) that tofus of different compositions could not be achieved by merely prolonging the boiling time of soymilk prior to coagulation. Saio et al. (1973) reported that tofu of 50% or less moisture content could be prepared from the 11 S protein fraction but this condition cannot be achieved by boiling procedures.

Recovery of Tofu and Its Components Table 3 shows the effects of the different times of boiling on the % recovery of tofu and its components. Table 4. Sensory qualities of tofu from pre-boiled soymilk l Boiling Time (min) Sensory parameters o 12 30 60 Mouthfeel 5.3 5.2 5.4 5.5 Flavor 5.0 4.9 5.3 5.1 Moistness 5.5 5.4 5.4 5.5 Acceptability 5.3 5.0 5.2 5.2 IMouthfeel, from I-very coarse to 7-very smooth; Flavor, from I-very strong off-flavor to 7-very pleasant flavor; Moistness, from I-very dry to 7-very moist; Acceptability, from I-dislike very much to 7-like very much.

Escueta et al. / 55

Boiling soymilk for 30 min instead of 12 min did not increase the recovery of tofu. The 070 recovery of tofu increased significantly after boiling for 60 min. Hence, the traditional tofu-making process which requires 10-15 min of boiling is reasonable from the viewpoint of economy and nutrition since trypsin inhibitors and other anti-nutritional factors in soymilk are destroyed by 10-15 min boiling (Liener, 1972; Rackis, 1974). The recovery of tofu can be increased by boiling for 60 min but the product will be significantly softer as shown in Table 1. The fat recovery increased significantly after 12 and 60 min of boiling. The protein and solids recovery were also significantly affected by the length of boiling. Protein and solids recoveries increased as the boiling time was increased. The increases in yield of tofu, protein and solids with increasing boiling time were obviously caused by the unavoidable concentration of the soymilk during boiling, as shown in Table 2.

Sensory Qualities Table 4 shows that the time of boiling soymilk prior to coagulation did not have a significant effect on the sensory qualities of tofu. The acceptability scores of about 5 given to all of the tofu samples are moderate even though tofu is a customary part of the Filipino diet. This suggests that the way the tofu is cooked or prepared has more influence on its acceptability than tofu as a product itself, which is bland in flavor and is rarely eaten alone. Future acceptability tests should be made with tofu prepared as a traditional Filipino dish.

Conclusion The composition and sensory qualities of tofu were not significantly affected by the time of boiling soymilk prior to coagulation. The yield increased slightly up to 30 min and then significantly after 60 min boiling. Tofu of acceptable sensory quality and chemical composition was made from pre-heated soymilk (0 min treatment). Prolonged boiling of soymilk gave no advantage in terms of composition and acceptability of tofu. The increase in the yield of tofu at the longer boiling periods has to be weighed carefully against the higher energy requirement of the longer boiling process. Boiling for 30 min and 60 min produced tofu that was softer and less chewy. Cohesiveness of tofu also decreased significantly after 60 min boiling. Springiness was the only textural property that was not affected by boiling treatment. Thus in terms of economy, textural quality and yield the 0 min pre-boiling treatment of soymilk prior to coagulation is sufficient but 12 min boiling of soymilk prior to coagulation is still appropriate in order to destroy the trypsin inhibitors and other anti-nutritional factors present in soymilk.

Acknowledgement This research project was supported by the National Science and Technology Authority (NSTA) formerly 56 / Escuata et al.

National Science Development Board of the Philippines.

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