An approach to saving energy in Kori-Tofu processing

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  • Energy in Agriculture, 6 (1987) 141-152 141 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

    An Approach to Saving Energy in Kori-Tofu Processing


    Department of Agricultural Chemistry, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113 (Japan)

    (Accepted 9 December 1986)


    Yano, T., Iibuchi, S., Lin, B.F., Miyawaki, O. and Torikata, Y., 1987. An approach to saving energy in kori-tofu processing. Energy Agric., 6: 141-152.

    Consumption of energy was analysed on a kori-tofu plant which processed 6 t of soybeans per day, involving 13 unit operations such as wet-milling, extraction-denaturation of soy protein, coagulation, compression, freezing, thawing, drying, and waste-water treatment. Total energy con- sumption per kg of the dry kori-tofu was 35.4-27.1 MJ of fuel energy and 8.3 MJ of electrical energy. The extraction-denaturation of soy protein consumed twice as much thermal energy as the drying. The waste-water treatment consumed more electrical energy than the whole manufac- turing process including the freezing and aging.

    Next the reduction of the thermal energy consumption in the extraction-denaturation of soy protein was attempted. The point was to reduce the amount of the extraction water that must be heated to at least 90C for denaturation of the soy protein. Decrease in the yield of extracted protein was overcome by using multi-extraction. The coagulation was not affected if the ratio of bound calcium to soy protein was controlled at a certain level. The consolidation became easier but its operation had to be adjusted to obtain the same quality of the consolidated cake. The series of investigations suggested that the thermal energy consumption in the extraction-denaturation of soy protein could be reduced to less than one half of the current energy consumption by only improving the extraction-denaturation of soy protein. Cost for a partially optimized sub-system is also discussed.


    After the first energy crisis in 1973, energy analyses were made intensively on food processings (for example, Unger, 1975; Brown and Batty, 1976; Henig and Schoen, 1976; Londahl, 1976; Rao, 1977; Schwarzberg, 1977; Naughton et

    The research was supported by a grant-in-aid for scientific research of the Ministry of Education, Science and Culture, Japan. *Present affiliation: Wayo Women's University, Ichikawa-shi, Chiba-ken, Japan. **Present affiliation: Tatung Institute of Technology, 22 Chungshan Noath Road, 3rd Sec., Taipei, Taiwan.

    0167-5826/87/$03.50 1987 Elsevier Science Publishers B.V.

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    al., 1979; Avlani et al., 1980; Beech, 1980; Beech and Craft-Lighty, 1980; Car- road et al., 1980; Chinnan et al., 1980; Craft-Lighty et al., 1980; Davis et al., 1980; Mayou and Singh, 1980; Singh et al., 1980; Steffe et al., 1980; Iibuchi et al., 1982; Gasparino Filho et al., 1984; Rumsey et al., 1984). Energy saving in food and other industries was accelerated, beginning with turning off electric lights, improving thermal insulation, recovering waste heat, and changing equipment to more energy-saving types. The supply of heavy fuel oil to the total manufacturing in Japan, 1984, came to one half of that in 1979, although the consumption of electric power was not reduced significantly (Statistics Bureau Management and Coordination Agency, 1986). Special project researches on energy have been also promoted by the Japanese government: 'Moon-Light Project' promoted by the Ministry of Industry and Trade 1978-; 'Green-Energy Project' by the Ministry of Agriculture, Forestry and Fishery, 1978-1988; and a project research in scientific level by the Ministry of Edu- cation, Science and Culture, 1978-1987. We have participated in the project research supported by the Ministry of Education, Science and Culture since 1980. This paper summarizes part of our research reports relevant to the energy saving in a soybean processing (Yano and Iibuchi, 1981, 1983a, 1983b; Yano, 1984, 1985; Iibuchi et al., 1982; Lin et al., 1987).


    Soybean processing

    A kori-tofu plant which processed 6 t of soybeans per day was subjected to the energy analysis. Kori-tofu is a dried soy protein product insolubilizing the protein by freeze-denaturation. It is made through 13 unit operations such as refining, washing and soaking, wet milling, extraction and heat denaturation of soy protein, filtration, coagulation, consolidation, desalting, freezing and aging, thawing and dewatering, softening the protein with NaHCO3, centrifug- ing and drying. Although the kori-tofu production is a minor food industry, it is interesting first in that the energy-intensive unit operations such as freezing and drying can be compared with the series of other unit operations in the whole processing, and secondly in that the energy saving in the kori-tofu pro- cessing can be applicable directly to the soy-milk beverage industry which has developed rapidly in the food market.

    Chemical analyses

    Nitrogen content was measured by the Kjeldahl method and crude-protein content was calculated as the nitrogen content multiplied by 5.71. Oil and fat content was measured by the Soxhlet extraction with ether (AOAC, 1980). Calcium content was measured by the EDTA titration with murexide as the

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    indicator (Tokyo Daigaku Nogakubu Nogeikagakka, 1970) after digesting the sample with nitric acid. Phosphorus content was measured by the molybdo- vanadate reagent method (AOAC, 1980). Moisture content was measured by drying the samples at 105C for 6-15 h. Solid content was obtained by sub- tracting the moisture content from the total weight. The carbohydrate and ash content was obtained by subtracting the crude protein and the oil and fat con- tents from the solid content.

    Extraction and heat denaturation of soy protein

    To reduce the amount of water to be heated the current method of extraction and three alternative methods were compared with laboratory experiments from the viewpoint of energy saving.

    (1) Standard extraction-denaturation ( R1 ). After wet milling at room tern- perature the soy protein was extracted once with water. The amount of extrac- tion water was varied to be 15 (traditional operation), 7, 5, and 4 times the weight of raw soybeans. For denaturation and further extraction of soy protein, the suspensions were boiled for 3 min, gently stirred for 90 min without heating and filtered through a 300-mesh filter to obtain the soymilk.

    (2) Multi-extraction ( R2 ). For effective extraction of soy protein with less water to be heated, a multi-extraction - twice in this study with half the water each - was studied. The extracted soy protein was heat-denaturated only for the first extraction.

    (3) High-temperature milling with cold water extraction (H1). For the extreme reduction of water to be heated the soy protein was heat-denatured with no extraction water - that is, wet soybeans soaked in water 3 times weight of the soybeans were milled at 95 C, followed by the extraction once with cold water.

    (4) High-temperature milling with double extractions ( H2 ). This is a mod- ification of the H1 operation for increasing the yield of soy protein. After the high-temperature milling, the extraction was done twice, each with half the water as in R2.

    Coagulation and binding equilibrium of calcium

    Extracted soy protein was coagulated by addition of CaCle solution. If I g of soy protein has n mole of sites for binding calcium, and the law of mass action is applicable for the binding equilibrium:

    [bound Ca] -K (1) [free Ca] {n" [protein] - [bound Ca] }

    where [bound Ca] and [free Ca] are the concentrations of bound and free

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    calcium ( mol/1 ), respectively, [ protein ] the concentration of soy protein (g/l), and K the equilibrium constant (1/mol). The value of K was determined from Klotz's plot (Klotz et al., 1946) as shown later in Fig. 5. To avoid heteroge- neous binding of calcium due to insufficient mixing, equivolumes of CaC12 solu- tion and soy milk were quickly mixed. The amount of calcium bound to the coagulated soy protein was calculated from the calcium concentration in the supernatant and the total calcium to be existed contained in the CaCle solution and the soy milk.


    Effects of the alteration in the extraction-denaturation operation and the coagulation condition on the subsequent consolidation of the coagulated pro- tein were studied by laboratory-scale experiments. The average consolidation ratio is given as following (Shirato and Murase, 1970) :

    L1-L o~ 8 /' (2N-1)znz~ Uc =-L1-L--2 1--g=a ~ (2N-1)ezc e e x p ~ ] 4 re (2)

    r~ =i2C~Oc/Wo e (3)

    where Ce is the modified coefficient of consolidation ( kg z m-4 s- 1 ), i the num- ber of drainage surface ( i= 2 in this case) (-), L the thickness of sample at a time 0c (m), Li and L2 the initial and the final values respectively of L (m), Uc the average consolidation ratio (-) , Wo the total solid mass per unit sec- tional area (kg m-2), 0 the time for consolidation (s), and r the nondimen- sional time defined by equation (3) (-).

    The value of Ce was calculated from the following equation (Shirato and Murase, 1970):

    C e =O.848WoZ/iZ09o (4)

    where 090 is the time when Ui = 0.9. The coagulated protein was packed in a cylinder and consolidated with a

    pressure of 25 g/cm z.

    Rheological behavior

    Quality of the consolidated soybean curds was compared through the creep experiment with a parallel-plate plastometer. The creep behaviors of the soy- bean curds were analysed by the following four-element model that is the com- bination of the Maxwell model and the Voigt model: