TOFU PRODUCTS EXCELLENT IN FREEZE RESISTANCE AND PROCESS FOR PRODUCING THE SAME

Abstract: A process for industrially producing tofu products which are excellent in taste and freeze resistance. This process comprises concentrating soybean milk extracted from uncooked soybean slurry or puree; adding one or more members selected from sugars, starch and transglutaminase to the concentrated soybean milk at an elevated temperature; adding a solidifying agent to prepare tofu; further heating the tofu in two steps; and then freezing.

Description: RELATED ART FIELD The present invention relates to frozen tofu products, and their production. Tofu (soybean curd) products (e.g., tofu, atsuage (thick fried tofu), etc.) have heretofore been said to be difficult to store for a long period of time, and have also been said to be difficult to subject to freezing storage. However, even when the tofu products of the present invention are frozen, and stored for a prescribed period of time, they are scarcely denatured by freezing. Further, when they are thawed and cooked for eating, they have excellent taste and cooking applicability, and are smooth. PRIOR ART As is generally known, tofu is produced by soaking soybeans in water for a certain period of time, grinding the soybeans, adding water thereto, heating the resultant slurry or puree and separating okara (soy pulp or tofu waste) to obtain hot soybean milk. To this soybean milk, a solidifying agent (coagulant) is added to obtain tofu. When this tofu is frozen, spongy tissue or layered tissue such as frozen tofu (koa tofu) is formed, thereby spoiling smooth mouthfeel before freezing. Therefore, various studies have been carried out to improve this. For example, JP 6-217729 A and JP 6-269257 A propose processes for producing tofu having freeze resistance, wherein transglutaminase, starch and the like are added to soybean milk. However, when tofu is produced by these processes on a large scale, it is very difficult to control the coagulation reaction because of the increase in viscosity of soybean milk, and there are problems in the production of tofu having good tissue. JP-A-10-127247 describes a process for producing frozen tofu in which soybean milk with a solids content of 10 to 18% is warmed to 75-95°C and saccharides, such as oligosaccharides are added. The coagulated tofu is roughly broken, then dehydrated and frozen. JP-A-61 254 158 describes a process for making frozen tofu in which a protein or starch is added to cooked soybean milk with a solids content of 4 to 30% at 70°C. In addition, JP 9-182571 A proposes a process for producing frozen tofu, wherein starch and the like are added to soybean milk concentrated to 14% to 20%. However, unless a solidifying agent is added to soybean milk at a temperature of 30°C or lower, taste and mouthfeel of tofu cannot be improved because of the increase in viscosity. Then, at present, an industrial process which can stably produce tofu having excellent freeze resistance has not yet been established. Further, after thawing frozen tofu stored for a prescribed period of time, cooking applicability of thawed tofu is spoiled and, when a lot of tofu is cooked in a large batch, it is crumbled or broken. A process for producing tofu maintaining excellent cooking applicability after thawing and cooking has not yet been established. DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION As described hereinabove, a method for efficiently improving freeze resistance of tofu has not yet become popular. The present inventors have studied freeze resistance of tofu. Thus, the present invention is to provide a process for industrially producing frozen tofu products which are not denatured even by freezing, and have the same mouthfeel and taste as those of fresh tofu even after thawing, and are excellent in cooking applicability. MEANS FOR SOLVING THE PROBLEMS The present invention provides a process for producing tofu products which are excellent in freeze resistance, said process comprising warming soybean milk having a viscosity at 10°C of not higher than 100mPa.s and a solids content of 10% to 16%, which is obtained by concentrating soybean milk extracted from uncooked soybean slurry or puree prepared from whole soybeans, to 60°C or higher; adding one or more members selected from the group consisting of sugars, starch and transglutaminase and adding a solidifying agent to the soybean milk whilst the temperature of the milk is 60°C or higher to prepare tofu; and then freezing; wherein, in the case of addition of starch, the soybean milk is warmed to a temperature from 60 to 70°C in the warming step. The present invention also provides a process wherein, after addition of one or more members selected from the group consisting of sugars, starch and transglutaminase and the solidifying agent to the soybean milk, the resultant mixture is heated to elevate its temperature stepwise. The present invention also relates to tofu products obtainable by the processes described herein. More specifically, in order to obtain soybean milk having low viscosity, concentrated soybean milk having low viscosity and a high solids content can be produced by extracting soybean milk from uncooked soybean slurry or puree prepared from whole soybeans, and concentrating the soybean milk so that the solids content after heating is 10% to 16%. To this concentrated soybean milk, 0.1 to 5% by weight of sugars or starch, or 0.01 to 2% by weight of transglutaminase, and the solidifying agent are added at a temperature of the concentrated soybean milk of 60°C or higher to prepare tofu. The tofu is heated to elevate its temperature stepwise to obtain tofu having elasticity. The tofu thus obtained has freeze resistance and is excellent in cooking applicability. EMBODIMENTS OF THE INVENTION Hereinafter, the present invention will be illustrated in detail. In the process for producing tofu products having excellent freeze resistance of the present invention, it is better to heat and concentrate uncooked soybean milk extracted from fresh soybean slurry or puree to obtain concentrated soybean milk having low viscosity and a high solids content. Further, in order to obtain tofu having excellent elasticity, the additive and the solidifying agent are added to the above concentrated soybean milk warmed to 60°C or higher, preferably followed by further heating the mixture stepwise (e.g., in two steps) to elevate the temperature. In a conventional process for producing tofu, generally soybean milk is extracted from cooked soybean slurry or puree. That is, soaked soybeans are ground together with water to obtain soybean slurry or puree. The soybean slurry or puree is heated with live steam, followed by separating okara therefrom to obtain soybean milk. When soybean slurry or puree is treated by heating in this manner, viscosity of the soybean milk is increased, which adversely affects the subsequent coagulation reaction. In contrast, in the present invention, soybean milk is extracted from uncooked soybean slurry or puree. That is, soaked soybeans are ground together with water to obtain soybean slurry or puree. Okara is separated from the soybean slurry or puree to obtain uncooked soybean milk. This is concentrated by heating (at 90°C or higher for 5 minutes or more). The resultant concentrated soybean milk has low viscosity, while it has a high solids content. It has been confirmed that good tofu is obtained by the subsequent coagulation reaction of this concentrated soybean milk. The term "low viscosity" used herein means that the viscosity of the soybean milk at 10°C is not higher than 100 mPa•s or lower, preferably, 50 mPa•s or lower. The concentration method is not limited to a specific one. For example, good concentrated soybean milk can efficiently be obtained by heat concentration under reduced pressure with a centrifugal thin membrane because excessive heating of soybean milk can be avoided. In the present invention the solids content of soybean milk is adjusted to 10 to 16% by weight. When the solids content is less than 10% by weight, mouthfeel of the resultant tofu becomes inferior, and tends to be denatured after freezing. When the solids content is over 16% by weight, the viscosity of the soybean milk is rapidly increased and it is difficult to subject the soybean milk to an appropriate coagulation reaction. Therefore, it is necessary to adjust the concentration of soybean milk within the above range. Next, to the concentrated soybean milk obtained by the above method, one or more members selected from the group consisting of sugars, starch and transglutaminase are added to prevent freeze denaturation. Examples of sugars include sucrose, fructose, xylose, sorbitol and the like, with oligosaccharides having low sweetness, sugar alcohol such as erythritol, maltitol, etc., trehalose, and the like being preferred. The starch may be cereal such as corn starch, wheat flour starch, etc., and their processed products such as α-starch, oxidized starch, etc., with starch which has freeze resistance without syneresis being preferred. Transglutaminase is an enzyme normally obtained by culturing a microorganism with starch, etc. as a fermentative raw material, and is an outstanding enzyme for crosslinking proteins in food to produce a close and smooth texture. In the present invention, a transglutaminase preparation containing an edible material such as starch, etc. may be used. The temperature of the soybean milk at which the solidifying agent is added to the concentrated soybean milk is 60°C or higher. Although bad taste of soybean milk can be somewhat improved by the above concentration, if the temperature is lower than 60°C, sufficient improvement of taste is scarcely expected and the resultant product has less of the specific taste of tofu and less elasticity. This may be due to too slow a reaction between proteins in the soybean milk and the solidifying agent. Further, in the case of addition of starch, when the temperature is higher than 70°C, the viscosity of the soybean milk is rapidly increased due to gelatinization of the starch and it is difficult to subject soybean milk to an appropriate coagulation reaction. Then, attention should be given to the temperature of the soybean milk. The solidifying agent to be used in the present invention includes a bittern normally used in tofu production, calcium salts such as calcium sulfate, etc., magnesium salts such a magnesium chloride, glucono delta-lactone and the like, with calcium sulfate whose coagulation reaction is gentle being preferred. Next, the tofu produced above is aged at 60 to 70°C for 10 to 60 minutes so that the transglutaminase acts on the tofu, and the coagulation reaction with the solidifying agent becomes gentle so as to facilitate the stable production of the tofu. Then, the tofu is heated to elevate its temperature stepwise to inactivate the transglutaminase and to promote the coagulation reaction in the tofu to provide elasticity. When the latter heating is carried out in combination with the above heating at 60 to 70°C by 2 steps, the tofu is heated at 80 to 90°C for 10 to 60 minutes. If the final heating is carried out lower than 80°C, the transglutaminase is insufficiently inactivated and elasticity of the tofu is also insufficient. If the heating is carried out at higher than 90°C, the tofu tissue is spongy and its mouthfeel is inferior. Then, the tofu is cut into a prescribed size. After cooling to 40°C or lower, the tofu is frozen with a quick freezer. Alternatively, after cutting into a prescribed size, the tofu may be fried to prepare so-called "atsuage" and then quickly frozen. The frozen tofu products thus obtained can keep the mouthfeel of tofu for more than 6 months. In addition, cooking applicability such as a lot of tofu can be cooked in a large batch without crumbling or breaking is desired in a market of tofu for business use, where there is much opportunity for a large quantity of tofu to be cooked in a large batch, such as in meals for schoolchildren, workers, etc. EXAMPLES The advantages of the present invention will be illustrated by the following examples but they are not to be construed to limit the technical ideas of the present invention. Example 1 Water (10°C) (15 kg) was added to whole soybeans (5 kg) and the soybeans were soaked in water for 14 hours. The mixture was separated into soaking whey and soaked soybeans with a 2.00 mm (10 mesh) sieve. Then, the soaked soybeans were ground together with water for grinding (20°C) (25 kg) by a grinder (manufactured by Nakagawa Kikai Seisaku-shoi; hereinafter the same grinder is used) to obtain soybean slurry. The slurry was separated into soybean milk (solids concentration: 9% by weight) and okara, by a separator (manufactured by K.K. Tofer; the same separator is used). The soybean milk was heated at 98°C for 5 minutes by an indirect heating apparatus (manufactured by K.K. Hoshidaka; hereinafter the same apparatus is used). The resultant tofu was adjusted to a solids concentration of 14% by weight by a concentrator (manufactured by K.K. Hisaka Seisaku-sho; hereinafter the same concentrator is used) under a vacuum of 13.33 kPa (100 torr). Viscosity of the resultant soybean milk was measured at 10°C with a viscometer (manufactured by TOKIMEC K.K.; hereinafter the same viscometer is used). Next, the concentrated soybean milk was adjusted to 70°C and, to this, calcium sulfate (0.3% by weight), potato starch (2% by weight, manufactured by Sanwa Starch K.K.; hereinafter the same starch is used) and a transglutaminase preparation (0.1% by weight, manufactured by Ajinomoto K.K.; hereinafter the same preparation is used) were added. The mixture was placed in a molding box (length: 150 mm, width: 100 mm and depth: 30 mm; hereinafter the same box is used). After standing for about 10 minutes, the box was heated with a steamer at 90°C for about 20 minutes to produce tofu. This was cut into a prescribed size and, after cooling to 40°C or lower, the tofu was quickly frozen in an atmosphere at -35°C. Example 2 According to the same manner as described in Example 1, tofu was prepared except that the soybean milk separated by the separator was concentrated to a solids concentration of 11% by weight. Example 3 According to the same manner as described in Example 1, tofu was prepared except that the soybean milk separated by the separator was concentrated to a solids concentration of 16% by weight. Example 4 According to the same manner as described in Example 1, tofu was prepared except that the amount of starch added to the concentrated soybean milk adjusted to 70°C was 0.2% by weight. Example 5 According to the same manner as described in Example 1, tofu was prepared except that the amount of transglutaminase added to the concentrated soybean milk adjusted to 70°C was 0.01% by weight. Comparative Example 1 According to the same manner as described in Example 1, tofu was prepared except that 32 kg of water for grinding was used, and the soybean milk separated by the separator was adjusted to a solids concentration of 6% by weight and used without concentration. Comparative Example 2 According to the same manner as described in Example 1, tofu was prepared except that the concentrated soybean milk was adjusted to 30°C and, after addition of the additives and the solidifying agent, the soybean milk was heated at 60°C for 30 minutes and further at 90°C for 30 minutes by the steamer. Comparative Example 3 According to the same manner as described in Example 1, tofu was prepared except that the concentrated soybean milk was adjusted to 80°C and, after addition of the additives and the solidifying agent, the soybean milk was held for 10 minutes. Comparative Example 4 The soybean slurry prepared with the grinder in Example 1 was heated with a steam heating pan (manufactured by K.K. Tofer; hereinafter the same pan is used) and the soybean slurry was separated into cooked soybean milk (solids concentration 9% by weight) and okara, by the separator. According to the same manner as described in Example 1, the resultant soybean milk was concentrated to prepare tofu. Comparative Example 5 According to the same manner as described in Comparative Example 4, tofu was prepared except that the concentrated soybean milk was adjusted to 30°C and, after addition of the same additives and solidifying agent as in Comparative Example 4, the soybean milk was heated at 60°C for 30 minutes with a steamer, followed by heating at 90°C for 30 minutes. Summary of conditions in Examples and Comparative Examples and results of measurement and evaluation:- The separation of soybean milk is represented by extraction of uncooked soybean slurry (U) or cooked soybean slurry (C). The measurement of solids concentration of soybean milk was carried out by an apparatus for measuring loss in weight on drying (105°C for 4 hours). Viscosity was measured by a viscometer. The state of the coagulation reaction of the soybean milk was evaluated by observation with the naked eyes of skilled persons. The results are shown by O (uniform gelation) and X (crumbled gel or no gelation). Cooking applicability was evaluated by observation of states of crumbling with the naked eyes of skilled cooks after cooking the frozen tofu in given stock for about 1 hour. The results are shown by O (good) and X (bad). Mouthfeel (organoleptic evaluation) and taste (organoleptic evaluation) of tofu were scored by 5 expert panelists after thawing frozen tofu according to the following 5 points (5: excellent, 4: good, 3: no good, 2: somewhat bad, 1: bad). The results are shown by the average and tofu having the score of 4 or more has marketability. The results are shown in Table 1. Overall evaluation based on Examples 1-5, Comparative Examples 1-5 and Table 1:- In view of overall evaluation, the product of Example 1 was the best and then the products of Examples 2, 3, 4 and 5 were similar to each other and better. All the products of Comparative Examples 1 to 5 were not desirable from the viewpoint of the objectives of the present invention. EFFECT OF THE INVENTION According to the present invention, tofu products excellent in freeze resistance can be stably produced on an industrial scale.

Claims: A process for producing tofu products which are excellent in freeze resistance, said process comprising warming soybean milk having a viscosity at 10°C of not higher than 100mPa.s and a solids content of 10% to 16%, which is obtained by concentrating soybean milk extracted from uncooked soybean slurry or puree prepared from whole soybeans, to 60°C or higher; adding one or more members selected from the group consisting of sugars, starch and transglutaminase and adding a solidifying agent to the soybean milk whilst the temperature of the milk is 60°C or higher to prepare tofu; and then freezing; wherein, in the case of addition of starch, the soybean milk is warmed to a temperature from 60 to 70°C in the warming step. The process according to claim 1 wherein, after addition of one or more members selected from the group consisting of sugars, starch and transglutaminase and the solidifying agent to the soybean milk, the resultant mixture is heated to elevate its temperature stepwise. Tofu products obtainable by the process of claim 1 or 2.

Inventors: ADACHI TOMOHIKO, AKASAKA TAKESHI, NAKATANI SHIN, OOMURA HIROKI, ADACHI, TOMOHIKO, AKASAKA, TAKESHI, NAKATANI, SHIN, OOMURA, HIROKI, ADACHI, Tomohiko Fuji Oil Company, Ltd., AKASAKA, Takeshi Fuji Oil Company, Ltd., NAKATANI, SHIN FUJI OIL COMPANY, LTD., OOMURA, HIROKI FUJI OIL COMPANY, LTD.
Applicants: FUJI OIL CO LTD, FUJI OIL CO., LTD.
Application year: 1999
Main classification: A23L1
IPC codes: A23C20