Method of manufacturing soybean curd

Abstract: Soybean curd is manufactured by mixing soybean milk with a coagulating agent, continuously filling the resulting liquid mixture into containers, continuously passing the containers through a zone in which the liquid mixture receives sufficient microwave energy to heat the interior of the mixture to a temperature in a range of about 75 - 85°C, removing the containers from the zone and maintaining the containers at a temperature in a range of about 80 - 90°C for about 20 -50 minutes or first at a temperature in a range of about 80 -90°C for about 10 - 15 minutes and immediately thereafter at a temperature of about 50 - 65°C for about 15 -25 minutes, and cooling and storing the resulting soybean curd in a refrigerated room.

Description: Background of the Invention The present invention relates to a method of manufacturing soybean curd employing microwave processing. Traditionally, soybean curd has been made by adding calcium sulfate, calcium chloride, or agents similar to these inorganic chemicals, to hot soybean milk. These coagulating agents have been found to be very effective. Coagulation is followed by pressing to expel the whey from the curd and to shape-form the curd. This traditional method of production is entirely batch oriented. A new acidic type coagulating agent, glucono-delta-lactone, (hereafter "GDL") has been developed in the 1960's. This agent produces an even, fine-grained soybean curd. Using GDL as the coagulation agent, the accepted practice of manufacturing soybean curd is as follows: After the soybean milk is boiled and filtered, it is cooled to room temperature. GDL is added to the soybean milk and thoroughly mixed therein. Packaging containers are filled with this soybean milk-coagulant mixture. These containers are immersed in a constant temperature hot water bath for a long period of time until the curd is formed. This process has been developed to a state of art by which continuous production of soybean curd is possible. Prior art relating to the use of GDL in the manufacture of soybean curd includes the following U. S. patents: 4,000,326; 4,140,811; 4,514,433; 4,537,789; 4,585,665; 4,636,398; 4,789,556; 4,826,701; and 4,828,869. GDL is derived from glucose and forms white, odorless crystals. It is used extensively in the food industry as an acidulant. In water, it is hydrolyzed and decomposes to give gluconic acid, which is the actual active agent as acidulant. Likewise, for the coagulation of soybean curd, gluconic acid is the active coagulation agent. GDL is stable in powder form. It is freely soluble in water. When added to water, it slowly hydrolyzes to form an equilibrium mixture of gluconic acid and its delta- and gamma-lactones, as shown in the following equation. At room temperature, complete hydrolysis occurs in about three hours. Figure 1 shows the extent of acid formation with increase in elapsed time of buffered aqueous GDL solutions at various concentration levels at 20°C. If curves No. 1 and No. 2 of Figure 1 are compared, it is noted that for solutions of same concentration, the initial hydrolysis rate is much higher for a more neutral solution than for a more acidic solution. Gluconic acid being the coagulant, a higher release rate of gluconic acid will give the coagulant a higher concentration immediately and, after any total elapsed time from the initial introduction of the gluconic acid, a longer period of higher concentration to diffuse throughout the soybean milk. It can also be concluded that in a non-buffered solution as hydrolysis proceeds the amount of gluconic acid that is formed will actually depress the subsequent rate of hydrolysis of GDL. But a depletion of gluconic acid, such as occurs when gluconic acid is removed from the soybean milk as it coagulates the soybean milk into soybean curd, will induce more hydrolysis of the lactones. Another notable property of the rate of hydrolysis of GDL can be studied by referring to Figure 2 which is a plot of degree of hydrolysis, represented in terms of pH value of the solution vs. elapsed time for a 1% GDL aqueous solution at temperatures of 5°, 20°, 50°, and 90°C. These curves all show inverted exponential characteristics, i.e., the rate increases rapidly during initial stage, reaches a maximum, and decreases to a minimum value, with this feature becoming more pronounced at higher temperatures of 50°C and 90°C. It can be seen that at 90°C an equilibrium pH value of 2.5 - 2.6 is reached after an elapsed time of only about 5 minutes. It is thus most advantageous to operate the coagulation at a temperature of about 90°C to achieve maximum rate of hydrolysis of GDL. The industrial practice in Japan for the production of soybean curd, or, by its Japanese name, "tofu," is to operate the coagulation step at 80° - 90°C by means of a hot water bath and with the immersion time being 40 - 50 minutes to give good coagulation results. However, the machinery for a hot water bath for continuous production is complex and costly to build. Also, the processing efficiency is lowered because of the time needed to conduct heat from the hot water bath into the contents of containers of soybean milk-coagulant liquid mixture to raise the temperature of the liquid. It has been suggested in Japanese Patent No. 61-19465, the subject matter of which is not embodied in any industrial process, to heat soybean milk-coagulant mixture by means of microwave energy, but without the purpose of making the heating more suitable to the properties of the coagulant than heating by means of a hot water bath. In Japanese Patent No. 61-19465 the coagulant is bittern or glucan, the latter being a polysaccharide that is a polymer of glucose. It is asserted that with preheating of the soybean milk to 65°C and mixing the preheated soybean milk with coagulant followed by a one minute application of microwave energy to heat the mixture to 85°C coagulation is completed. It is also asserted that, alternatively, the hot, freshly made soybean milk may first be cooled down to a temperature below which coagulation occurs (about 10°C for bittern and about 25°C for glucan), then mixed with the coagulant and, finally, subjected to microwave energy for about two minutes after having been filled into containers or while being pumped through a pipe, the mixture thereby being heated to 80°C to complete coagulation. This patent does not define the process conditions to be observed to assure a good product. For example, there is no data given on the amount of microwave energy to be imparted to the liquid mass of whatever weight. Moreover, it is not plausible that acceptable tofu can be produced with a one or two minute coagulation process. In U.K. Patent Application 2 155 751A, GDL is added to soybean milk. With the object of maintaining the mixture at 80 to 90°C for 20 to 30 minutes thereby to form curd, the mixture is boiled for one to two minutes and left to stand for 20 minutes. Pieces of the curd mixed with soybean milk are coagulated, for example by the same procedure as the initial coagulation. The invention is set out in claim 1 and optional features are defined in the dependent claims. Microwave energy in a short burst amounting to an appropriate dosage will rapidly raise the temperature of coagulant-soybean milk liquid to a desired level. From the dielectric and thermal properties of the soybean milk the amount of microwave energy to be applied can be calculated. The objective of microwave heating is to bring the liquid rapidly to temperature levels at which the rate of hydrolysis of GDL is high enough to release significant amount of gluconic acid. There is a temperature distribution in the liquid, which is a result of application of microwave energy in a short burst, but this has no adverse effect on the process. Maintenance of the temperature for the continuation and completion of coagulation can be done, for example, by one of the following three alternative processes of supplying heat: (1) Apply continuously a rather minimal level of microwave energy necessary for the aforementioned temperature maintenance. (2) A constant temperature hot water bath. (3) A warm room with storage shelves in which hot air of constant temperature is circulated. Another alternative is to use a combination of (2) and (3) of the above processes. The choice is to be determined by an economic analysis of investment and production costs of a process. Brief Description of the Drawings Fig. 1 is a heretofore published set of plots of hydrolysis of aqueous solutions of GDL at 20°C measured as variation in percent gluconic acid with time; Fig. 2 is a heretofore published set of plots of degree of hydrolysis of GDL measured as variation in pH with time for a 1% GDL solution at various temperatures; and Fig. 3 is a flow chart of a process according to the invention. Detailed Description of Preferred Embodiments of the Invention As an illustrative example, this invention in the manufacturing of soybean curd can take the form of a production scheme composed of process steps represented in Figure 3. Dry soybeans are washed by jets of water or in a suitable washing tank, step 1 in Figure 3. They are then put in a soaking tank with sufficient water added and left for a period of time sufficiently long for the beans to swell to maximum size, step 2. After the beans have swollen to maximum size, a large quantity of water is added. This mass is fed to a grinding machine to be ground, step 3, and the ground beans go to a storage tank, which also serves as a heating vat. The ground mass is heated to boiling point and then kept at 100°C for another 3 - 8 minutes, step 4, and afterwards is filtered by means of a rotary drum, vibrating screen, or other type of machinery, step 5, to separate the solids from the liquid, the liquid being the soybean milk. In step 6, the solids are washed with water and are heated together with wash water, and after filtration the liquor is combined with the soybean milk from step 5. The soybean milk is then sent to a storage tank where it is cooled and its solids content adjusted to a desired concentration level by adding more water to the soybean milk, step 7. In the next step, 8, the soybean milk is introduced into a mixing tank into which an appropriate amount of coagulant or coagulants is also introduced, followed by thorough mixing. The coagulant-soybean milk mixture can now be pumped continuously to a packaging machine, step 9, wherein open plastic packaging tubs or containers, in a size suitable for supermarket sale, are filled with the coagulant-soybean milk mixture and then each is sealed with a plastic film and topped with a snap-on plastic cover. The filled tubs are transported by conveyor belt at predetermined speed through a microwave heating tunnel oven, step 10. In place of a tunnel, box type microwave ovens arranged in series can carry out the job equally well and in a continuous manner. Constant temperature heating for the coagulation and formation of soybean curd, step 11, is carried out by sending the products of step 10 to a series of two warm rooms maintained at certain temperatures as mentioned hereinabove and by keeping them in these warm rooms for predetermined times also as mentioned hereinabove. The finished products are then cooled and stored in refrigerated room, step 12. In all the experiments carried out in making this invention, the soybean milk used for each experiment was made in the following identical way. 454 g. (1 lb.) of dry soybean was weighed and placed in a tank. Water was added in sufficient quantity and the mass was left standing for a sufficient time for the soybeans to swell to a maximum. More water was then added to the mass so that wet grinding of the swollen soybeans could be effected. The ground mass was heated to boiling and then kept at 100°C for another 3 - 8 minutes. This was followed by filtration while the mass was still hot and the filtrate was retained as the soybean milk. A quantity of water was added to the filter cake and the resultant mixture was heated to boiling to effect secondary extraction. Filtrate from the mixture was combined into the primary batch of soybean milk. In all, this series of steps used a volume of water four times the volume of dry soybean. Before the soybean milk was used to produce soybean curd, its solids content was tested and adjusted. From a series of control experiments it had been determined that solids contents of about 10 -14%, especially 10.5 - 13.8%, gave excellent quality soybean curd products. Regarding the amount of the coagulant GDL used in each experiment to produce soybean curd, the guideline of manufacturers of GDL was adopted, i.e., an amount of GDL equal to about 0.3 - 0.5% by weight of the weight of soybean milk was used. It was found that some improvement in processing conditions and product quality could be obtained when a mixture of GDL and at least one divalent metal salt was used as coagulant; in particular, a firmer soybean curd having a less acidic taste is obtained. Preferred such salts are calcium sulfate, calcium chloride, calcium phosphate, and magnesium chloride. These could be used in addition to the about 0.3 - 0.5% by weight of GDL or used to replace part of the GDL amount. Typical compositions of a mixture of coagulants consist of about 70% GDL, about 20 - 25% calcium sulfate and about 10 - 5% magnesium chloride. Other typical compositions are about 70 - 75% GDL and about 30 - 25% calcium sulfate. A series of control experiments was carried out to produce soybean curd by heating soybean milk with coagulant (or a coagulant mixture) in a convection oven at constant temperature, without any microwave heating. Satisfactory conditions were found to be heating at a constant temperature level in the about 80 - 90°C range for a time period of about 40 - 50 minutes. Typically, it was found 50 minutes would be required for heating at 80°C, and 40 minutes would be required for heating at 90°C, to produce good quality soybean curd. The size of the test sample was varied in the range 57 - 284 g. (2 - 10 oz.) by weight. For the series of experiments undertaken to demonstrate the present invention, a microwave oven and a convection oven were used in series, the convection oven simulating a warm room. The microwave power given out in the microwave oven was 350 watts. As in the control experiments, the size of the test samples of soybean milk-coagulant mixture was varied in the 57 - 284 g. (2 - 10 oz.) weight range. The time period each sample was subject to microwave energy was calculated to be such that the temperature of the interior of the soybean milk would rise to about the 75° - 85°C range. Any variation in the spot temperatures in the liquid was found not to have affected the final result of soybean curd formation. An average value of time exposure of 80 seconds for each 99 g. (3.5 oz.) of soybean milk yielded good results, and an operating range of about 60 - 100 seconds gave good quality product. When a sample left the microwave oven it could be observed that the soybean milk had thickened considerably to become a heavy paste. The sample was next introduced into the convection oven. The temperature inside the convection oven was set at 85°C. In the convection oven, the coagulation of soybean milk was completed and soybean curd formed, after which the sample could be put into refrigerated storage. It was found that a heating period of about 20 - 35 minutes in the convection oven yielded good quality soybean curd. It was also found that 28 - 30 minutes was the optimum time period. Furthermore, it was determined that variation in the size of the sample did not require changes in temperature level in the convection oven or in the residence time in the convection oven. In another set of experiments two samples were used. One sample followed the procedures above. The other sample was subject to the following process variation of the coagulation step after the microwave heating step. The sample was first put in a convection oven operated at 85°C for 15 minutes. After this, it was placed in a second convection oven operated at 60°C for 15 minutes. Both samples yielded good quality soybean curd.

Claims: A method of manufacturing soybean curd, comprising mixing soybean milk with a coagulating agent comprising glucono-delta-lactone, continuously filling the resulting liquid mixture into containers, continuously passing the containers through a zone in which the liquid mixture receives sufficient microwave energy from microwave energy sources of sufficient power relative to the weight of the soybean milk in the liquid mixture and for a sufficient period of time to heat the interior of the mixture rapidly to a temperature in a range of 75° to 85°C, thereby to initiate coagulation of the mixture, conveying the containers from the zone to a first warm room and keeping the containers in the first warm room at a temperature in a range of 80°C to 90°C for a period of 10 to 15 minutes thereby to continue coagulation of the mixture and immediately thereafter conveying the containers to and keeping the containers in a second warm room at a temperature in a range of 50°C to 65°C for a period of 15 to 25 minutes thereby to complete coagulation of the mixture in the containers to form soybean curd, and cooling and storing the soybean curd in a refrigerated room. A method according to Claim 1, in which the power of the microwave energy sources relative to the weight of the soybean milk is about 350 watts per 99 g. (3.5 ounces),by weight, of the soybean milk and the period of time in which the liquid mixture receives the microwave energy is 60 to 100 seconds. A method according to Claim 1 or Claim 2, wherein the coagulating agent consists of glucono-delta-lactone. A method according to Claim 1 or Claim 2, wherein the coagulating agent consists of a mixture of glucono-delta-lactone and at least one divalent metal salt. A method according to Claim 4, in which the proportion of glucono-delta-lactone in the coagulating agent mixture is 70 to 75% by weight based on the weight of the coagulating agent mixture. A method according to Claim 4 or Claim 5, in which the divalent metal salt(s) is or are selected from the group consisting of calcium chloride, calcium phosphate, calcium sulfate, and magnesium chloride. A method according to any preceding claim, in which the proportion of glucono-delta-lactone mixed with the soybean milk is 0.3 to 0.5% by weight based on the weight of the soybean milk.

Inventors: CHEN RICHARD K, CHEN, RICHARD K.
Applicants: AEONIAN HOLDINGS LTD, AEONIAN HOLDINGS LTD.
Application year: 1992
Main classification: A23C20
IPC codes: A23C20, A23L1