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Guideline for the production of soybean milk and soybean curd at the village level
Guideline for the production of soybean milk and soybean curd at the village level
Keith H. Steinkraus
Cornell University, Geneva, New York, USA
I. SOYBEAN MILK
The essential steps in producing soybean milk at the village level are the following:
Cleaning
The beans used for soymilk should be sound, dry, and free of mould, dirt, and extraneous matter. The beans should be washed with clean water to remove dirt. Damaged or mouldy beans should be removed by hand and discarded.
Soaking (hydration)
The soybeans should be soaked in approximately three times their weight or volume of water until they have about doubled their dry weight. The major purpose of hydration is to facilitate grinding. In the tropics, it is very difficult to soak soybeans for more than a few hours without an acid bacterial fermentation beginning. It is desirable to soak the beans in cool, running water to lessen the chance of bacterial growth that will sour the flavour. Soaking should last only for the minimum time required to about double the original dry weight. Hydration can be hastened by the use of soak water heated to a temperature of 50 or 60°C. This also decreases the chance that souring will occur.
Grinding
Grinding the soaked beans in eight parts water to produce a bean and water slurry is a challenge at the village level. It has been solved, however, by the Chinese who use traditional stone mills. The stone mill is excellent for grinding soaked soybeans with water added in order to produce either soybean milk or soybean curd.
If a Chinese stone mill is not available, then the best grinder available at the village level is likely to be a corn mill or "Corona" mill, designed for grinding dry corn or cereals between two adjustable metal grinding surfaces-one stationary and the other rotating against it.
In general, the finer the grind, the better the extraction of soy proteins, lipids, and other components of nutritional importance in the milk produced.
Filtration
The soluble, soymilk emulsion is readily separated from the insoluble residue by passing the ground slurry through a fine cloth filter. The milk is the principal product, but the residue does contain about 15 per cent protein of good quality. The residue can be steamed and fermented to make tempeh, or it can be consumed in soups as an additional source of nutrients, or it can be fed to animals.
Boiling
The soybean slurry can be boiled before or after filtration. In any case, the milk should be boiled about thirty minutes to ensure destruction of trypsin inhibitor and to retain the maximum nutritive value of the protein. Boiling also improves the flavour.
Flavouring
Many Chinese like the normal beany flavour of soy milk. Consumers less familiar with soy foods may prefer a sweetened milk. Five per cent sugar added to the milk will mask most of the beany flavour. It also adds calories. Other flavourings, such as vanilla or chocolate, can be added if the cost is not prohibitive.
Consumption
Freshly boiled soymilk will keep only a few hours without refrigeration in the tropics. With refrigeration, storage time may be increased up to three days. The milk can be sterilized in a pressure cooker by placing it in capped soft-drink bottles and cooking for 15 minutes at 15 psi, or 20 minutes at 10 psi (steam pressure). The sterilized milk will keep for at least a year without refrigeration.
II. SOYBEAN MILK CURD (Japanese tofu, Chinese tahu, Philippine tokua)
The first step in making soybean milk curd is to produce the milk as described above, but without adding flavouring. Immediately following boiling (and if the milk is to be used for curd production it is only just brought to boiling), it is subjected to a series of steps, as follows.
1. The curd is precipitated with calcium or magnesium salts; lemon juice can also be used. While the filtered milk is still hot (above 80, and preferably at 90°C), the salt is added in a concentration of 0.035 per cent. Calcium sulfate (plaster of parts) or magnesium sulfate (epsom salts) or chlorides can be used. Nigari, the concentrate left after manufacture of sea salt, is particularly valued as a precipitant. Calcium sulfate is useful because it does not add any flavour to the curd and it contributes calcium to the consumer's diet.
The method of adding the salt is considered somewhat of an art, and techniques vary. It is useful to dissolve, or suspend in the case of CaSO4, the salt in hot water and sprinkle it slowly over the surface and around the edges of the milk. At the same time, the milk can be slowly and very gently stirred to distribute the salt. The gentler the stirring, the larger the curds that will be produced. The whey should separate readily and be a clear yellow. If the whey remains milky or cloudy, additional salt should be added until it becomes clear.
When the whey becomes clear, the precipitating curd should be covered and allowed to agglomerate for about ten minutes. Then a portion of the whey should be removed and the curd transferred to a cloth-lined wooden box punched at intervals with holes to allow escape of the whey. The box should be of a size suitable for producing the size curd cakes desired. The cloth should be folded over the top of the curd and weights should be placed on the curd to squeeze out the whey. The lighter the pressing weights, the more gelatinous the curd will be. The heavier the weights, the harder the curd will be.
2. The curd cake should be cut into pieces of the desired size and stored in clean, cool water until consumed. Keeping time is only one day without refrigeration.
3. Soybean curd can be consumed without further cooking. The Japanese dip tofu in soy sauce and eat it directly. Soybean curd is also deep-fat fried until crisp on the outside and tender inside. Curd is also used as an ingredient in soup.