By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

Tofu is the soft, white curd prepared by coagulating water soluble soya proteins. It has a gel structure that resembles fresh cheese or firm set yoghurt. Tofu has long been the most popular way of consuming soya beans in East Asia and its use dates back to 164 BC. Since its introduction to the West, tofu has gradually been accepted as an alternative protein source. Tofu is inexpensive, nutritious and versatile, and gaining popularity amongst vegetarians and the environmentally conscious. It can be served as a meat or cheese substitute. This month we look at the traditional and commercial methods of producing tofu.

We thank the ARC Institute for Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.

The preparation of tofu is a complex interaction of many factors. It is therefore not easy to produce tofu with consistent quality and yield, even under well-controlled processing conditions. The preparation of tofu can be divided into three main steps, namely the extraction of soya milk, the coagulation of soya milk and the pressing and packaging of the curd. The wide range of tofu products that are for sale is the direct result of variations within each of these three steps.

Traditional method of producing tofu

The traditional Eastern method for preparing tofu is outlined here:

Soaking of whole soya beans

The soya beans are washed and soaked in water overnight. The ratio of the water to beans is approximately 2:1. Soaking raises the moisture content, making it easier to grind.

Rinsing of soaked soya beans

The water is discarded, and the beans are rinsed with fresh water until the rinse water runs clear.

Grinding of soya beans

The soya beans are ground with fresh water. The water to bean ratio is approximately 6:1. Stone grinders are used.

Filtrating the slurry

The slurry is filtered through (muslin) cloth. The residue is known as soya pulp or okara, and the filtrate is the raw soya milk. The okara can be washed again and remixed with the slurry to obtain maximum protein extraction.

The concentration of the raw soya milk is a very important factor in the preparation of tofu. It is measured in Brix (°B) with a refractometer. With remixing and washing of okara, the total volume of the final raw milk is about eight to ten times that of the original volume.

Boiling of soya milk

The soya milk is boiled for ten minutes prior to processing or storage. Boiling inactivates enzymes and improves the nutritional quality of the soya milk, reduces the bean flavour and denatures the proteins for subsequent coagulation.

Cooling of milk

The boiling hot milk is transferred to a coagulation vat where it is left to cool before the coagulant is added.

Preparation of coagulant

A coagulant solution is prepared by mixing powdered gypsum (calcium
sulphate) or nigari (magnesium chloride) with hot water and stirring well to form a suspension.

Coagulation of soya milk

The coagulant is mixed with the milk as soon as the milk reaches 78°C. The mixture is vigorously stirred for 30 seconds and then left and covered to allow coagulation to take place for the next 30 minutes.

Pressing of soya curds

The coagulated milk or curds are stirred to break the gel structure. This causes the whey to be released. The broken curds are transferred to a shallow perforated box lined with cloth. Once the box is filled, the cloth is pulled up and folded over the curds. The box is then covered, and pressure is applied to the box by stacking bricks on top of the lid. The amount of pressure applied determines the firmness of the tofu. The curds are only pressed for approximately 30 minutes, during which time whey seeps out of the box.

Packaging and storage of tofu

The pressed tofu “cake” is left to cool and then removed from the box before being cut into slices or pieces. The fresh tofu is ready for use and can be stored for a limited period. Tofu pieces were traditionally stored in water for up to 14 days, depending on the season’s temperatures. Today, refrigerated storage is used and the storage water is changed occasionally.

Commercial production of tofu

The traditional process has been adapted for continuous, large-scale production of tofu, but the basic principles remain the same. There are, however, many different types of commercial methods being used, which makes it difficult to make a general summary of commercial tofu production. These methods differ with regard to the way the milk is extracted, the way the proteins are coagulated and the way the curd is pressed and packed. All of these variations affect the tofu quality and yield, resulting in a wide variety of tofu products on the market today. Based on the water content and textural properties, tofu can be marketed as “soft” (silken), “firm” or “extra firm”.

Soft tofu types contain 88 to 90% moisture and 6% protein. It is prepared from rich soya milk containing 10 to 12% solids. The milk is mixed with a low concentration of calcium sulphate and allowed to coagulate at 65 to 70°C for 30 to 60 minutes. The result is a fine, smooth curd that is firm enough to retain its shape after slicing. The curd is not broken or pressed and thus contains the whey. This type of tofu has the highest yield. To produce firm and extra firm tofu, the curds are broken and pressed. The heavier the weight or the higher the pressure applied, the firmer the texture. Firmer tofu is easier to handle and keeps its shape during cooking. It is therefore ideal for frying and dicing into foods or soups.

Further processing of tofu

Tofu can be further processed in various ways, thereby adding to its versatility:

Frozen tofu: Freezing causes dramatic changes in the texture and flavour of tofu. Upon thawing, the tofu has a tough chewing texture and is resilient like a sponge.

Dried and frozen: This greatly extends the shelf life of tofu and produces a concentrated source of protein and energy.

Deep-fried: Frying imparts a meaty texture, flavour and colour to tofu while extending its shelf life. It also eliminates the need for storage in water. Deep-fried tofu is especially appealing to Western consumers.

Fermented tofu or “Sufu” is also known as Chinese cheese.

Next month we shall look at the production of soya yoghurt.

Published with the acknowledgement to the ARC Institute for Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.

LITERATURE SOURCES

1. Applewhite, TH. 1989. Vegetable Protein Utilization in Human Food and Animal Food Stuffs.
2. Lui, KeShun. 1999. Soybeans: Chemistry, Technology and Utilization. Gaithersburg: Aspen Publishers, Inc.
3. Snyder, HE & Kwan, TW. 1987. Soybean Utilization.
4. Tanteeratarm, K. 1992. Soybean Processing for Food Uses.