For The Love Of Tofu

A block of tofu on a plate with peppers an broccoli as health foods.
Photo by hanul choi c/o Pixabay.

The desire for protein is ever rising and tofu is one of those products that might just deliver. Tofu itself is a fantastic meat and dairy substitute and has been used for this purpose for many years.

Some product developers call tofu ‘the cheese of Asia’. It is a highly nutritious, protein-rich bean curd manufactured by coagulating soy milk.  The process relies on coagulation using divalent cations such as magnesium and calcium (Mg2+, Ca2+). Soy milk has numerous health benefits and most of these extend to tofu as well as acquiring a few new ones !

The tofu might also be described as a true protein gel rather than a curd in some circles.

Tofu comes in all sorts of textures and flavours, including extra firm, firm, soft, packed and silken tofu (Saio, 1979). Extra firm and firm tofu have textures similar to cooked meat and raw meat, respectively. Acceptable soft tofu requires a bland, almost non-descript taste and fine texture with 88 to 90% moisture content. Soft tofu has a soft cheese like texture but is firm enough to retain shape after slicing (Tsai et al., 1981)

History

The product fist appeared in China. Legend has it that a Han dynasty prince called Liu An produced it during the 2nd century BCE. The food has been eaten throughout that time but wasn’t recognised as the nutritional food it really is until relatively recently.  It began to really take off in its current guise in the 1960s when the Japanese Food Research Institute started updating and standardizing tofu production in Japan.

Production Of Tofu

Production begins by soaking and grinding soyabeans.  Soybeans are soaked for anywhere between 9 and 10 hours at lower temperatures (22°C) or 4 to 6 hours at higher temperatures (32°C). If the beans are ground prior to soaking, the soaking time can be reduced.

The processors use soft and malleable rotating rubber rollers that removes the hulls before the beans are cooked. The beans are cooked often by steaming  and then ground to produce soy milk and soy fiber or pulp. This latter fraction is usually called okara and is an important nutritional product in its own right.

The sugar from the soybean steaming and/or boiling process is generally named “soybean oligosaccharides.” which consist of stachyose, raffinose and sucrose

Microfiltration is needed to separate the soy milk and the soy pulp.  The next step is separation of the protein from the oil.

The separation process is influenced by various processing factors which includes the heating process and type of coagulant used.

Soy milk is the portion needed for producing the tofu. To achieve this there is a heating stage which produces thermal denaturation of the soybean proteins.  Soymilk is conventionally heated with a one-stage steam heating process. In some instances, a two-stage ohmic heating process has been applied with some success (Wang et al., 2007).

The most critical factor is the type of coagulant used which affects the quality of the final product. Coagulants encourage protein liquids in this case to become solids. The other key factor is the addition of various natural oils.

Type of Coagulants Used

The two most commonly used coagulants are calcium sulphate and magnesium chloride. Calcium sulphate is the most widely used because it does not mask the taste of the soybeans.  Instead it helps preserve and highlight the flavors. Magnesium chloride and calcium chloride are more soluble and produce smoother-textured tofu.

In some cases, an edible acid might be used. the most often applied is glucono delta-lactone (GDL). This acid is used primarily to produce soft or silken tofu because the coagulation rate is very quick. It means silken tofu can form inside its storage container without the need for an air gap. This prevents the silken tofu from breaking down when it is being transported.

Sea water is used on some of the Japanese southern islands like Okinawa where it makes one of the best tasting tofu known. The sea water is gathered from a clean source and never from the mouth of any river. In Japan, nigari, which consists primarily of magnesium chloride, is produced from seawater. Sodium chloride is removed and water is evaporated to yield the nigari white powder. Nigari may also contain small amounts of magnesium sulfate, potassium chloride, and calcium chloride depending on how it is made. The variable composition of nigari can be a challenge for large-scale tofu manufacturing.

Pasteurisation is often used to extend the shelf-life of tofu. This type of heat treatment to produce a more microbially stable product has introduced consumers to a new type of product offering. We now can choose to eat tofu hot dogs, tofu pasta and tofu burgers. You can also eat a tofu ice-cream but that isn’t recommended.

Other heat treatments include ultra high temperature (UHT) processing which is more effective but has greater sensory impact. This process is often used for milk. The product can also be packaged aseptically which extends shelf-life even further.

Quality Of Tofu

The quality of tofu depends essentially on its textural properties and that is often a judgement of texture as opposed to flavour. The main components are soybean and primarily this is protein with phytate. Protein is probably the most influential and there are two key proteins based on their sedimentation criteria called glycinin (11S)  and β-conglyciniin (7S) which have most impact. Indeed it is the 11S/7S ratio which is the most important measure associated with firmness. 

However, whilst protein ratios are fundamental these levels are dependent too on the type of cultivar or variety of soybean used (Shen et al., 1991; Sun and Breene, 1991) . So as well as processing methods and types of coagulant, various soybeans have been explored to obtain the best type of tofu required.

Soybeans have a tendency to split and develop seed coat cracks. If the seed split is close to 20% then good quality tofu ir rarely produced.

The colour of tofu and indeed soy milk mirror each other. Both products start as a creamy yellow initially but over time develop a beige tinge and is a minor but not insignificant quality factor. Lipid oxidation and nonenzymatic browning reactions are probably mots likely here (Hou and Chang, 1998, 2005). 

References

Hou, H.J.Chang, K.C. (1998) Yield and quality of soft tofu as affected by soybean physical damage and storageJ. Agric. Food Chem. 46 pp. 4798805 (Article)

Saio, K. 1979. Tofu-relationships between texture and fine structure. Cereal Food World 24(8) pp. 342–354

Shen, C.F., DeMan, L., Buzzell, R.I., and de Man, J.M. (1991) Yield and quality of tofu as affected by soybean and soymilk characteristics: glucono delta lactone coagulant. J. Food Sci. 56 pp. 109–112

Tsai, S.J., Lan, C.Y., Kao, C.S., and Chen, S.C. (1981) Studies on the yield and quality characteristics of tofu. J. Food Sci. 46 pp. 1734–1737

Wang, L. J., Li, D., Tatsumi, E., Liu, Z. S., Chen, X. D., & Li, L. T. (2007). Application of two-stage ohmic heating to tofu processing. Chemical Engineering and Processing: Process Intensification46(5), pp. 486-490.

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