Strictly speaking, "soymilk" is a water extract of whole soybeans. It is an off-white emulsion/suspension containing the water soluble proteins and carbohydrates, and most of the oil of the soybeans. Soymilk has been produced traditionally in China, and to a lesser extent elsewhere in East Asia, but was never a predominant element in the popular diet. The concept of a milk-like food from soybeans was introduced to Europe in the beginning of our century. A patent for soymilk production was issued in 1910, to Li Yu-ying, a Chinese living in France. Small-scale industrial production of "artificial milk" was started soon after, with limited commercial success.

Soymilk was "re-introduced" to China, in the 1930s, by an American physician-missionary, Dr. Harry W. Miller, in an attempt to improve the nutritional status of the population, and particularly of infants and small children. Consumer resistance to this "substitute food" was apparently not easy to overcome, despite the food shortage and Dr. Miller's enthusiasm. Production was discontinued, due to the destruction of the plant in 1937, in the China-Japan war.

A different marketing approach was more successful in Hong Kong. A soymilk plant, the Hong Kong Soymilk Factory, was established in 1940 by K.S. Lo. After World War II, the company, now named the Hong Kong Soya Bean Products Co. Ltd., started to market its product, under the name of Vitasoy, not as an artificial milk, but rather as a soft beverage. The marketing channels and even the shape of the bottle, clearly indicated that Vitasoy was competing with the popular "sodas", not with milk. By 1974, sales of Vitasoy exceeded those of Coca Cola in Hong Kong.

Attempts to reproduce the "Vitasoy Story" outside East Asia ( e.g. Saci by Coca Cola in Brazil ) failed to attain the same level of market penetration. It should be noted that Vitasoy has a marked beany taste, which is apparently not objectionable to the consumer in Hong Kong, but is unacceptable to the general public in the West.

"Beany" flavour is indeed the principal inconvenience of traditional soymilk. This objectionable flavour comes from some ketones and aldehydes, particularly hexanals and heptanals, produced through lipoxidase-catalyzed oxidation of soybean oil. These compounds are not present in sound, dry soybeans but are produced as soon as the beans are wetted and ground.

Several approaches have been used to overcome the problem of off flavours in soymilk. These are:

a: Heat inactivation of lipoxidase in the whole dry bean or during the wet grinding process.

b: Starting with defatted materials ( defatted soy flour, soybean protein concentrates or even isolated soybean protein).

c: Removing the flavour compounds by evaporation (deodorization) , after they have been formed in the soymilk.

d: Masking the bitterness and off-flavour by sweetening and flavouring ( e.g. with chocolate or coffee flavour ).

e: Developing through genetic engineering varieties of soybeans devoid of lipoxidase activity.

A new wave of technology development based on some of these or similar approaches occurred since the late 1970s. The technology packages being offered today range from cottage-industry operations to fairly large-scale soymilk factories, featuring the most modern techniques of dairy technology.

Today, the manufacture of soymilk and related products constitutes an important part of the edible soy protein industry. It is difficult to obtain up-to-date production statistics. World production in 1983 was estimated to be close to one million tons of soymilk, representing approximately 130,000 tons of soybeans. Since then, consumption has risen considerably. The largest growth has occurred in Japan, where soymilk manufacture is primarily in the hands of gigantic companies who are also in the dairy industry business.

8.2 Composition and classification

With the exception of Japan where a Japanese Agricultural Standard defines and classifies soymilk, there is no widely accepted official standard of identity for this product. The custom to write "soymilk" in one word is probably based on the need to avoid involvement in the heavily regulated area of dairy product nomenclature. Soymilks and related products can be formulated rather freely and may have widely differing compositions.

Commercial soymilks and related products may be classified, according to their composition, as follows:

a- Plain (or traditional) soymilk: Made by water extraction of whole soybeans, using a bean to water ratio of 1:5. Contains approximately 4% protein.

b- Dairy-type soymilk: Formulated so as to have a composition roughly similar to that of dairy milk. Bean to water ratio 1:7. Protein content 3.5%. Slightly sweetened, contains added oil, salt. May contain imitation milk flavour.

c- Soy beverages: Sweetened and flavoured drinks, containing about 1% protein. Bean to water ratio 1:20.

d- Cultured products: Any of the above type after lactic fermentation or acidification with lactic acid.

e- Blends: Mixtures of soymilk and other vegetable or dairy milks.

In addition, a product made by grinding dehulled, heat-treated soybeans to an ultra-fine powder has been suggested as a solid soymilk base. It contains all the insoluble components of dehulled soybeans and its composition is therefore, markedly different from that of soymilk preparations made by water extraction.

Composition and nutritional value date for a European soymilk , as advertised by the manufacturer,is given in Table 8-1. The emphasis on the absence of cholesterol and the preponderance of poly-unsaturated fatty acid hints to the health implications of soymilk composition.

Source: Vandermoortele (UK) Ltd. (ALPRO soymilk)

8.3 Production processes

This is the method by which soymilk is made for daily consumption , particularly in China. The soybeans are washed, soaked overnight and then ground to a slurry. Cold water is added. after thorough mixing, the dilute slurry is strained through cheese cloth and pressed. The extract (soymilk) is boiled, strained again and filled into various types of containers. This is a heavy, quite gritty suspension and has a strong beany flavour and chalky mouthfeel.

Commercial soybeans contain varying amounts of foreign material such as dirt, dust, stones etc. It is vital for the production of high quality soymilk that this foreign material be removed.

Just as important is the elimination of damaged soybeans from the supply to the soymilk processing plant. This is necessary because the enzyme lipoxidase ... will have acted on the fatty acids in the damaged soybean cell tissue, producing compounds with the characteristic beany flavour.

Soybean hulls contain unwanted substances. Also the hulls are an obstruction to continued processing, especially in the decanter. Soil bacteria are present in the soybean hulls. The hulls should therefore be removed to reduce bacteria count in the soymilk, resulting in better flavour and shelf-life. Soybean hulls contain polysaccharides which should be removed to avoid off-flavours and processing problems caused by foaming. The holding time for heat treatment of the soybeans to inactivate undesired enzymes can be shortened when using dehulled soybeans. This will decrease protein denaturation and browning of the soymilk.

Dehulled soybeans produce a white, attractive, appetizing soymilk.

Blanching of the soybeans in a solution of sodium bicarbonate at high temperature starts the inactivation of the bitter taste causing enzyme lipoxidase. This treatment also washes out water soluble oligosaccharides (flatulence causing) and starts the inactivation of trypsin inhibitor (reducing digestibility).

Grinding in a hot water solution of sodium bicarbonate converts the soybeans into a colloid solution (soyslurry) without unwanted enzyme activity.

In order to avoid chalkiness and obtain a good mouthfeel of the soymilk, the insoluble fibres are filtered away. To achieve optimal results, a decanter centrifuge is used....

To remove unwanted volatile off-flavours the clarified soymilk base is deaerated by means of vacuum and parboiling in the deodorizer.

Water is added to the soymilk base to obtain the desired protein content of the soymilk...

One of the keys to widespread acceptance of soymilk is proper formulation, using sweetening and flavouring agents of the types and in the amounts suited to local tastes. The addition of oil to soymilk results in increased richness and creaminess (good mouthfeel) and adds calories....

Soymilk for school-feeding programs in developing countries is preferably fortified with vitamins and minerals, e.g. vitamin B-12 and calcium. Soymilk has only 1/5 of the calcium in cow's milk (but 1/2 of that in mother's milk).

Homogenization breaks down fat globules into very fine particles by forcing them under great pressure through minute valve openings, and distributes them evenly throughout the soymilk. Otherwise the fats would tend to lump together, rise to the surface, and separate as a distinct layer. Homogenization gives soymilk a creamier, more uniform consistency.

Direct method UHT treatment of soymilk serves two purposes:

(1) to inactivate bacteria thereby prolonging shelf life.

(2) to deodorize the soymilk..........

....The UHT treated product should be aseptically packed."

As it seen from the above description, the STS process makes use of advanced dairy processing techniques, such as Ultra-high temperature (UHT) sterilization and aseptic packaging. It is not surprising, therefore, that the process has been adopted by the Danish Turnkey Dairies Ltd., a company specializing in the supply and erection of dairy and related plants on a world wide basis. Fig. 37 shows a tri-dimensional layout of a standard 4,000 litre per hour soymilk plant supplied by the company.

The "dairy type" approach is widely applied in Japan. It has been reported that more than 75% of the soymilk in Japan is UHT processed and aseptically packaged. The price of soymilk in Japan is 25% higher than that of cow's milk.

As in all water extraction based processes for the production of soy milk, the STS process generates a by-product, namely the insoluble extraction residue or "pulp". This material comes out of the solids discharge of the decanter and contains the insoluble proteins and fibre as well as an entrapped portion of the extract. The yield of soy milk can be increased by providing an additional stage for the extraction of the residue with water, followed by an additional stage of separation in a second decanter. The economic feasibility of this second stage depend on the cost of a decanter against the price of soybeans. The determination of the break-even point is illustrated in Fig. 38. The pulp is not a waste but a useful by-product which can be sold as a feedstuff or a food ingredient with a high dietary fibre content and a good protein concentration.

The distribution of protein between the product and the residue is shown in Fig. 39. ( In this diagram, the residue is called by its Japanese name: "Okara".)

The development of this process resulted from research done at the University of Illinois at Urbana-Champaign in the early 1970s and was picked up later by INTSOY (International Soybean Program). INTSOY is a program supported by U.S. and international agencies and managed by the University.

The "Illinois process" is basically similar to the STS process described above. Actually, the basic process of lipoxidase inactivation by blanching the whole beans in a hot dilute solution of bicarbonate originated in Illinois. The soybeans are cleaned, sized, oven dried by forced air, split and dehulled. The cotyledons are blanched in boiling water containing sodium bicarbonate, drained and ground in boiling water . After thorough mixing, the slurry is strained. The filtrate (milk) is heated, homogenized, pasteurized and packaged.

In contrast to the STS Process,the INTSOY process has been designed for small-scale plants. These plants are sold by BAR Export/Import, Inc. a company based in Urbana, Illinois. A price quotation for a complete plant for 220 litres of soymilk per hour is appended. (See Annexes).

Buhler Ltd. from Switzerland presents its patented process for the manufacture of dehulled, micro-milled, full-fat soy flour as an alternative or as a supplement to conventional soymilk production. Two features distinguish the process from ordinary full-fat soy flour manufacturing : the hot dehulling process and micromilling of the cotyledon particles to an impalpable powder by means of a six-roller mill. Hot dehulling, mentioned and described in Chapter 3, is said to reduce bitterness. Micromilling is claimed to break the cells, liberating the protein bodies and fat globules and making them more available for extraction.

A flow diagram of the Buhler Process is shown in Fig. 40.

According to Buhler publications, the fine powder is easily dispersed in beverages such as chocolate drinks and can replace milk solids in such products. A suspension of the micro-pulverized full-fat soy powder in water is referred to as "soymilk", although it has all the fibre of dehulled soybeans. If a fibre-free normal soymilk is desired, the micro-milled flour can serve as the starting material . It is claimed that the yield of soymilk per unit weight of soybeans is almost doubled, due to the fact that micro-milling releases the proteins and fats, rendering their extraction more complete. The residue (okara) now consists of a relatively small amount of cell-wall debris, its quantity being only 1/8 of that of okara in the traditional extraction process. ( Fig. 41).

8.4 Soymilk related products

Following the commercial success of soymilk, imitation dairy products based on soymilk or mixtures of soy and cow's milk have been developed.

Soymilk is curdled when acidified by natural lactic acid fermentation or by adding acid. Through careful acidification it is possible to obtain gel like products resembling cultured dairy products such as yogurt. Since soymilk does not contain lactose, the normal lactic acid bacteria cultures used in the dairy industry do not ferment soymilk. This difficulty can be overcome by adding lactose, whey solids or cow's milk. Another approach, still at the research stage, is to develop , through genetic engineering, strains of bacteria that will ferment the natural carbohydrates of soymilk and produce lactic acid. Soy yogurts of acceptable quality are now available in various countries.

Another soymilk derivative is soy "ice cream". This product is being well accepted by the vegetarian and cholesterol-conscious population.

Anon. (1987)
INTSOY Develops New Techniques for Commercial Soymilk Processing INTSOY Newsletter No. 36 University of Illinois at Urbana-Champaign

Gavin M. and A. Wettstein (1990)
"Soymilk and other soya products- From the Traditional Method of Production to the New Manufacturing Processes." Buhler Ltd. Uzwil, Switzerland

Hauman, B.F. (1984)
Soymilk. New Processing, Packaging Expand Markets J. Amer. Oil Chem. Soc. 61: 1784

Nelson, A.I., M.P.Steinberg and L.S. Wei (1976)
Illinois Process for Preparation of Soymilk J. Food Sci. 41: 57

STS-Soya Technology Systems Ltd. (1986)
"Soymilk in Brief" STS Ltd. Singapore