By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

People who are allergic to animal milk and suffer from lactose intolerance can use soya milk as an alternative. It is also a source of complete protein and contains all nine essential amino acids. Soya milk is the aqueous extract of soya beans and closely resembles dairy milk in appearance and composition.

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

Product description: Soya milk

Soya milk has been consumed in Eastern countries for centuries. Soya milk prepared by traditional, Eastern methods, has a flavour that is often described as “beany”, “rancid” or “bitter” by consumers of the Western world. For this reason, the West did not readily accept soya milk until production techniques were sufficiently adapted to improve the flavour.

Modern soya milk extraction techniques were also able to increase the product yield, making it more economic for processors and producers alike.

Soya milk is consumed in much the same way as dairy milk. It is also a very useful food source for babies who are allergic to dairy milk. It can be formulated and fortified to be consumed as a beverage or food supplement. Soya milk can be concentrated by evaporation or spray drying. It can also serve as the substrate to prepare dairy analogues such as “cheese” (tofu), soya yoghurt and soya ice cream.

Process description – Traditional (Oriental) method for extracting soya milk

Soaking of whole soya beans

The soya beans are soaked in water overnight. Just enough water is used to cover the beans. Soaking raises the moisture content and cleans the beans.

Rinsing of soaked soya beans

The soak 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 between 8:1 and 10:1. 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 soya milk.

Boiling of soya milk

The soya milk is boiled for a few minutes and cooled before consumption or storage. Boiling inactivates enzymes and improves the storage life of the soya milk.

Process description – Modern method for extracting soya milk

Modern soya milk preparation methods seek to eliminate off-flavour development and improve the flavour with better extraction techniques. To do this successfully, an understanding of the chemistry of the “beany” flavour formation is required. The characteristic flavour of soya milk results mainly from reactions catalysed by an enzyme known as lipoxygenase.

These enzymes need to be released to come into contact with its target chemicals. They also require the presence of water. These conditions are typically found when soya beans are ground (enzymes released) and mixed with water to form a slurry. The temperature of the soya slurry has a great effect on the rate of flavour development. As the temperature increases, so the number and volume of volatiles decrease. The enzymes are inactivated at 80°C and therefore it is critical that the slurry be heated to 80°C or above as soon as possible after (or even during) wet grinding. Alternatively, the dehulled soya beans can be blanched prior to grinding.

Several different techniques have been developed by research institutions. Most commercial soya milk plants employ a combination of these techniques or variations of the original techniques. Regardless of variation in the methods, the basic steps and principles of soya milk extraction are essentially the same. They include raw material selection, incorporation of water, grinding, extracting soya milk, heat treatment, formulation, fortification, packaging and several optional procedures. A representative procedure of commercial soya milk processing will be discussed but take note that many other methods are employed in the industry.

Raw material selection

The selection of the right variety of soya beans is essential to successful soya milk production. Not only does it influence the quality of the end product, but also the economy of the process. High yield and protein extraction are desired from the cultivar. The typical soya bean for milk extraction should yield 80% or more protein and between 55 to 65% total solids.

Cleaning of soya beans

The first step in preparing the soya beans for processing is to remove foreign materials, including plant rests, stones and dust. Cleaning is performed by a series of vibrating screens followed by aspirators and cyclone separators.

Dehulling of soya beans for milk extraction (optional)

Most manufacturers choose not to remove the hulls of soya beans prior to milk extraction, because dehulling can damage the cotyledon and release oxidative enzymes that cause off-flavours. There are various arguments for and against dehulling.

If dehulling is performed, it should be:

• done in-line with grinding, with no time in between;
• with minimum heating/heat-hold time;
• in the absence of water; and
• with minimum possible damage to cotyledons.

Soaking of soya beans (optional)

There are several ways of incorporating water to beans to obtain an aqueous extract. Soaking is one of the ways in which to hydrate soya beans. Traditionally it was done overnight in cold water, but the soaking time can be greatly reduced by increasing the temperature of the soak water. The soya bean can increase in weight 2 to 2,3 times during soaking.

The benefits of soaking include:

• reduced powder input requirements for grinding;
• less erosion of millstones/blades;
• reduced blanching/cooking requirements;
• increased dispersion of “milk” solids; and
• increased yield.

Blanching of soya beans (optional)

Blanching is performed to inactivate the enzymes that are responsible for off-flavour development. Blanching also reduces the surface microbial load of soya beans. Anti-nutritional factors such as trypsin inhibitors are also partially deactivated. It also hydrates the beans if they have not been soaked. Blanching is commonly performed in two steps with or without the addition of sodium bicarbonate.

Step 1: The beans are heated for five minutes in boiling water containing 0,25% sodium bicarbonate.

Step 2: The beans are drained, rinsed and boiled again for five minutes in water containing 0,05% sodium bicarbonate. The addition of sodium bicarbonate to the blanching water is optional, but serves the following purposes:

• Softening of hulls (if beans were not dehulled) and tenderising of beans
• Shortening of blanching time
• Reduces energy input required during grinding and homogenisation
• Raises the pH, making soya proteins more soluble and thus increases protein and solid recovery

Disadvantages of the use of sodium bicarbonate include:

• The possible addition of acid to neutralise the pH
• The destruction of some key nutrients
• The possibility of a soapy aftertaste

Grinding of soybeans for milk production

The purpose of grinding is to disrupt the tissues and release the proteins, lipids and other solids for solubilisation and suspension in water. The method of grinding used depends largely on the preceding hydration and preparation steps. Grinding may be performed dry or wet, with hot or cold water.

Dry grinding

This method is used by some processors to bypass soaking and blanching. The dry flour is then mixed with water to form a slurry.

Cold, wet grinding

Soaked soya beans were traditionally ground at ambient temperatures.

Hot, wet grinding

This is the most commonly used method. Grinding and hydration is performed in one step. Hot water and controlled conditions are required to coincide with grinding to prevent off-flavour development. The air may be saturated with steam to reduce the available oxygen and inactivate lipoxygenase enzymes.

Grinding devices include the traditional stone mills, large blenders, hammer mills, pill mills and large commercial grinders.

Next month we shall look at the filtration process of soya milk and soya milk extraction

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.