By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

Last month’s article introduced the production of soya milk through the processing of soya beans. This month we look at the remaining processes, to wit filtration, extraction and packaging of soya milk.

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

Filtration and soya milk extraction

The efficiency of extraction directly determines soya milk yield. Soya milk is extracted from the slurry by:

• Filtration through cloth or vibrating screens
• Pressing sacks
• Centrifugation

Extraction methods may be repeated or combined to improve extraction efficiency. The insoluble solid residue (soya pulp or okara) still contains some protein and other solids. To maximise the yield, the okara is often washed with hot water and filtered again.

Filtration is omitted altogether in certain cases to produce a soya milk with 100% solids content. This naturally delivers a product with the highest yield but requires proper homogenisation and may have an offensive “chalky” mouthfeel.

Cooking of soya milk (optional)

The filtrate (soya milk) is cooked to increase the storage life of the product by inactivating microorganisms and enzymes that cause spoilage. It improves the flavour of the product by inactivating the enzymes responsible for off-flavour development. It improves the nutritional value of the product by inactivating anti-nutritional factors such as trypsin inhibitors and lectins. Heat also denatures the soya proteins, making them more easily digestible. Cooking is necessary for soya milk that is intended for tofu because it alters the soya protein properties and maximises coagulation.

The extent of cooking (if any) needed depends on the previous heat treatments given to the soya bean prior to milk extraction. However, heating at 85 to 87°C for 20 minutes is common. Overheating should be avoided to prevent Maillard browning, (a reaction between sugars and proteins causing deterioration of food), loss of vitamins and the development of a cooked flavour.

Deodorisation of soya milk (optional)

Soya milk can be deodorised to improve the flavour or to remove offensive off-flavours that were formed during the production of the soya milk. This is done in a vacuum pan at high temperatures and high vacuum conditions (115°C and 0,526 mm/Hg). It is a very expensive process but strips the milk from all volatiles and produces a very bland product.

Formulation of soya milk (optional)

Formulation involves the addition of sweeteners and flavours to soya milk to produce a more acceptable product for drinking. Ingredients that are commonly added include honey, maple syrup, sugar, vanilla extract, cocoa powder, orange juice, malt, coffee, almond extract, oil and lecithin.

Fortification of soya milk (optional)

Fortification aims at improving the nutritional value (or profile) of a food product by the addition of nutrients such as vitamins and minerals. Soya milk that is used for baby formulas is usually fortified with calcium, B vitamins and essential amino acids (methionine and lysine).

Homogenisation of soya milk (optional)

Homogenisation is the process whereby a fluid is forced through a very narrow opening under high pressure. It aims at breaking up fat globules, reducing them in size and thus ensuring their suspension throughout the liquid. Homogenisation prevents the separation of fat and aqueous phases and produces a stable emulsion.

Soya milk contains aqueous and fat phases as well as large solids (and sometimes added ingredients) that need to be mixed thoroughly to form a homogenous substance. This is normally done at 13 800 to 24 150 kPa (2 000 to 3 500) psi at a temperature of 90°C. The homogenised soya milk has a smooth, creamy texture (mouthfeel) and whiter appearance. Homogenisation is essential for unfiltered soya milk and soya milk formulations.

Stabilisation and packaging of soya milk

Soya milk is a highly nutritious food product and is an ideal growth medium for bacteria. Commercial soya milk requires a heat stabilisation treatment such as pasteurisation or sterilisation to make it safe for consumption and to extend the shelf life. The heat stabilisation treatment is given just before or after packaging or can even be part of the packaging process as in the case of aseptic packaging. There are basically three options available to the processor:

Pasteurisation

Pasteurisation involves heating the milk to 75°C for 15 seconds in a heat exchanger to ensure that all vegetative bacteria that cause spoilage are destroyed. The pasteurised product is packed in glass bottles, plastic pouches, or gable-top cartons. It has a shelf life of seven days and requires refrigerated storage. It is the best method for preservation of nutrients and original flavour.

Sterilisation

Sterilisation involves heating the milk to 121°C for 20 minutes in an autoclave to ensure that all forms of bacteria and their spores are killed. The milk is filled and sealed into cans, glass bottles or retort pouches prior to sterilisation. The sterilised product has shelf life of two years and does not require refrigerated storage. The high temperatures and long processing time involved in the sterilisation process causes significant vitamin loss, browning and flavour deterioration.

UHT Processing

UHT processing involves heating the milk to 140 to 150°C for two to eight seconds in an aseptic system followed by flash cooling in a vacuum system prior to aseptic packaging in laminated box cartons, plastic bottles or pouches. The process kills all vegetative bacteria and their spores with minimal thermal damage to the product. The product has a shelf life of six to eight months and requires no refrigerated storage. An added advantage of the process for unflavoured milk is the deodorisation that takes place during flash cooling.

Next month we shall look at the production of tofu.

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.