By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

The protein extracted from soy beans is a very high quality protein and contributes significantly to muscle growth. It also increases the nutritional value of certain foods because it contains a complete amino acid profile. Moreover, it also reduces the changes of heart disease and the risk of certain cancers and osteoporosis. This month we take a closer look at the process of extracting this product.

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

Product description – Protein concentrate

Defatted soy meal consists of 30 to 35% carbohydrates. By removing the soluble carbohydrate fraction along with some flavour components, the protein concentration of the soy meal is increased.

Three different methods can be used to concentrate soy meal:

1. Acid leaching method (pH 4,5)
2. Aqueous ethanol method (60 to 80 %)
3. Moist heat – water leaching method

All three methods cause the proteins to become insoluble, while some of the carbohydrates are solubilised in the medium used for separation. The solid and liquid fractions are then separated by decanting and/or centrifugation. The remaining solids (concentrate) are dispersed in water and neutralised to pH 7 if necessary. The concentrate is finally dried to produce a powder known as soy protein concentrate. The protein content and functional properties of the concentrate differ according to the method used.

Process description – Protein Concentrate

Method 1: Acid leaching process

Defatted soy flakes are mixed with acidified water at isoelectric pH 4,5 to remove soluble carbohydrates. The ration of flakes to water is from 10 to 20:1. Extraction is achieved at 40° C for between 30 to 40 minutes during which time the carbohydrate leaches out into the water.

Thereafter, the insoluble residue (mainly protein) is removed using a decanter and/or centrifuge. The protein residue is neutralised to pH 7 and spray dried to produce a protein concentrate with high nitrogen solubility and low microbial count.

Method 2: Aqueous alcohol extraction

This method extracts alcohol soluble carbohydrates from defatted soy grits using a counter-current stream of aqueous alcohol in a plateless column. The flakes containing insoluble carbohydrates are continuously removed at the end of the extraction column.

This method requires the desolventising of the flakes prior to drying.

The alcohol is also recovered from the solvent-carbohydrate mixtures for re-use. This method produces a bland soy protein concentrate with low nitrogen solubility.

Method 3: Moist heat and water leach method

Moist heat is applied to defatted soy flakes to denature the proteins and cause them to precipitate in water. The soluble carbohydrates and salts leach out in the water and are separated from the precipitate by decanting and centrifugation. The protein rich precipitate is dried to produce a soy protein concentrate with low nitrogen solubility.

Full-fat soy flour is prepared by grinding dehulled soy beans. The soy beans may be given a steam treatment prior to grinding to produce toasted full fat soy flour. Steaming inactivates the lipoxygenase that is largely responsible for beany off-flavours. Alternatively, enzyme-active full-fat soy flour can be produced by omitting the steam treatment. The enzymes of this type of soy flour are used to bleach wheat flour and act as dough conditioners.

Toasted full-fat soy flour is used in a wide range of baked products and baby formulas. It can also be further processed to produce meat analogues through a thermoplastic extrusion process. Meat analogues or substitutes are soy products with a meat-like texture. They are prepared by mixing soy flour, water and additives to form dough. The dough is then extruded under high pressure to obtain a fibrous texture.

LITERATURE SOURCES

1. Lui, KeShun. 1999. Soy beans: chemistry, technology and utilisation. Gaithersburg: Aspen Publishers, Inc.

Next month we shall look at an oil seed product which is more familiar to us, namely soy sauce.

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