Soy Protein Production
Soy protein is made from dehulled, defatted soybean meal. The concentration of protein is achieved by removing most of the soluble non-protein compounds. These compounds are mainly soluble carbohydrates and some nitrogenous substances and minerals. Due to this process much the undesirable beany flavour and oligocharides (raffinose and stachyose) are removed. Soy protein concentrates also have a bland taste.
There are three methods to produce soy protein concentrate.
- Aqueous alcohol wash process
- Acid wash process
- Water wash process with heat denaturation
These methods will yield soy protein concentrate with similar composition: 70 percent protein, 20 percent carbohydrates, 6 percent ash and about 1 percent oil. The solubility of the protein can differ between the different extraction processes. One ton of defatted soybean flakes will give about 750 kg (75 percent yield) of soybean protein concentrate.
Aqueous alcohol wash process
With this process the sugars are dissolved with alcohols (methanol, ethanol or isopropyl alcohol) in a batch or a continuous process. These alcohols do not dissolve the soy proteins. Defatted soy flakes are used as raw material. After the extraction of the sugars, the alcohol is recovered and re-used. This recovery is accomplished by evaporation and rectification in a distillation column. The final flakes are dried with hot air and milled.
The soybean protein becomes insoluble in water when the pH is adjusted to 4.2. With this low pH, it is possible to dissolve the sugars without the use of special solvents. This process is a lot safer due to the absence of flammable solvents. On the other hand, it is more difficult to remove the water from the soy protein. Most of the water is removed with rotary vacuum filters or centrifuges. The obtained solids wet milled and spray dried.
Water extraction process with heat denaturation
With this process, the soy proteins of defatted soy meal are first rendered insoluble by thermal denaturation. The meal is heat treated and then extracted with hot water to remove the sugars. The process is similar to the acid-wash process.