Value-added soy protein
Consumers are increasingly interested in including proteins from plant sources in their diet, to a large extent because of the beneficial health effects attributed to them. Throughout the world, protein from soy is the most widely consumed plant protein, accounting for 68% of total consumption of this type of protein, according to data for 2009. Depending on the type of process used to extract and purify the soy protein, the isolate (90% protein content or more) obtained may still contain varying amounts of undesirable compounds, such as trypsin inhibitors and phytic acid. Trypsin is an enzyme that allows protein digestion, and trypsin inhibitors prevent the enzyme from functioning properly. It is fairly easy to eliminate trypsin inhibitors by heating soy flakes or flour before extracting their protein. It is a different story in the case of phytic acid, which binds to the protein and is not destroyed by heat. The presence of phytic acid in the isolate can reduce calcium absorption in the intestines by up to 90% and interferes with protein assimilation. In adults with a varied diet, this is not a problem, but the situation can be different for nursing infants, children and adolescents who are still growing, as well as for the elderly, who have higher calcium and protein requirements. That said, most soy protein isolates currently on the market are high in phytic acid.
Electricity and filtration to the rescue!
How can the phytic acid content of isolates be reduced? Basically, it's a matter of releasing the phytic acid by lowering the pH of the soy extract and then filtering the acid out. Electrodialysis with bipolar membranes is used to acidify the environment and release the phytic acid, and is followed by ultrafiltration (Figure 1).
Electrodialysis with bipolar membranes electrically splits the water molecules, generating H+ and OH- ions and directing them to two compartments separated by a membrane: the H+ ions on one side, and the OH- ions on the other. The addition of H+ ions acidifies the environment in the compartment and lowers the pH from 9 to 6, and it is in this compartment that the soy extract is placed. The beauty of this approach is that it is the water present in the soy protein extract that is used to generate the H+ and OH- ions. This minimizes the volume of water required and hence the volume of effluent generated by the process. The ultrafiltration step works something like a colander to separate the soy proteins from the undesirable molecules based on their size differences. Most of the proteins are retained by the membrane, while most of the sugars, minerals and phytic acid pass through and are eliminated. Ultrafiltration has proven to be most effective at a pH of 6 because links between phytic acid, calcium and proteins are weak at this pH.
In addition to being easy to digest, most proteins obtained by this process (Figure 2) are soluble in an acidic environment (pH 2 to 3.5), which is another significant advantage in the preparation of such beverages as fruit juice-based protein drinks.
Report a problem on this page
- Date modified: