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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).

Figure 1: A. Bipolar membranes electrodialysis - CM: Cationic membrane, BPM: Bipolar membrane. B. Ultrafiltration
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Description - Figure 1

The first phase of the diagram depicts a bipolar membrane electrodialysis cell pair. A cell pair is composed of a bipolar membrane (BPM) and two cationic membranes (CM). In an electrodialysis stack, it is typical to have up to 200 cell pairs stacked between the electrodes. Upon the application of a direct electrical potential, the bipolar membranes allow the electro-dissociation of water molecules into proton (H+) and hydroxyl (OH-) ions. The soy protein extract to be acidified is fed into the acid compartment receiving the H+ generated by the bipolar membranes, which come into contact with the proteins stream, decreasing their pH. At the same time, cations present in the proteins stream, permeate through the cationic membranes and form the corresponding bases with the OH- generated by the bipolar membranes.

The second phase depicts an ultrafiltration membrane. Ultrafiltration consists in the application of a transmembrane pressure to force a solution through a semipermeable membrane with a pore size ranging from 1 to 100 nm. Water and solutes in the feed stream that are below the nominal molecular weight cut-off of the membrane are allowed to permeate through the membrane pores into a permeate stream, while the larger feed components are retained by the membrane. For the soy protein extract purification, most of the proteins are retained by the membrane, while a significant fraction of the other components such as carbohydrates, minerals and phytic acid will permeate the ultrafiltration membrane.

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.

Figure 2: A. Soyflakes prior to proteins extraction. B. Low phytic acid soy protein isolate.
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