2.3.2 Application of ultrafiltration membrane method
Omasaiye et al. (Omosaiye et al., 1978)prepared a full-fat SPC by continuous filtration from an aqueous soybean extract using an ultrafiltration membrane method. It was found that this Diafiltration method was effective in removing oligosaccharides from the full-fat soybean extract. Shallo et al. (Shallo et al., 2001) enzymatically digested defatted soybean meal with commercial pectinase and diafiltered it using a porous stainless steel ultrafiltration membrane system. They obtained a soybean concentrate with a protein content of 78.5%, which had a reduced level of phytic acid. This process produced a soy concentrate with a protein recovery of 17% to 26% higher than current commercial processes. Kim et al. (Kim and Kim, 2015) used 100 kDa molecular weight ultrafiltration membranes to extract soybean protein concentrate from whole and defatted soybean meal. The protein content of full-fat soybean protein concentrate and defatted soybean protein concentrate was 68.6% and 80.0%, respectively. It was found that the membrane-treated SPC was lighter in weight and more yellow than the acid precipitated protein concentrate. The solubility, emulsification and stability, and foaming ability of the membrane-treated soybean protein concentrate were significantly higher than that of the acid precipitated protein concentrate. The amino acid profiles of the membrane treated and acid precipitated soy bean protein concentrates were comparable. This suggests that ultrafiltration membrane treatment can be used as a gentle and chemical-free process for soy protein extraction. It was demonstrated that the use of an 80 kDa ultrafiltration membrane resulted in an improved separation efficiency, yielding a SPI with a protein content of 90.0%, and also reduced membrane fouling. Furthermore, the total isoflavones in the soy protein product were reduced to 70.0 mg/kg. Due to its better solubility and lower content of anti-nutritional factors, the SPI product obtained by this process has potential applications in infant formulae (Yang et al., 2014). The content of anti-nutrients, phytic acid, in the soy protein was reduced by a series of ultrafiltration and percolation steps. After extraction, the soy protein was purified by sequential ultrafiltration and Diafiltration without pH adjustment or by adjusting the pH to 6.5. This purification method showed the lowest phosphorus to protein ratio (4.4 ± 0.3 mg P/g protein) and reduced membrane contamination compared to aqueous extraction conditions. This study demonstrates the feasibility of ultrafiltration membrane technology for the production of SPI with low phytic acid content. Studies have shown that phosphorus removal can be improved by combining bipolar membrane electrodialysis with ultrafiltration compared to using ultrafiltration membranes alone. This extraction method retains the whey-like proteins lost during conventional isoelectric precipitation. The ultrafiltration membrane extraction results in improved solubility of the isolate in the pH range of 2.0 to 4.5 and lower phytic acid content. Since the pH of liquid foods is around 3.5, this isolate has the potential for use in juice drinks (Ali et al., 2010). Sharapova and Moresoli (Mondor et al., 2010) compared the differences in infiltration time and final product composition between electro-acidified (pH 6) and non-electro-acidified (pH 9) soy proteins using high shear tangential flow hollow fiber ultrafiltration membranes with a cut-off molecular weight of 100 kDa. They observed higher removal of calcium, magnesium, and phytic acid during filtration of electro-acidified proteins compared to non-electro-acidified proteins. pH adjustment from 9 to 6 not only reduced the permeate flux of the ultrafiltration membranes but also resulted in more severe membrane contamination and longer filtration times. It was found that discontinuous filtration increased the removal of carbohydrates and minerals, resulting in a higher protein content product, but did not improve the permeate flux of electroacidified proteins. Wu et al. (Wu et al., 1998) modified soybean isolates with protein hydrolases and then ultrafiltered them to separate these proteins into peptides of controlled molecular size. The hydrolysates were ultrafiltered using stirred cell and disc membranes (100, 50, and 20 kDa molecular weight cut-offs) and further fractionated into one retention (R100) and three permeates (P100, P50 and P20). The results showed that the soy protein peptides prepared from soy isolates modified by papain and ultrafiltrated had a lower molecular weight, higher solubility, and emulsification. Due to these properties, they have the potential for application in the cosmetic and health food industries. Goodnight et al. have patented a process for ultrafiltration membranes that produce soy protein with improved digestibility, low phytic acid content, improved functional properties, high water solubility, and absence of soybean flavor and improved palatability. Phytic acid is removed by extracting defatted soybeans and separating insoluble material with a pH above 10.1. SPC is recovered by ultrafiltration to obtain fractions with antioxidant activity in different media. The low molecular weight fractions were the most active and were free radical scavengers. Protein hydrolysis increased the antioxidant activity of the >30 kDa fractions, although the heat treatment following protein hydrolysis may lead to protein aggregation, which has an impact on free radical scavenging capacity (Wieser, 2007).