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).