2.3 Membrane ultrafiltration extraction
Among other new and unconventional processes, the purification of
proteins using ultrafiltration membranes is an attractive alternative to
the traditional isoelectric precipitation (NICHOLS D J, 1981,). Membrane
ultrafiltration systems were first used in the early to mid-1970s for
the separation of soy protein (Lawhon, 1978a). Lawhon et al. (Lawhon,
1978b) used a discontinuous percolation or re-ultrafiltration process to
produce a soybean product with a protein content of approximately 90%
(dry basis), while Olsen (S., 1978) concentrated defatted soybean
extract from 5.6% to 25% of total solids by direct ultrafiltration to
produce a soybean product with a protein content of 88% (dry basis).
Ultrafiltration not only separates proteins from salt and sugar but also
each other (M., 1992). The partial hydrolysis of SPI produces proteins
with different molecular weight sizes (Zhang Y, 1996), which are
separated by ultrafiltration membranes of different pore sizes.
Depending on the difference in molecular size between proteins and other
components, membrane ultrafiltration selectively separates and removes
undesirable components, such as soy oligosaccharides (Endres, 2001),
from soy. In addition, most of the protein in soy is recovered without
producing a whey-like by-product.
Positively charged cations can interact with proteins (Pearson, 1983).
Proteins are strongly negatively charged at this pH and therefore do not
allow them to pass through the ultrafiltration membrane. In addition, at
this pH, the phosphorus present in the soybean in the form of phytic
acid interacts with the proteins (Garcia et al., 1997) and calcium to
form a ternary complex (Grynspan and Cheryan, 1989), preventing the
phytic acid and calcium from penetrating the ultrafiltration membrane
together. The protein-mineral interaction reduces the amount of protein
in the final product and limits its solubility after rehydration
(Grynspan and Cheryan, 1989). on top of this, the permeate flux of
protein decreases with time. This decrease is attributed to the
accumulation of feed components in the membrane pores and on the
membrane surface. When the reduction in flux is very large, membrane
permeation is not attractive for protein separation.