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.