Enzymatic Modification of Soy Proteins in an Ultrafiltration - Enzyme Reactor System

134 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.Enzymatic modification offers a safe, mild, and efficient means of improving functional properties of proteins. Current methods for producing enzyme hydrolyzates have several problems, among them (a) the high cost of the enzyme which can be used only once in the traditional batch process, (b) the added cost of inactivating the enzyme either by changing pH or heat treatment at the end of the reaction, (c) they are energy and labor intensive, (d) because of their discontinuous nature, use equipment very inefficiently, and (e) the extent of reaction and thus the uniformity of product is difficult to control, which can lead to off-flavors and bitterness in batch hydrolyzates.An alternative method of enzyme modification was investigated here to overcome some of these problems. An ultrafiltration-enzyme reactor system was designed for the continuous hydrolysis of proteins. A continuous stirred-tanked reactor (CSTR) was coupled to a hollow fiber ultrafiltration module in a semi-closed loop configuration. The system was designed such that the bulk of the reaction occurred in the CSTR, which was kept physically separate from the separation unit. This enabled each of the two major components--the CSTR and the ultrafiltration unit--to be characterized separately and then merged together and optimized.Residence time distribution studies showed that the overall UF Reactor system could be modeled on the basis of an ideal CSTR. The effect of performance variables such as enzyme concentration, substrate concentration, flux (flow rate), and reactor volume on substrate conversion and reactor productivity were evaluated. These variables were combined into a single term called space time and correlated with kinetic parameters, based on the Michaelis-Menten model for enzyme reactions. This kinetic model fit the experimental data well, except at high conversions where it overpredicted conversion slightly. A statistical model based on curvilinear regression analysis was also developed.Long-term operational stability of the reactor was affected by thermal inactivation of enzyme, enzyme leakage, and product and/or unconverted substrate build up in the CSTR. Membrane adsorption and the high shear rates in the hollow fibers had little or no effect. Reactor decay could easily be compensated for by adjusting space time variables. The productivity of the continuous ultrafiltration reactor system was several times higher than the traditional batch process, and yields were typically 90% or greater.The hydrolyzate product from a reactor using soy protein isolate as feed, a 10,000 molecular weight cut-off membrane and Pronase('(REGTM)) as the enzyme contained 90% protein nitrogen (N x 6.25), dry basis, and about 8% ash. The major proportion of peptides in the hydrolyzate were 2,400 molecular weight or less. They hydrolyzate was easily dispersible and completely soluble over the entire pH range from 2-9. It was clear in solution (at 1% w/v) at all pH values and did not possess the bitterness usually attributed to extensively hydrolyzed proteins. It also showed significantly greater water adsorption ability as compared to the soy protein isolate. Foam stability and emulsification properties were poor. Molecular weight distribution of product shifted slightly during long-term reactor operation due to changes in reaction kinetics and conversion. These changes, although slight, were shown to sometimes markedly affect functional properties.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD