Whey protein isolate gel for separation: A formation, characterization, and application study

Novel microporous membranes made of whey protein isolate (WPI) were developed. Aggregates of WPI comprised the bulk of the membrane, the size and packing density of which were varied by changing CaCl₂ concentration (0.05–0.3M) and WPI concentration (30–40wt%), respectively. Aggregate sizes of the membranes made with 0.3M, 0.1M, 0.05M CaCl₂ were roughly 1.5μm, 1μm, and 0.8μm, respectively. Skin layer of thickness about 0.5μm was found on either side of the membrane, but the thickness could reach 5μm at 0.3M CaCl₂. Additionally, the porosity of the skin layer was shown to be modifiable with the addition of surfactant. Membranes were stable in hexane with flux values on the order of 1–1000gal/ft 2·d depending on the morphology of the membrane. The molecular weight cutoffs (MWCOs) of the WPI membranes with skins were evaluated using two different methods: (i) dextran marker method and (ii) protein/vitamin marker method. Membranes were found to have MWCOs of 1,000 or greater with variations when the concentration of salt used to control aggregate size, or surfactant used to modify skin properties were selected.; The microporous WPI gel was also used as a cation exchanger and a hydrophobic adsorbent. The WPI cation exchanger has a maximum capacity of 68mg cupric chloride per gram dry WPI gel at neutral pH and can be regenerated effectively by reducing the pH of the solution. The WPI gel has also been found to be an excellent adsorbent for total phenolic compounds from grape extract with a partition coefficient higher than 1000 in aqueous system. The mechanism for total phenolic compounds adsorption is believed to be physical sorption, particularly sorption/condensation of total phenolic compounds in the pores and on all surfaces of WPI gel. The gel has a low extractables of 1ng/ml.g gel, and has an isoelectric point of 5.5. Although WPI gel was made into a monolith for continuous bed chromatography, channeling problems have made it very hard to evaluate the performance of WPI monolith for continuous bed chromatography. Nonetheless the WPI monolith may be used to separate different protein molecules based on protein-protein interaction between WPI and protein molecules to be separated.