Production of Liquid Milk Protein Concentrate with Antioxidant Capacity, Angiotensin Converting Enzyme Inhibitory Activity, Antibacterial Activity, and Hypoallergenic Property by Membrane Filtration and Enzymatic Modification of Proteins

Liquid milk protein concentrate with different beneficial values was prepared by membrane filtration and enzymatic modification of proteins in a sequential way. In the first step, milk protein concentrate was produced from ultra-heat-treated skimmed milk by removing milk serum as permeate. A tubular ceramic-made membrane with filtration area 5 x 10(-3)m(2)and pore size 5 nm, placed in a cross-flow membrane house, was adopted. Superior operational strategy in filtration process was herein: trans-membrane pressure 3 bar, retention flow rate 100 L.h(-1), and implementation of a static turbulence promoter within the tubular membrane. Milk with concentrated proteins from retentate side was treated with the different concentrations of trypsin, ranging from 0.008-0.064 g.L(-1)in individual batch-mode operations at temperature 40 degrees C for 10 min. Subsequently, inactivation of trypsin in reaction was done at a temperature of 70 degrees C for 30 min of incubation. Antioxidant capacity in enzyme-treated liquid milk protein concentrate was measured with the Ferric reducing ability of plasma assay. The reduction of angiotensin converting enzyme activity by enzyme-treated liquid milk protein concentrate was measured with substrate (Abz-FRK(Dnp)-P) and recombinant angiotensin converting enzyme. The antibacterial activity of enzyme-treated liquid milk protein concentrate towardsBacillus cereusandStaphylococcus aureuswas tested. Antioxidant capacity, anti-angiotensin converting enzyme activity, and antibacterial activity were increased with the increase of trypsin concentration in proteolytic reaction. Immune-reactive proteins in enzyme-treated liquid milk protein concentrate were identified with clinically proved milk positive pooled human serum and peroxidase-labelled anti-human Immunoglobulin E. The reduction of allergenicity in milk protein concentrate was enzyme dose-dependent.