Interactions of γ-aminobutyric acid and whey proteins/caseins during fortified milk production

The potential interactions between milk proteins (whey proteins and caseins) and gamma-aminobutyric acid (GABA), alone and in combination, and the physiochemical properties of GABA-fortified milk were investigated in the present study. After mixing 0.05-1.0% (w/w, based on the solution) GABA with 0.6% (w/v) whey proteins and with 2.6% (w/v) caseins, sequential preheating (60 degrees C), homogenization (20 MPa), and pasteurization (72 degrees C for 15 s and 138 degrees C for 2 s) processes were used to simulate practical processing conditions for GABA-fortified milk production. GABA mainly cross-linked beta-lactoglobulin (but had no remarkable cross-linking effect on caseins), adducted with alpha-lactalbumin and alpha(s1)-casein fractions (not excluding other fractions in whey proteins/caseins) via the amide linkage under the sequential processing conditions, which was confirmed by electrophoresis, mass spectroscopy, and amino acid composition analysis. Characterization of the physiochemical properties (protein solubility, relative surface hydrophobicity, zeta-potential, particle size, and fluorescence intensity) of the GABA-fortified protein solutions substantiated the presence of the cross-links and adduction reactions. In addition, GABA tended to form its dimer, trimer (line-like structure) and 3-8 membered ring structure oligomers. The interaction between GABA and whey proteins/caseins provides insight into the evaluation of the food quality and chemical safety (due to protein/GABA derivatives) of GABA-fortified milk.