MODIFICATION OF BETA-LACTOGLOBULIN BY ALIPHATIC-ALDEHYDES IN AQUEOUS-SOLUTION

Each of the secondary lipid oxidation products pentanal, hexanal and h eptanal was found to react with beta-lactoglobulin (beta-lg) in a two- phase model system (aqueous phosphate buffer-1-octanol) yielding fluor escent condensation products (emission maximum, 410 nm; excitation max imum, 350 nm). Protein polymers were detected by size-exclusion HPLC, and the rate of reaction paralleled the formation of fluorescent produ cts, with the reactivity being pentanal > hexanal > heptanal. Simultan eously, the reaction also changed the intrinsic fluorescence of beta-l g, and in particular pentanal reduced the intensity of tryptophan fluo rescence (emission maximum, 332 nm; excitation maximum, 288 nm) by 30% . These findings are discussed with reference to the effect of peroxid izing lipids on the physical properties of whey proteins and the use o f protein fluorescence (induced by the reaction with aldehydes) as mar ker for the oxidative status of milk and whey protein products.