Studies on the susceptibility of membrane-derived proteins to proteolysis as related to changes in their emulsifying properties

Abstract

Milk fat globule membrane (MFGM) isolates obtained from fresh unheated cream and industrial buttermilk were hydrolyzed using trypsin and chymotrypsin. The protein composition of these hydrolyzed samples was determined by gel electrophoresis. MFGM proteins obtained from industrial buttermilk were fully hydrolyzed after a short incubation time. In contrast, the MFGM isolates from unheated cream did not undergo complete hydrolysis, and intact proteins were still observed after 3 h of incubation. Thus, heat treatment of the cream caused profound changes in the composition and structure of the MFGM, so that, after the heating it became more susceptible to hydrolysis by trypsin and chymotrypsin. When proteolysis was carried out on emulsions prepared with MFGM isolates, neither the rate of hydrolysis nor the composition of the residual MFGM protein changed from that of MFGM isolates treated with enzymes in solution. The emulsifying properties of the MFGM isolates improved with proteolysis, as estimated by the size distribution of the emulsion droplets. A similar improvement in the droplet size distribution of emulsions was also observed when the hydrolysis was carried out after homogenization. In this case, the large particles formed during homogenization disappeared after proteolysis, suggesting that MFGM isolates, when present in less than saturating amounts, caused bridging flocculation of the emulsion. The improvement in the emulsifying properties resulting from proteolysis was substantial for MFGM isolates from unheated cream, and even greater for isolates derived from industrial buttermilk.