Lipolysis of milk lipids by Mucor miehei enzymatic preparation: what are the kinetics levers?
Résumé
Prelipolized milk lipids are interesting flavouring ingredients and Mucor miehei
enzymatic preparation is frequently used as lipolyzing agent. This study aimed at
determining whether technological treatments such as high pressure homogenization
and heat treatment of milk emulsion might affect their susceptibility to lipolysis. Indeed,
such treatments modify quantitatively (increase in lipid specific surface) but also
qualitatively (protein interfacial composition and conformation) the emulsion interface. If
it is well known that an increase in lipid specific surface induces an increase in the
kinetics of lipolysis of the milk emulsion, the effect of qualitative variations of interfacial
composition (load of amphiphiles such as casein micelles or whey proteins, state of
aggregation of these amphiphiles) on the kinetics of lipolysis remains to be elucidated.
To reach this aim, 5% (wt/wt) anhydrous milk fat oil-in-water model emulsions stabilized with 3.5% (wt/wt) milk proteins were elaborated. 3 types of milk protein blends were selected. They differed by their native phophocaseinate (PPCN)/whey protein (WPI) weight ratios: 0:100, 80:20, 100:0. The PPCN/WPI 80:20 wt/wt stabilized emulsions were subjected to a heat treatment (72°C, 2 min) and compared with a similarly
standardized (5 % fat, 3.5 % protein) milk emulsion. The model and milk emulsions all presented the same lipid specific surface and their granulometric stability was monitored. The emulsions were hydrolyzed by a Mucor miehei lipase preparation (30°C,pH 7.0) and the evolution of lipid hydrolysis was measured by pH-stat. Our results indicated that: i) the WPI stabilized emulsions presented a longer initial lag phase as compared to other model emulsions (PPCN, unheated PPCN/WPI 80:20 or heated PPCN/WPI 80:20) or milk emulsion; ii) after this initial lag phase, the rates of lipid hydrolysis did not differ greatly between PPCN and WPI-stabilized emulsions, but it did differ between heated emulsions as compared to unheated emulsions. This study
showed that the level of denaturation of proteins at the interface of milk emulsions is a predominant parameter to control their lipolysis as compared to the nature of the protein present at the interface. And yet this parameter is a less effective lever than the quantity of interface available for lipase adsorption.