Zinc-Induced Precipitation of Milk Fat Globule Membranes: A Simple Method for the Preparation of Fat-Free Whey Protein Isolate

摘要

A simple method to isolate milk fat globule membrane (MFGM) from cheese whey has been developed. The method was based on the premise that divalent cations should be able to form insoluble complexes with MFGM under certain solution conditions; this ability, however, is dependent on their coordination chemistry. Incubation of cheese whey with 0-50 mm CaCl2 or MgCl2 at 30 °C did not cause precipitation of MFGM, whereas incubation with Zn(Ac)2 under similar conditions induced selective precipitation of MFGM in a concentration-dependent manner at pH 5.2, with complete precipitation occurring above 20 mm Zn(Ac)2. The whey proteins remained soluble in the supernatant under these conditions. The ability or inability of a cation to induce precipitation of MFGM is related to the ionic radius as well as its coordination geometry. Calcium and magnesium ions have a strong tendency to form hexa-coordinated (n = 6) complexes in a regular octahedral geometry, whereas zinc prefers to form a tetra-coordinated complex in a tetrahedral geometry with MFGM phosphate groups. It is proposed that the tetrahedral geometry of zinc coordination in the zinc-MFGM complex permits hydrophobic interaction between MFGM particles, resulting in precipitation at 30 °C. Further processing of the supernatant using membrane ultrafiltration/diafiltration resulted in a fat-free whey protein isolate.