The Coalescence Characteristics And The Mechnism Of Milk Fat Globules

Coalescence of fat globules is the most common quality defect of milk.It seriously affects the sensory property and quality of dairy products,and remains a serious headache of dairy industries.During processing,several tremens such as heating and curding would change the colloidal stability of the emulsion system,which affect the coalescence of fat globules.Previous researches have speculated that the lipids and proteins of membrane may be related to the breaking of membrane,which could contribute to the coalescence of the globules.However,the fundamental understanding of the composition and spatial arrangement consulting the milk fat globule and their effect on the coalescence of globules remains scarce.In the present study,we take fat globules with different sizes as the study object.By studying the coalescence state of fat globules in different processing system of milk as well as analyzing the molecular structural characterization of fat globules,we try to explore the mechanisms of the coalescence of fat globules.The study of the composition-structure-property relationships of milk fat globule membrane and the coalescence of fat globules would reveal the stability mechanism of fat globules in milk emulsion system at molecular level,which could be of tremendous importance to inhibit the creaming of fat globules and improve the quality of dairy products.The coalescence of fat globules in the dairy processing system was studied.The processing treatments induced the increase of the size and zeta-potential of fat globule as well as the free fatty acid of milk(P<0.05).The effect of stirring was rather weak and only induced the flocculation and partial coalescence of a small number of fat globules.Cooling induced the partial coalescence of fat globules.Heating induced the large areas of flocculation of fat globules.Acidification contributed to the coalescences of fat globules and casein micelles.Brining induced the fat globules to coalescence into a bigger globule or to be irregular shape.The coalescence degree of fat globules increased with the increasing size of fat globules.The coalescence of fat globules in the rennet coagulation system was studied.By using the two-stage gravity separation combined with centrifugation scheme,we separated the globules into two fractions with the average diameters of 1.907 ?m and 4.277 ?m,respectively.The average diameters of globules in whey of curd made of milk enriched smaller globules was 3.737 ?m,while the value was 198.8?m of the larger fat globules.This result indicated a obvious coalescence of fat globules happened during renneting,which was more significant with the increase of globule sizes(P<0.05).By using the confocal laser scanning microscopy,we found the gel network of curd made of small fat globules fraction was more homogeneous.The coalescence and creaming of larger globules were more significant,which broke the network of curd and turned into a less homogeneous curd.The release speed of caseinomacropeptide and the accordingly storage modulus of the curd decreased with the increase of globule sizes.The time when the storage modulus reached 1 Pa significantly increased with the increase of globule sizes(P<0.05).The molecular structural characterization of fat globules was analyzed.The proportion of ?crystals of fat globules increased with the increase of globule sizes.Using cryo scanning electron microscopy,casein micelles with diameters of 40?-60nm were found adsorbed onto the native milk fat globule membrane(MFGM).The surface roughness of MFGM and the surface protein coverage of milk fat globule significantly increased with the increase of globule sizes(P<0.05).The diameters of the adsorbed casein micelles decreased with the increase of globule sizes.The main polar lipids found in the fat globules with the diameter of 4.277 ?m were choline-containing polar lipids,including PC(24.70%)and SM(28.60%),whereas the major polar lipid found in the globules with the diameter 1.907 ?m was PE(23.50%).The surface cholesterol content of globules significantly increased with the increase of globule sizes(P<0.05).The area proportions of the lipid rafts in the MFGM were quantified to be 28.8,37.3 and 54.6%for fat globules of 1.3,4.4 and 6.5?m in diameter,respectively.The Young's modules of MFGM significantly increased with the increase of globule sizes(P<0.05),showing a stiffest MFGM(472 kPa)of globule with diameter of 8 ?m.The fragility of MFGM increased with globules sizes.Taken together,these results suggest that the solid fraction of globules,the adsorbed casein micelles in MFGM and the area of lipid rafts of MFGM along with its impact on the nano-mechanical properties are the key factors which determine the coalescence of globules.