| Milk fat provides energy, essential fatty acid, fat-soluble vitamins and other nutrients for human body as an important part of milk. Milk fat products are applied to baking industry and catering industry increasingly because of their natural milk flavor and decent sensory properties. Milk fat is mainly composed of triglycerides (TAGs), which account for 95%-98%. The positional distribution of fatty acids in TAGs has a great influence on the digestion and absorption of milk fat and can also affect physical, chemical and processing properties of milk fat products, such as butter. Therefore, it has a great significance to analyze the positional distribution of fatty acids in TAGs in milk of different species for developing humanized infant formula and exploring the correlation between structural composition and physical and chemical properties of butter.1. We established enzymolysis method and LC-MS method analyzing positional distribution of fatty acids in TAGs in cow milk. The data obtained from enzymolysis method and LC-MS method were compared. Two methods had the same detection effect, but the latter could directly provide the composition information and content of TAGs. So we choose LC-MS method as detection method for the follow-up experiments.2. We analyzed TAGs composition in holstein cow milk, jersey cow milk, yak milk, goat milk, human milk and five commerical infant formulas (1) by LC-MS method.213 kinds of TAGs were detected in five milk of different species with the molecular weight of 498-918 and double bond number of 0-6. There were great differences between human milk and other animal milk in TAGs composition in milk fat:large molecular weight TAGs accounted for more than 95% in human milk fat, it was significantly higher than that of other animal milk. SU2 (SUU and USU) type TAGs accounted for 45% in human milk fat as the highest level TAGs, however, S3 type TAGs is the highest level in other animals milk (67%-85%). Top ten triglycerides (mol%)in human milk was OPO, OPL, POP, LaPO, SPO, PLP, OOO, LPL, OLO, OMO, which were lower in content or not detected in other animal milk.121 kinds of TAGs were detected in five infant formulas with the molecular weight of526-912 and double bond number of 0-8. The TAGs composition in the infant formulas tended to be more close to that of human milk, but there was still a certain gap. OPO, POP and 000, which had higher content in human milk, were added in the infant formulas.3. Commercial butter was fractionated by short-path molecular distillation at different distillation temperatures of 150℃,165℃,180℃,190℃,200℃,210℃,225℃,240℃. The relationship of distillation temperatures and distillation effect was analyzed, the chemical composition of different fractions was also detected. The lowest temperature that butter could be fractionated was 165 ℃, distillate yield increased with the distillation temperature increased. The distillates were enriched in short- and medium-chain fatty acids (40%-60%) and small molecular weight TAGs, while the retentates were enriched in long-chain fatty acids (65%-75%), unsaturated fatty acids (about 30%) and large molecular weight TAGs. Distillation temperatures had a significant effect on the composition of fatty acids and TAGs of butter fractions.4. Acid value, iodine value, peroxide value of butter fractions were measured and the correlation between physical and chemical properties such as slipping melting point, DSC thermograms of melting and crystallization and polarized light micrographs and structural composition of TAGs in fractions. Short-path molecular distillation did not cause breaking down fatty acids from the glycerol skeleton of butter and also did not cause fat oxidation. Relative to milk fat, the slipping melting point of the distillate was depressed and that of the retentate was augmented, while both of them showed a increased trend as the distillation temperature increased. There was a significant positive correlation (P<0.01) between the concentration of long-chain fatty acids in the fractions and their slipping melting point, r=0.977. DSC results showed that the melting and crystallization peaks of fractions were offset to the high temperature area as the increase of distillation temperature which indicated that the melting and crystallization properties of fractions were different but changed regularly. Polarized light micrographs showed that the distillate and retentate had different fat crystals, the crystals formed on cooling at 5 ℃ for 24h corresponded to spherulites for the retentate whereas the solid fat phase of the distillate corresponded to needle-shape crystals. slipping melting point, DSC thermograms of melting and crystallization and polarized light micrographs of butter fractions were related to the content of large molecular weight TAGs in butter. |
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