Study On Fatty Acid Profile Of Raw Milk And The Effects Of Processing

Milk fat and fatty acid(FA)composition are important indexes to evaluate nutritional value and functional quality of milk.In order to improve the nutritional quality of milk,scientists have done a lot of research on animal nutrition and genetic modification.To scientifically evaluate dietary regulation and nutritional quality of milk,it is necessary to fully understand milk FA composition.However,due to the limitations of the determination method,it is difficult to deal with the diversity and complexity of FA in milk fat,which restricts the systematic study of milk FA composition.This study aims to establish a sensitive,accurate,and stable high-throughput determination method.The FA composition of different animals’ raw milk has been studied comprehensively and systematically,the FA fingerprints has been further established and their composition characteristics has been analyzed.The effect of degreasing and heat treating on milk FA composition has also been discussed.Trial Ⅰ: A high-throughput method for the determination of milk FA.In this study,the chromatographic separation of FA was improved by optimizing the gas chromatography(GC)conditions,and the detection sensitivity was improved by increasing the dwell time of the characteristic ions using GC-MS selected ion monitoring mode.The standard curve for each FA was established and a wide linear range was set to increase the applicability.The proposed method was then validated.The results showed that 69 FA were simultaneously quantified by this method,and which had LOQs ranged from 0.3 μg/L ~14.3 μg/L,recovery rates of 76.6% ~ 109.3%,and variable coefficient of 0.5% ~ 11.7%.Therefore,the proposed method has good accuracy and precision,and has advantages over existing detection methods in high-sensitivity,high-throughput,and wide linear ranges.Trial Ⅱ: FA composition of animal raw milk and their fingerprints.In this study,FA profiles of raw milk samples from Holstein cow,Jersey cow,buffalo,yak,human,goat,donkey,and camel,were determined by the proposed GC-MS method,followed by a comprehensive analysis and comparison.The results showed that the FA profile of milk from Holstein cow,Jersey cow,goat,and buffalo were similar,and that from yak and camel were similar,but human and donkey milk were quite distinct from these ruminant milks.Holstein cow,Jersey cow,goat,and buffalo milks contained high content of even-chain saturated fatty acid(ECSFA,57.3%~58.8%),while goat milk had higher content of medium-and short-chain fatty acid(MSCFA).The major FA included C4:0,C6:0,C8:0,C10:0,C12:0,C14:0,C15:0,C16:0,C18:0,C14:1 C9,C16:1 C9,C18:1,C18:2 c9c12(linoleic acid,LA)and C18:2 c9t11(conjugated linoleic acid n7,CLA n7),etc.Yak and camel milk are potential functional foods due to their high levels of odd-and branched-chain fatty acid(OBCFA)and low ratio of n6/n3 polyunsaturated fatty acid(PUFA).Camel milk also contained high content of C16:1 c9,while its MSCFA content was very low.Donkey milk contained low levels of monounsaturated fatty acid(MUFA)and high levels of PUFA and MSCFA.Human milk contained lower levels of saturated fatty acid(SFA),including OBCFA.Human milk was rich in MUFA and PUFA,such as much higher contents of DHA and LA,than other types of milk.The ratio of n6/n3 PUFA was up to 13.75,while the content of MSCFA and CLA was low.In addition,based on characteristic FA ratios,a set of dual index discriminant analysis method is established to identify adulteration of other animal milk.And the multi-index confidence interval radar diagram was further developed for milk falsification identification.Trial Ⅲ: Effects of degreasing on FA composition of milk.In this study,the fat layer of raw milk was removed by high-speed centrifugation,and the residual FA composition was compared with raw milk data.The FA compositions of low fat milk,skim milk and skim milk powder in the market was also determined and analyzed.The results showed that there was a great difference in milk FA composition before and after degreasing,and the changes differed greatly among ruminant milk,donkey milk and human milk.In terms of cow milk,the relative content of C14-C18 FA decreased significantly after degreasing,while that of C18-C24 FA increased significantly.After degreasing,the relative content of C22:1 c13 in milk increased sharply,which can be regarded as the characteristic FA of degreasing milk.Trial Ⅳ: Effects of heat treating on FA composition of milk.In this study,raw milk sample was treated with pilot test at 72℃~120℃ for 15 s,and then determined FA composition and compared with that of raw milk.The FA composition of fresh raw milk,commercial Pasteurized(PAS)and ultra-high temperature treated(UHT)milks were also determined and analyzed.The results showed that heating could oxidate and degrade long-chain fatty acids(LCFA)in milk,especially high content of SFA such as C14:0,C16:0 and C18:0.The higher the heating intensity,the more complete degradation and the longer the carbon-chain of degraded FA.In addition,ultra high temperature leads additional oxidation,degradation,and isomerization of PUFA.After moderate heating,the contents of long-chain saturated fatty acids(LCSFA)and OBCFA decreased,while the contents of MCSFA,MUFA and PUFA increased.Furthermore,after ultrahigh temperature heating,the content of n6 PUFA decreased sharply,while the content of OBCFA and CLA increased.In this study,we have established a high-throughput method for the determination of milk FA,which provides a technical means for microscopic description in milk composition.We have also systematically compared the FA composition of 8 kinds of animal milk samples and established their fingerprints,which provides data support and reference frame for evaluating dietary regulation and nutritional quality of different animal milk,and the technical means for authenticity identification of milk.Moreover,the effect of degrease treatment on milk FA composition was studied for the first time,and the effect of heat treatment on milk FA composition was also analyzed and clarified,which provide basis for evaluating the quality of dairy products and have guiding significance for production.