| Goat milk,which is rich in nutritional value, is delicate and digestible. It is an excellently completeprotein food. Due to the dairy goat rearing’s small scale and low production, fresh goat milk cannot bequickly used for processing after milking. It needs one day to several days of cold storage andtransportation before the goat milk can be used for the dairy producation’s processing. The flavor ofgoat milk mainly depends on the composition and content of fat (and free fatty acids). The freshness ofgoat milk has a direct effect on the flavor and the quality of dairy products.Therefore, it has a greatsignificance to analysis the mechanisms and rules of goaty flavor and quality changes during goatmilk’s refrigeration. It can help to discuss the way to control the deterioration of goaty flavor and theadverse effects it had on the refrigerated goat milk.In the paper, we seperated, purified and identified psychropHile from goat milk firstly. In order toknow how did the microbial lipase affect the goat milk, experiments were conducted to find out theexogenous lipases’ production rules, heat resistance and enzymatic properties.Secondly, different goatmilk (Guanzhong colosrtum, Guanzhong milk, Sannen milk) and Holstein cow milk were chosen as thestudy objects. The activities of lipases (lipoprotein lipase, microbial lipase), the contents of fatty acids(TFA, FFA) and the capric flavor were tested. Through the analysis we can know the difference of milkflavor, the major lipase in goat milk and the relationship between the FFAs and the goaty flavor. Finally,through the refrigeration experiment on Guanzhong goat milk,we can get the variation rules of lipaseactivity, FFA cotents and goaty flavor. The main results are as follows:(1) The main Psychrophile in milk were Pseudomonas, Arthrobacter, Flavobacterium, Bacillus,Acinetobacter, Exiguobacterium, Paenibacillus, Enterococcus, Enterobacter, most of them wereGram-negative bacteria. The optimum pH of lipase produced by Pseudomonas were9.5, and theoptimum temperature were50℃. After100℃,5minutes heat treatment, the Pseudomonas lipaseresidual was80.43%.(2) The differences of lipoprotein lipase activity in milk are as follows: There was no significantdifference between goat milk of different species(Guanzhong, Sannen); However, it was significantlydifferent between goat milk of different stages of lactation (colostrums, normal milk), the activity in theformer milk was about1.73times of the activity in the latter milk;The LPL activity in Holstein milk wassignificantly higher than that in Guanzhong normal milk, which was about1.58times of the latter; Theratio of LPL activity between skim milk phrase and fat phase in goat milk (Guanzhongcolostrums,Guanzhong normal milk, Sannen normal milk) was1:1. However, the ratio was2.24:1inHolstein cow milk.(3) The difference of different milk’s flavor are as follows: the flavor of Holstein cow milk andGuanzhong normal were significantly different, while it was not significantly different betweendifferent species’ goat milk (Guanzhong, Sannen); There is a big significance between Guanzhongcolosturm and Guanzhong normal milk.(4)The contents of short-medium chain free fatty acids had a great revelance with the goaty flavor among different milk. The difference between goat milk and cattle milk was related to C6:0～C14:0free fatty acids; There was a great revelation between the goaty flavor (Guanzhong milk and Sannenmilk) and the C8:0free fatty acid; Different lactation stages Guanzhong goat milk’s flavor differencewas related to the C6:0～C14:0free fatty acids.(5) The major lipase was lipoprotein lipase during the Guanzhong goat milk’s refrigeration.TheLPL activity in goat skim milk was decreasing from0d to9d. However, the LPL activity in fatty phasegradually increased during the same time. The lipoprotein lipase was rising again with the goat milk’sgoing bad after9d.(6) During the refrigeration, the FFAs made the largest contribution to the goaty flavor changes,especially capric acid, decanoic acid and caproic acid. With the refrigerating, the content of capric acid,decanoic acid and caproic acid increased, which would caused a heavy goaty flavor. |
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