| Pasteurized milk is favored by consumers at home and abroad because of its mild pasteurization process,which can kill most pathogenic bacteria in raw milk and keep the main nutrients and flavor of fresh milk to the maximum.However,due to the inactivation of all microorganisms and enzymes in raw milk,some of the remaining heat-resistant bacteria and heat-resistant enzymes will remain in the final product,resulting in a short shelf life of pasteurized milk,which can not be stored and sold at room temperature,seriously restricting the production,sale and consumption of pasteurized milk.Studies have shown that sterilization conditions are closely related to the residual heat-resistant microorganisms and enzymes.In this study,the raw milk obtained on the day of normal process in large domestic dairy enterprises was used as control.The pasteurization was carried out at 62?,65?,68? for 20 minutes,25 minutes,30 minutes and 35 minutes,72?,75?,80?,83? and 85? for 15 seconds,20 seconds and 30 seconds respectively,and at 93?,95? and 97? for 15 seconds,30 seconds,60 seconds,2 minutes and 5 minutes respectively.The obtained dairy products were analyzed by measuring its protein,fat,non-fat milk solids(SNF),lactose index,physical and chemical properties by measuring acidity,pH value and color difference,flavor quality of pasteurized milk samples under different sterilization conditions was analyzed by electronic tongue and flavor substance technology,and then amino acid automatic analyzer and GC were used.-The free amino acids and fatty acids of pasteurized milk were analyzed and determined by MS and other techniques.Finally,the community formation rules of residual heat-resistant microorganisms in pasteurized milk samples under different sterilization conditions and the mechanism of their effects on dairy flavor quality were analyzed by using macrogenomics techniques.The results were as follows.(1)Firstly,the nutritional components and physicochemical properties of milk under different heat treatment conditions were studied.It was found that the changes of fat were more obvious under the sterilization condition of 68?,and the acidity and astringency were significantly higher than those of raw milk under the sterilization condition of low temperature.(2)Secondly,the components of flavor substances in milk under different heat treatment conditions were analyzed.There were 19 flavor substances in raw milk,11 substances disappeared after heating,and 8 substances changed in content under the influence of temperature and time conditions.In the gradients of low,medium and high temperature,42,66 and 101 new substances were produced respectively.A total of 8 flavor substances were detected,including 15 alcohols,19 acids,24 esters,9 ketones,8 aldehydes,22 alkanes,4 phenols and other flavor substances.Among them,alkanes are susceptible to the influence of low temperature heat treatment conditions,and the number of alkanes varies most obviously.Under the condition of pasteurization at 72?,the only cyclopropionic acid in raw milk disappeared when heated,and six ketones were added,and the trend of change was affected by temperature.For pasteurized milk treated at high temperature,the acid and ester substances changed obviously with the increase of temperature,but the quantity fluctuated with the prolongation of sterilization time.(3)The effects of different heat treatments on the contents of amino acids and fatty acids in milk were investigated.A total of 16 amino acids were detected,8 of which were essential for human body.Under three pasteurization treatments,glutamic acid(Glu)was the most abundant in milk,followed by glycine(Gly).Under high temperature,the content of polyunsaturated fatty acids in milk decreased with the increase of temperature,while the content of short and medium chain fatty acids increased.The ratio of three fatty acids is close to 1:1:1 when heated at 85? for 15 seconds,which is the ideal proportion model of fatty acids.(4)Microorganisms in milk under different pasteurization conditions were studied.It was found that Pseudomonas-brenneri and Pseudomonas were the dominant bacteria in raw milk.There were 24 bacterial genera in three groups of pasteurization conditions.Streptococcus accounted for a large proportion of bacteria when they were sterilized for 15 and 30 seconds at 93?.Pseudomonas was the dominant bacteria in the other high temperature pasteurization treatments.When pasteurized at low temperature for a long time,it was found that the number of bacteria decreased compared with raw milk.The species of Prototheca-zopfil in pasteurized milk treated at 72?,75? and 80? decreased with the prolongation of pasteurization time.With the increase of sterilization temperature,the proportion of Streptococcus-uberis strains in the pasteurized milk under this treatment condition increases obviously.At the same time,the proportion of Streptococcus-agalactiae and Streptococcus mammarius in 6 samples increased when pasteurized milk was treated at 93?,95? and 97?for 15 and 30 seconds.It can be concluded that these two strains may belong to Streptococcus thermophilus and remain in pasteurized milk at high temperature and at high temperature.Shorter processing time will lead to species residue,while longer treatment will kill them,which provides a good reference value for pasteurization production and processing. |
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