| The balanced of China's liquid milk market was hindered by pasteurized milk,due to the short shelf life of them.Extending the shelf life of pasteurized milk is of great significance for improving the regional consumption imbalance of liquid milk in our country.However,the available sterilization process in dairy industry for producing extended shelf-life(ESL)milk would damage the bioactive protein in the milk and seriously reduce the nutritive value of the milk.Therefore,it is vital to extend the shelf life of pasteurized milk while retaining the bioactive protein to the maximum by optimizing the sterilization methods,and this could improve the quality of liquid milk and low-temperature milk market,and promote the balance of national nutrition and health.In this study,skim milk was used and the effects of microfiltration combined with ultraviolet(UV-C)and ultrasonication on shelf life,bioactive proteins and microorganisms were researched.After that,the two non-thermal processes were compared with two commonly applied processes for ESL milk in dairy industry,and qualities during shelf life,bioactive protein and volatile flavor compounds were mainly studied.The main contents were as follows:First,the effect of microfiltration combined with different doses of UV-C on the shelf life of skim milk and bioactive proteins were studied.The results showed that more reduction of vegetative bacterial load and longer microbial shelf life of skimmed milk was found along with the higher dose of UV-C during storage of 4°C.When the UV-C dose was up to 39.3 m J/cm~2,the vegetative bacterial load of skim milk was undetectable,and the shelf life could be extended to at least 40 days.After UV-C treatments,the retention of immunoglobulin,lactoferrin(LTF),low-abundance bioactive whey proteins,and the retention activity of lactoperoxidase(LPO)and xanthine oxidase(XO)were more than 80%,significantly higher than the high-temperature-short-time(HTST)pasteurization.Moreover,the increase of UV-C dose had no significant effect on bioactive proteins.Then,the effects of microfiltration combined with different energy densities of ultrasonication on the microbial shelf life of skim milk and bioactive proteins were studied.The results showed that more reduction of vegetative bacterial load of skim milk was found along with the stronger energy density of ultrasonication.When the energy density was up to 1296J/m L,the vegetative bacterial load of skim milk was undetectable,and the shelf life of skim milk could be extended to at least 40 days.In addition,ultrasonication did not change the high abundance of proteins in whey,such as?-lactalbumin(?-La)and?-lactoglobulin(?-Lg).Due to the partly high temperature caused by cavitation of ultrasonication,a higher energy density could lead to denaturation and inactivation of some bioactive proteins,such as LTF,Ig A,Ig M,complement factor B,XO,etc.Furthermore,ultrasonication could increase the abundance of some whey proteins because of mechanical physics.The effects on bioactive proteins of ultrasonic treatments was less than HTST pasteurization.Finally,the effect of different treatments on the qualities of ESL skim milk during the shelf life was studied.Immunoglobulins,LTF and activity of LPO and XO were almost completely destroyed by ultra-pasteurization,and protein oxidation,as well as sulphides and more aldehyde flavor compounds were found after ultra-pasteurization.Microfiltration combined with HTST retained 40%-80%of the bioactive proteins,while microfiltration combined with UV-C and ultrasonication retained minimum of 80%of bioactive protein.No significant protein oxidation was found in skim milk treated by the above three treatments.In addition,more pyrolytic hydrocarbons were produced because of the cavitation of ultrasonication,and might lead to irritating odor of skim milk.However,microfiltration combined with UV-C had little effects on the volatile flavor compounds.The shelf life of skim milk could be extended to 40 days after the four treatments for processing ESL skim milk.ESL skim milk treated by microfiltration combined with UV-C and ultrasonication respectively had a similar proteolysis rate during shelf life compared to that treated by microfiltration combined with HTST pasteurization.In summary,this study combined microfiltration with UV-C and ultrasonication respectively,compared to HTST pasteurization,these two treatments extended the shelf life,and improved the retention of bioactive proteins.Since microfiltration combined with UV-C had less impacts on the flavor of skim milk,it may have higher commercial values and potential for achievements transformation.This research provided insights for improving the quality of liquid milk and improving processing technology of dairy industry. |
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