Effect Of Thermo-sonication Treatment On Pathogens Inactivation And Physicochemical Properties Of Liquid Milk

Due to its high nutritional value,milk is recognized as an indispensable nutri-tional food,but it is very susceptible to various pathogenic microorganisms.Among them,Listeria monocytogenes and Salmonella typhimurium are the representative pathogenic bacteria in milk.The mian traditional sterilization method for liquid milk is heat sterilization,the shelf life of ultra-high temperature sterilization milk can be up to 9-12 months at room temperature,but high sterilization temperature will negatively influence the sensory quality of milk.Alternatively,pasteurized milk is favored by consumers because of its lower nutritional losses and good sensory quality processed under relatively mild sterilization condition,however,the pasteurization is not com-pleted and its shelf life is too short.Therefore,it is necessary to develop a novel steri-lization technology which can not only avoid the adverse effects on dairy products caused by traditional heat sterilization but also could effectively inactivate pathogenic and spoilage microorganism.Thermo-sonication treatment(TS)is one of promising bactericidal technology,which can achieve sound inactivication efficacy at relative low temperature,maintaining the sensory quality of milk to some extent.Raw milk and reconstituted milk(whole milk and skim milk)were used as object of this study.Thermo-sonication sterilization process for L.monocytogenes and S.typhimurium in liquid milk was optimized and the bactericidal mechanism was researched,then the effects of thermo-sonication sterilization and traditional pasteurization on the physi-cochemical properties of liquid milk as well as the quality changes during storage were also compared.The main results are as follows:1.Increasing ultrasonic time,power and temperature,the inactivication efficacy of TS on L.monocytogenes and S.typhimurium in skim milk or whole milk increased,among that,temperature has the greatest effect.The inactivication curves of these two pathogens showed similar trends,but the inactivication efficacy of TS on S.typhimurium was higher than on L.monocytogenes.The Decimal reduction time of L.monocytogenes and S.typhimurium in milk was about 50 s and 43 s,respectively,un-der the sterilization condition of 400 W.50?.Moreover,milk fat has shown some protection for L.monocytogenes against TS.In order to ensure food safety,the FDA recommended that the inactivation efficacy of pathogens should be greater than 5 log CFU/mL.Take it as a goal,the optimal sterilizing condition for L.monocytogenes in skim milk recommended by response surface analysis is 12.6 min,413 W,53 ? and 10.3 min,300 W,50 ? for S.typhimurium in skim milk;when it comes to L.monocytogenes in whole milk,the optimal sterilization condition is 13.7 min,312 W,54 ? and 8.8 min,307 W,50 ?for S.typhimurium in whole milk.Compared with raw milk,the colonies number and heat-resistant bacteria number decreased signifi-cantly after sterilization,and these indicators reached the requirements of national standards.The TS treatment had better inactivication efficacy on the microorganisms in raw milk compared with pasteurization and could kill heat-resistant bacteria more easily.2.Study on the mechanism of Thermo-sonication sterilization found:The intra-cellular protein leakage of L.monocytogenes and S.typhimurium was 108.99 ?g/mL and 66.64 ?g/mL respectively after treated by TS.The results of scanning electron microscopy and transmission electron microscopy showed that the ultramicrostructure of bacterial cells after TS treatment was severely damaged.The extent of damage to S.typhimurium was greater than that of L.monocytogenes,indicating that significant differences in the cell walls of Gram-negative bacteria(GR-)and Gram-positive bacteria(GR+)may make the ultrastructure of GR-more easily be damaged.Both of these two pathogens are less damaged in milk than in water,which indicated that the protein,fat and other solids in milk could greatly help the bacteria to resist the action of ultrasound under strong thermo-sonication treatment,among which fat has the greatest protective effect.The TTC(2,3,5-Triphenyl tetrazolium chlo-ride)-dehydrogenase relative activity of L.monocytogenes and S.typhimurium de-creased by 47.17%and 36.59%respectively after TS treatment.The results above in-dicated that TS could damage the cell structure 5 result in the loss of cell content and decrease the activity of intracellular enzymes which may inhibit the respiration of cells,then generate the lethal effect on microorganisms.3.There was no significant difference between TS and control group on pH,ti-tration acidity,L*and a*value,but b*value appeared significant reduction;the loss rate of Vitamin C(Vc)was 4.47%(skim milk)and 10.6%(whole milk),significantly lower than pasteurization(30%-60%);the diameter of fat globules in raw milk de-creased from 9.6?m to 2.2 ?m,significantly lower than that of pasteurization and were more uniform.Compared with pasteurization,the effect of TS on colloidal ca-sein particle size in reconstituted milk was the smallest and the casein colloid was the most stable while in raw milk,colloidal casein particle size increased significantly and the degree of whey protein denaturation was great.The protein analysis of infra-red spectroscopy also showed that TS was more devastating to protein structure than pasteurization but could result in the increase of Zeta potential and improve the stabil-ity of casein colloid.Compared with pasteurization,TS can extend the shelf life of liquid milk.After treated by TS of 50?,400 W,10 min and 10 days storage under 4 ?,the pH,titration acidity,colloidal casein particle size,Zeta potential and other indicators of liquid milk were all very stable,that is,its shelf life can be up to 10 days,which is 3 days more than the shelf life of pasteurized milk.