Effects Of Ultrasound、Heat Combined With Lysozyme On Inactivation Of Bacillus Subtilis Spores And And Its Application In Sterilization Of Pear Juice And Carrot Juice

Bacterial spores are common spoilage factors in food,which can cause food spoilage and foodborne diseases.Ultrasound is an emerging green processing technology that can kill bacteria in a short time,reduce microbial contamination,and reduce damage to food quality.Compared with ultrasonic processing alone,the effect of ultrasound、heat combined with lysozyme treatment was better.Therefore,the inactivation effect of ultrasound、heat combined with lysozyme treatment on Bacillus subtilis spores and its kinetic model were studied in this paper,and then the changes of spore structure and key substances were discussed.The inactivation mechanism of ultrasound、heat combined with lysozyme treatment on spores was comprehensively expounded by the changes of spore structure and key substances,and the effect of ultrasound、heat combined with lysozyme treatment on the quality of fruit and vegetable juice was analyzed and discussed.The main research contents and results are as follows:(1)Bacillus subtilis spores were treated with ultrasound(300 W,600 W)of different power combined with heat(60℃,75℃,90℃)and lysozyme(0.2%,0.4%)for 10～50 min,the effects and kinetics of inactivating spores under different treatment conditions were studied,and the correlation between spore inactivation and OD value of spore suspension(OD600,OD260,OD280)was explored.The results showed that low power ultrasound and low power ultrasound combined with low temperature had almost no inactivating effect on Bacillus subtilis spores in a short period of time.Ultrasound,heat and lysozyme had synergistic inactivation effect on spores.Increasing ultrasonic power,treatment temperature and lysozyme concentration had more significant inactivation effect on spores.At the same time,with the increase of treatment time,the spore inactivation curve showed a downward trend,and the fitting degree of Logistic model was better.In addition,the amount of spore inactivation was significantly correlated with OD600,OD260 and OD280 values,However,the correlation with OD600 was more significant,the OD600 value was easy to measure and could be used to predict the inactivation effect of spores quickly and reliably.(2)The changes of the spore inner membrane structure were analyzed by detecting the release of spore contents combined with flow cytometry,the changes of spore morphology was analyzed by particle size changes and scanning electron microscopy results.The results showed that after 30 min of ultrasound、heat combined with lysozyme treatment,the release of spore’s contents increased,the leakage of nucleic acids and proteins increased,the spore inner membrane was damaged,the permeability increased,the particle size became smaller,the outer wall of the spores fell off,dented and ruptured,and its integrity was destroyed.Based on the above results,ultrasound、heat combined with lysozyme destroys the spore structure,which in turn lead to inactivation of spores.(3)The effects of ultrasound、heat combined with lysozyme treatment on key substances of spores were analysed by measuring the release of DPA,changes in Na+/K+-ATPase activity,and changes in FTIR infrared spectra after 30 min of treatment.The results showed that after 30 min of ultrasound、heat combined with lysozyme treatment,the release of DPA increased,the Na+/K+-ATPase activity decreased significantly,and the resistance of spores decreased and their energy metabolism was disturbed.At the same time,FTIR spectroscopy results indicated that the spore inner membrane fluidity,nucleic acid backbone stability and protein structure were altered.In summary,ultrasound、heat combined with lysozyme kills spores by damaging their key substances.(4)The inactivation effect of ultrasound、heat combined with 0.2%lysozyme treatment for 30 min on Bacillus subtilis spores in pear and carrot juice was evaluated by the total number of colonies,and the changes of the quality of pear and carrot juice were explored by the determination results of pH,soluble solids(TSS),Vc contents,color,browning degree and other indicators.The results showed that ultrasound、heat combined with lysozyme treatment had a significant inactivation effects on the spores in pear juice and carrot juice,and had no obvious effects on the pH and TSS of pear juice and carrot juice,but it could reduce the VC content.Lysozyme basically does not affect the quality of pear juice and carrots,but it can slow down the degree of browning of pear juice and carrot juice in a certain extent.