Inactivation Of Osmotoletant Yeast In Apple Juice Using Cold Plasma Treatment

China is the largest apple producer in the world.The export of apple juice not only plays an important role in China's food industry,but also it directly influence the interests of tens of millions of fruit growers.However,the presence of osmotolerant yeast may result in the generation of gas in the cider during the storage process,leading to peculiar smell or even swelling of the packing bags,causing great economic loss,which becomes a big problem disturbing the cider processing industry in China.Therefore,the establishment of a suitable method to inactivate the osmotolerant yeast is very crucial to apple juice in terms of the quality and safety.As a new way of nonthermal sterilization,the nonthermal plasma technology has the advantages of high sterilization efficiency and less time consumption in treatment under low temperature,etc.However,the treatment capacity is so small that it becomes the main obstacle for promoting the use of this technology on large scale.To tackle this problem,a plasma reactor is designed in this paper and fabricated for inactivation of microorganisms in liquid food,with the inactivating effect in liquid being measured,and the impact of cryogenic plasma treatment on the quality of apple juice being analyzed.Based on the test,a numerical model of plasma inactivation of microorganism is established by the use of the finite element analysis software COMSOL Multiphysics to identify the gas-liquid mass transfer and the microorganism inactivation process during the treatment for seeking the optimal solution.The main results of the research are as follows:?1?The discharge of the gas-phase surface-discharge plasma reactor is a typical micro-discharge process.In the treatment process,the pH value of the sterile water decreases significantly along with the increase of treatment time.Given the condition of discharge under 21kV,the conductivity after 30 minutes of treatment increases from the initial 12?s/cm to 142?s/cm,with the temperature only increased slightly by 0.7?,O₃ and H₂O₂concentrations in the sterile water are 3.14 mg/L and 4.39 mg/L respectively.After 30min of plasma treatment,the population of Z.rouxii LB decreases by 5.6log₁₀,and the number of Z.rouxii 1130 by 6.82log₁₀.The lethal kinetics curves of osmotolerant yeast Z.rouxii LB and Z.rouxii 1130 well follow the Weibull model.Due to the differences in cell wall composition,various strains demonstrate different sensitivity to plasma treatment.It takes longer time to inactivate the same amount of LB.?2?Based on the features of gas-phase surface-discharge plasma in microorganisms inactivation,a simulation model is established with COMSOL Multiphysics.The inactivation process is simplified to the process of bubble transport and dilution species transport in the turbulence.The validated model can be further used to study the effect of initial yeast concentration and reactor size on the inactivation of yeast by plasma activation.Both the experimental and simulation results show that the increase of initial concentration has a negative effect on the yeast deactivation,but the change of reactor size has no significant impact on yeast deactivation.?3?The plasma spray reactor has been designed and fabricated to improve the mass transfer efficiency and avoid the generation of bubbles in the liquid.The data show that,the concentrations of O₃ and H₂O₂ in the sterile water are 3.84 mg/L and 5.20 mg/L,respectively after the plasma treatment for 30min,which slightly higher than that in the gas-phase surface-discharge plasma reactor.The ozone concentration at the outlet is about 10mg/L,which is not significantly different from that of the gas-phase surface-discharge plasma reactor.At the peak discharge under voltage of 21kV,the number of Z.rouxii LB cells decreases by 5.92log₁₀0 and that of Z.rouxii 1130 decreases by 6.9log₁₀ after plasma treatment for 30min.The comparison of D values demonstrates that the plasma spray reactor can activate microorganisms in water faster than the gas-phase surface-discharge plasma reactor.?4?A computer model for inactivation of yeast solution by plasma spray reactor is developed.The yeast survival curve and ozone concentration curve obtained by the computer modelling match well the experimental results measured under different discharge voltages.The verified model can be further used to study the impact of initial yeast concentration on the inactivation.The paramenters of process can be optimized for better plasma inactivation by taking these simulation results as a basis.?5?The study shows that after 30min treatment in a plasma spray reactor,the 6.6-6.8log₁₀ reductions of Z.rouxii LB and Z.rouxii 1130 can be achieved in 0.5L apple juice.Maximum inactivation rate can be reached when gas flow is 150 L/h with liquid flow 9.0 L/h.SEM images show that the plasma treated cells ruptured and their contents oozed out.The compounds produced by the plasma,such as O₃ and H₂O₂,as well as active substances in the liquid phase,are the main cause of the inactivation of microorganisms.?6?When the concentration of soluble solids in the apple juice increases,the time required for the inactivation yeast in the plasma spray reactor also increases.After plasma treatment,the pH value of apple juice increases only slightly while the concentrations of reducing sugar and titrable acid remain stable.After 30 min processing by plasma spray reactor,the total color difference in 12,36 and 60°Brix apple juice between the treated and untreated samples is 9.53,7.97 and 3.86.No significant change is detected in the concentration of the main components of volatile compounds,such as alcohol,ester,aldehyde and ketone in apple juice concentrate and fresh apple juice.