Optimization of Zygosaccharomyces rouxii sterilization from concentrated apple juice by radio frequency heating

Abstract: As the production of concentrated apple juice becomes larger in recent years, the spoilage of osmotolerant yeasts is increasingly serious, and the safety and quality of concentrated apple juice in China has been under serious threat. Zygosaccharomyces rouxii (Z. rouxii) is a kind of osmotolerant yeast which possesses great resistance to osmosis, acid and preservative and might result in spoilage for concentrated apple juice. This is not only a risk to consumers' health, but also bringing huge losses to the production enterprises. Therefore, it is necessary to seek a new sterilization method which can sterilize Z. rouxii quickly, as well as exert less impact on apple juice quality. Radio frequency (RF) heating sterilization is a novel dielectric heating sterilization method, by which the internal heating is rapidly generated as a result of molecular friction in response to an applied alternating electric ?eld with the frequency between 3 kHz and 300 MHz. This technology can offer the advantages of rapid heating, high penetration depth, heating homogeneity and so on. This article used Z. rouxii as model microorganism, and studied the influence of different factors on apple juice heating and Z. rouxii sterilization under the RF heating treatment. To achieve the goal, single-factor analysis was conducted to measure the effects of RF heating time, polar plates' spacing, juice volume, juice concentration on the sterilization efficacy. Combining single-factor experiment results and practice, RF heating time, polar plates' spacing and juice volume were further researched using a three-variable three-level Box-Benhnken design to optimize the sterilization conditions. A quadratic polynomial model for the logarithmic reduction of viable count for Z. rouxii was established, and the results indicated that the interaction effects of RF heating time and polar plates' spacing, RF heating time and juice volume, and polar plates' spacing and juice volume were extremely significant (P<0.01). Moreover, the order of significant degree of 3 factors was as follows: polar plates' spacing > juice volume > RF heating time. When the concentration of apple juice was selected as 70%, the optimized parameters were determined by response surface experiment and they were as follows: polar plates' spacing of 110 mm, juice volume of 45 mL and heating time of 70 s. Under this optimal condition, at least a reduction of logarithm value of 6 for viable count of Z. rouxii could be achieved. In addition, the verification test showed that there was a good fit between the experimental and the predicted values. Meanwhile, this study also compared the variations of juice physical indicators and juice flavor between RF heating and traditional water bath by electronic nose when achieving the same sterilizing effect. The results indicated that the sterilization rate of RF heating (70 s) was outstandingly faster than water bath (330 s), and furthermore, its impacts on the quality and flavor of apple juice were less than water bath treatment. Therefore, the results indicate that the method of RF heating can efficiently sterilize Z. rouxii in concentrated apple juice. The research provides a theoretical basis for further study on the RF heating sterilization and its application in sterilizing Z. rouxii in concentrated apple juice in industry.