Effect Of Pulsed Electric Fields On Blueberry Juice And Sterilization Mechanism Of PEF

Blueberry is a kind of very healthful food with a variety of physiological health functions, it has a broad prospect for development. To achieve the development and utilization of blueberry products, in this paper, PEF technology was used to process fresh blueberry juice, which is a kind of non-thermal processing technology, to research the change of microorganisms, nutrients and aroma components and the change of various physical and chemical indicators during storage. Combined with bionic technology and GC-MS technique, the changes of blueberry Juice flavor were analyzed. And then microorganisms in blueberry juice were processed by PEF, in order to explore the microbial passivation mechanism of PEF. The main results were as follows:1. PEF technology can effectively kill microorganisms in blueberry juice, and does not affect the color, the flavor and nutrients of blueberry juice. When the electric field strength was 30 kV/cm and the processing time was 60 μs, the inactivation rate of blueberry juice reached more than 99% already. The total number of colonies was 1.3 logarithms after 30 days of storage under 4℃, it meets the requirements of commercial sterility (GB19297-2010).2. PEF technology has little effect on anthocyanins which are important active ingredient in the blueberry juice. The retention rate of anthocyanins in blueberry juice via PEF treatment reached 84.84%; The Vc retention rate of blueberry juice via PEF treatment reached 87.87%,13.27% higher than via the heat treatment; Impact on the reducing sugar, total acid, total phenols, Brix content of blueberry juice via both PEF treatment and heat treatment was small; The value of the color difference ΔE*of blueberry juice after the heat treatment nearly three times higher than after PEF treatment, indicating that color change of blueberry juice via heat treatment was greater than PEF treatment.3. Electronic nose for non-destructive testing technique was suitable for detection of blueberry juice, the total contribution rate of PCA analysis was 97.943%, the correct diagnosis rate of DFA analysis was 100%, SIMCA analysis showed that blueberry juice via PEF treatment and the control group were a class of substances. Detecting blueberry juice by Electronic Tongue found that the biggest change of flavor was the richness (fresh aftertaste) after three different treatments, sour, salty, astringent were different with the control group. After heat treatment, the taste of blueberry juice had the biggest change.4. Using SPME to improve the aroma and GC-MS online retrieval method to detect blueberry juice via heat treatment and PEF treatment, it showed that the type and content of alcohols, ketones, aldehydes, esters, acids and alkanes changed (p<0.05); But the main aroma components changed little, which gave blueberry juice unique aroma, such as fruit, flowers, rosin, camphor incense.5. With the increasement of the electric field strength and the processing time, the killing effect of PEF to E. coli, Staphylococcus aureus and S. cerevisiae enhanced(p<0.05). When the electric field strength was 35 kV/cm, the processing time was 90 μs, the inactivation rates of E. coli, Staphylococcus aureus, S, cerevisiae were 5.12,4.64,5.30 logarithms. Under the same electric field strength and processing time, the fatality rate of S. cerevisiae was greater than E. coli than Staphylococcus aureus. Hulsheger dynamics model can effectively analyze the dynamic relationship between PEF and various microorganisms. The critical field strengths of E. coli, Staphylococcus aureus, S. cerevisiae were 11.01kV/cm,16.55kV/cm,11.25 kV/cm respectively.6. There were sublethal cells of yeast and E. coli after PEF treatment. Through scanning electron microscopic observation, it showed that, after PEF processing, tooth marks of S. cerevisiae increased, the cell surface appeared rough wrinkles, depressions and even holes. Transmission electron microscopy observation found that S. cerevisiae after PEF treatment showed protoplast reunion, missing; cytoplasm and cell wall separation; nuclei obviously missing. Through PI staining, it was found that the number of E. coli and S. cerevisiae’s cell damage increased with the increasing of electric field strength and treatment time, and S. cerevisiae was easier to cause membrane injury than E. coli. After PEF treatment, the intracellular proteins of S. cerevisiae and E. coli occurred to leak, the secondary structure of protein was also damaged. AO staining found that DNA of S. cerevisiae and E. coli denatured. The mitochondrial membrane of S. cerevisiae potential damaged. These phenomenon supports the theory "membrane perforation", and microorganisms after PEF treatment not only cause membrane perforation, but intracellular proteins, mitochondria, genetic material and so on were all damaged, it can cause the death of the microorganisms, too.