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Zhao F.,China Agricultural University | Zhao F.,Chinese National Engineering Research Center for Fruit and Vegetable Processing | Zhao F.,Key Laboratory of Fruit and Vegetable Processing | Bi X.,China Agricultural University | And 8 more authors.

The viable but nonculturable (VBNC) state is a survival strategy adopted by many pathogens when exposed to harsh environmental stresses. In this study, we investigated for the first time that whether high pressure CO2 (HPCD), one of the nonthermal pasteurization techniques, can induce Escherichia coli O157:H7 into the VBNC state. By measuring plate counts, viable cell counts and total cell counts, E. coli O157:H7 in 0.85% NaCl solution (pH 7.0) was able to enter the VBNC state by HPCD treatment at 5 MPa and four temperatures (25°C, 31°C, 34°C and 37°C). Meanwhile, with the improvement of treatment temperature, the time required for E. coli O157:H7 to enter VBNC state would shorten. Enzymatic activities in these VBNC cells were lower than those in the exponential-phase cells by using API ZYM kit, which were also reduced with increasing the treatment temperature, but the mechanical resistance of the VBNC cells to sonication was enhanced. These results further confirmed VBNC state was a self-protection mechanism for some bacteria, which minimized cellular energetic requirements and increased the cell resistance. When incubated in tryptic soy broth at 37°C, the VBNC cells induced by HPCD treatment at 25°C, 31°C and 34°C achieved resuscitation, but their resuscitation capabilities decreased with increasing the treatment temperature. Furthermore, electron microscopy revealed changes in the morphology and interior structure of the VBNC cells and the resuscitated cells. These results demonstrated that HPCD could induce E. coli O157:H7 into the VBNC state. Therefore, it is necessary to detect if there exist VBNC microorganisms in HPCD-treated products by molecular-based methods for food safety. © 2013 Zhao et al. Source

Wang S.,China Agricultural University | Wang S.,Chinese National Engineering Research Center for Fruit and Vegetable Processing | Wang S.,Key Laboratory of Fruit and Vegetable Processing | Lin T.,China Agricultural University | And 17 more authors.
Food and Bioprocess Technology

The effects of 0.2 % ascorbic acid + 0.2 % citric acid (AA + CA), 0.2 % ascorbic acid + 0.2 % citric acid + N2 (AA + CA + N2) and 0.2 % ascorbic acid + 0.2 % citric acid + CO2 (AA + CA + CO2) on the quality of banana smoothies were investigated. An obvious anti-browning effect was observed after each treatment, which resulted from a significant decrease in the polyphenol oxidase activity (p < 0.05) and the exclusion of oxygen by infusion with inert N2 or CO2. The AA + CA + N2 treatment exhibited the lowest change of browning index (ΔBI) indicating the best anti-browning effect on the banana smoothies. Compared with the Herschel-Bulkley model, the Power Law was less complex and more suitable for describing the rheological properties of the banana smoothies. AA + CA, AA + CA + N2 and AA + CA + CO2 treatments all significantly increased the pectin methylesterase activity of the banana smoothies (p < 0.05). The AA + CA treatment increased the cumulative volume of the larger particles in the banana smoothies. The viscosity of the banana smoothies decreased after the three different treatments. In terms of the comprehensive results, the inclusion of AA + CA + N2 in the production of banana smoothies was the best processing method applied. © 2013 Springer Science+Business Media New York. Source

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