Influence of Pressurization Rate and Mode on Inactivation of Natural Microorganisms in Purple Sweet Potato Nectar by High Hydrostatic Pressure

Analyzing the pressure resistant, sublethal injury and resuscitable viable but non-culturable state population of Escherichia coli, Staphylococcus aureus, Bacillus amyloliquefaciens and Lactiplantibacillus plantarum under high pressure processing

This study aimed to analyze and reduce the pressure resistance (PR), sublethal injury (SLI), and viable but non-culturable (VBNC) populations during HPP. Escherichia coli, Staphylococcus aureus, Bacillus amyloliquefaciens and Lactiplantibacillus plantarum were selected for evaluation of PR, SLI and VBNC cell counts and proportions during HPP. The results revealed that the bactericidal efficiency against these strains gradually improved as the processing pressure increased. However, viable bacteria could still be detected, suggesting that there may involve the presence of resistant population that difficult to be killed or revived from SLI. Further detecting the quantity and proportion of PR, SLI and VBNC bacteria found that these state of cells were present during whole HPP treatment. Additionally, the more resistant a bacterial species was to high pressure, the fewer SLI and more resuscitable VBNC (RVBNC) populations it generated, and vice versa. Therefore, correlation analysis was also employed to make the relationship between log reduction, SLI and RVBNC population ratios clearer. The results demonstrated that the log reduction was highly positive correlation with SLI population ratios, and negative correlation with RVBNC population within our detected species at 500 MPa. Furthermore, CO2 and Nisin were employed to combined with HPP to reduce these survivors. Comparing with 233, 218, 241 and 259 MPa for HPP treatment, it took 37, 89, 135 and 229 MPa for HPP + CO2, and 189, 161, 199 and 292 MPa for HPP + Nisin to the first decimal reduction for E. coli, S.aureus, B. amyloliquefaciens and L. plantarum, respectively. The results indicated that HPP combined with CO2 or Nisin could significantly reduce the quantity of PR, SLI, and RVBNC cells during HPP, and provide better bactericidal effects. In conclusion, we quantified the presence of PR, SLI, and VBNC bacteria after high pressure treatment and investigate the effectiveness of HPP combined with CO2 or Nisin to enhance the inactivation of bacteria and reduce the occurrence of PR, SLI, and RVBNC bacteria.

Influence of pressurization rate and mode on cell damage of Escherichia coli and Staphyloccocus aureus by high hydrostatic pressure

As a non-thermal technology, high hydrostatic pressure (HHP) has been widely investigated for inactivating microorganisms in food. Few studies have been presented on the pressurization/depressurization rate and mode of microbial inactivation. In this study, effect of pressurization rate and mode on Escherichia coli and Staphylococcus aureus cell damage during HHP treatment was investigated. The results showed that fast pressurization + linear mode (FL) treatment has the best bactericidal effect on E. coli and S. aureus, followed by fast pressurization + stepwise mode (FS) and slow pressurization + stepwise mode (SS) treatments. FL treatment caused more morphological damage to the cell wall, cell membrane, and cytoplasmic components compared with FS and SS treatment detected by SEM and TEM. Additionally, the damage to membrane permeability of them was also enhanced after FL treatment. Therefore, our results indicated that FL treatment could be applied to enhance the bactericidal effect of HHP on bacteria by increasing the damage to cell morphological structure and membrane integrity.

Model of Fungal Development in Stored Barley Ecosystems as a Prognostic Auxiliary Tool for Postharvest Preservation Systems

Postharvest preservation and storage have a crucial impact on the technological quality and safety of grain. The important threat to stored grain quality and nutritional safety of cereal products is mould development and their toxic metabolites, mycotoxins. Models based on predictive microbiology, which are able to estimate the kinetics of fungal growth, and thus, the risks of mycotoxin accumulation in a mass of grain are promising prognostic tools that can be applied in postharvest management systems. The study developed a modelling approach to describe total fungal growth in barley ecosystems stored at different temperatures (T = 12–30 °C) and water activity in grain (aw = 0.78–0.96). As the pattern of fungal growth curves was sigmoidal, the experimental data were modelled using the modified Gompertz equation, in which constant coefficients reflecting biological parameters of mould development (i.e. lag phase duration (τlag), maximum growth rate (μmax) and the maximum increase in fungal population level (Δmaxlog(CFU)) were expressed as functions of storage conditions, i.e. aw and T. The criteria used to evaluate the overall model performance indicated its good precision (R2 = 0.95; RMSE = 0.23) and high prediction accuracy (bias factor and accuracy factor Bf = 1.004, Af = 1.035). The formulated model is able to estimate the extension of fungal contamination in a bulk of grain versus time by monitoring temperature and intergranular relative humidity that are readily measurable in practice parameters; therefore, it may be used as a prognostic support tool in modern postharvest management systems.

Application of a roller conveyor type plasma disinfection device with fungus-contaminated citrus fruits

Efficient methods to achieve the safe decontamination of agricultural products are needed. Here, we investigated the decontamination of citrus fruits to test the antifungal potential of a novel non-thermal gas plasma apparatus, termed a roller conveyer plasma instrument. This instrument generates an atmospheric pressure dielectric barrier discharge (APDBP) plasma on a set of rollers. Penicillium venetum was spotted onto the surface of the fruit or pericarps, as well as an aluminium plate to act as a control, before performing the plasma treatment. The results showed that viable cell number of P. venetum decreased with a decimal reduction time (D value or estimated treatment time required to reduce viable cell number by 90%) of 0.967 min on the aluminium plate, 2.90 min and 1.88 min on the pericarps of ‘Kiyomi’ (Citrus unshiu × C. sinensis) and ‘Kawano-natsudaidai’ (C. natsudaidai) respectively, and 2.42 min on the surface of ‘Unshu-mikan’ (C. unshiu). These findings confirmed a fungicidal effect of the plasma not only on an abiotic surface (aluminium plate) but also on a biotic surface (citrus fruit). Further development of the instrument by combining sorting systems with the plasma device promises an efficient means of disinfecting citrus fruits during food processing.

Optimization of high pressure processing for microbial load reduction in Diospyros kaki 'Fuyu' pulp using response surface methodology

Diospyros kaki L., cv. Fuyu is a non-astringent seasonally available persimmon variety from New Zealand having short shelf life. Most of the current preservation techniques like pasteurization, spray drying etc. use high temperature for microbial inactivation, which results in quality reduction. In the present study, response surface methodology having Box-Behnken design used to explore the consequence of pressure (200-600 MPa), time (5-15 min) and temperature (20-45 °C) for controlling microbial load in fruit pulp. A mathematical model created to envision the responses, and the R² value indicated that the established model proved highly accurate in the prediction of response. The optimization process advocated non-thermal minimal processing of persimmon pulp by high pressure processing at low temperature 20 °C, 400 MPa pressure for 5 min holding time for reducing total plate count and yeast mould count.