I. Inactivation of foodborne pathogens in mixed vegetables packed in plastic containers using hydrogen peroxide combined with in-package cold plasma treatment
The effects of hydrogen peroxide (H₂O₂) combined with cold plasma (CP) treatment (HCP treatment) on the growth of pathogen, microbiome in mixed vegetables, and concentration of endocrine disruptor chemicals from polyethylene terephthalate (PET) container were investigated. Mixed vegetables was packaged in a H₂O₂ pad-attached PET container and CP-treated at 24.5 kV for 3 min. Cell populations of indigenous aerobic bacteria and yeast and molds in mixed vegetables tended to increase regardless of treatment and maintained a certain differences cell population between HCP treated and not treated mixed vegetables, during storage at 4 ℃ and 10 ℃. The cell population of Escherichia coli O157:H7 in mixed vegetables decreased and the differences in the counts of Listeria monocytogenes in mixed vegetables with or without HCP treatment increased during storage from 1.4 log CFU/g to 1.9 log CFU/g, without affecting the antioxidant activity, respiration rate, and color of mixed vegetables during storage at 4 ℃ for 14 days. The proportion of Pseudomonas decreased after HCP-treated, confirming the effectiveness of HCP treatment on the microbiome. HCP treatment did not affect the concentration of bisphenol A and phthalates from the PET container. The results revealed that HCP treatment was effective to inhibit the microbial growth and influenced the microbiome of mixed vegetables without affecting its quality properties and the concentration of endocrine disruptor from PET container.
II. Optimization of treatment conditions of ultrasound treatment combined with plasma-activated water for inactivation of indigenous bacteria in fresh-cut celery
In this study the effect of ultrasound (US) combined with plasma-activated water (PAW) treatment was developed for the inactivation of indigenous bacteria in fresh-cut celery. The response surface method was used to determinate the optimum treatment conditions of US-PAW (UP) treatment for inhibition of indigenous bacteria in celery. The optimal UP treatment conditions were determined to be 61.5 minutes of discharge time and 338 seconds of washing time, and 2.7 log CFU/g of indigenous aerobic bacteria was reduced. The cell lipid peroxidation value, active oxygen species production value, and the number of non-viable cells with damaged membranes increased after US and PAW treatment, but there were much higher after UP treatment. This study demonstrated that UP treatment showed the a higher inhibition rate compared to each individual treatment and demonstrated the potential of applying microbial decontamination of fresh-cut celery.
III. Effects of ultrasound combined plasma-activated water treatment on celery during storage
The effects of ultrasound (US) combined plasma-activated water (PAW) treatment on the growth of indigenous bacteria, quality properties, and microbiome were investigated. PAW discharge time was 61.5 min and US washing time was 338 sec. Comparing the cell population of the indigenous bacteria in celery according to NaClO, US, PAW, and US-PAW (UP) treatment, it was found that it showed the lowest cell population after UP treatment and grew while maintaining the degree of inhibition during stored at 4 ℃ or 10 ℃. There was no significant differences color, respiration rate, firmness, and sensory properties between untreated and UP-treated celery during storage at 4 ℃ or 10 ℃ for 7 days. The proportion of proteobacteria and actinobacteria of celery which was stored at 4 ℃ for 3 days after UP treated decreased confirming the storage after UP treatment affects the microbial community of celery. The physicochemical properites of PAW were changed during storage at 4 ℃ and 25 ℃ for 7 days, but these changes were less than 7% of initial value. This study shows that US combined PAW treatment can effectively inhibit indigenous bacteria in celery during storage with minimal impact on quality properties of celery.