| Fresh-cut vegetables,a convenience food for today’s fast-paced lifestyle,have expanded the vegetable industry.Because a large portion of such produce is consumed raw,an increase in foodborne outbreaks has been observed.Therefore,disinfection is a key postharvest operation that must be performed before fresh produce is packaged for sale.To that end,chemical sanitizers have been widely used,owing to their low-cost,easy-to-use characteristics.They are divided into two categories—oxidizing agents and organic acids.The disinfection efficacy of minimal processing is limited because only one type sanitizer is used.Ozone and most organic acids were approved as generally recognized as safe(GRAS)by FDA.The objective of this work was to improve the efficacy of organic acids,reduce processing time,and lower the cost of fresh-cut lettuce disinfection by employing mixtures of aqueous ozone(AO)and acid.In addition,while the disinfection efficacyof minimal processing time is traditionally evaluated with respect to microbial count reduction after disinfection,changes in microbial composition are not usually considered.In this study,a 16 S r RNA technique was employed to determine the changes in microbial composition after disinfection.We compared the disinfection efficacy of different organic acids and evaluated their effect on bacterial composition.Seven GRAS organic acids(lactic,tartaric,acetic,propionic,malic,succinic,and citric acids)were selected for this experiment.The results indicate that the disinfection efficacy of lactic acid(LA)was higher than that of the other six GRAS organic acids.Acid disinfection did not affect microbial richness.However,the relative abundance of the plant pathogen Xanthomonas was significantly increased by malic and propionic acid;more specifically,propionic acid caused the relative abundance of Xanthomonas to increase from 24.73% to 47.53%.Microbial interaction analyses revealed co-exclusion between Xanthomonas and other core taxa,suggesting that the microbial distribution on lettuce surfaces after disinfection carries an increased risk of quality loss.Therefore,the difference in disinfection efficacy of sanitizers was reflected in both microbial counts and bacterial community changes.Based on results from the above experiment in which LA produced the highest microbial reduction,a novel disinfection strategy using LA plus AO washing was proposed.Compared to the traditional disinfection strategy(100 mg/L free chlorine(120 s)or 1% LA(120 s)plus tap water(30 s)and 2 mg/L AO(150 s)),he use of 1%LA for 90 s + 1 or 2 mg/L AO for 30 s resulted in the highest microbial reduction(i.e.,Escherichia coli O157:H7,aerobic mesophilic counts,aerobic psychrophilic counts,molds,and yeasts).Quality analysis(color,sensory properties,electrolyte leakage,polyphenolic content,and weight loss)showed that compared with tap water treatment,LAplus AO did not cause additional quality loss.To enhance the disinfection efficacy of LA and lower its cost,the use of an acid mixture(LA + acetic acid)was proposed.Microbial count analysis(E.coli O157:H7,aerobic mesophilic counts,aerobic psychrophilic counts,molds,and yeasts)indicated that the disinfection efficacy of the acid mixture was consistent with that of LA and higher than that of acetic acid;but at the end of the storage period,the disinfection efficacy of the acid mixture was higher than those of LA and acetic acid.Quality analysis results indicated that the visual quality of fresh-cut lettuce was poor when acetic acid concentrations exceeded 0.8%.Microbial richness and evenness did not decrease in the individual acid-and acid mixture–treated groups.However,microbial richness and evenness significantly decreased and were lower in the test groups than in the control group,after storage.During storage,the acid mixture– and individual acid–treated groups exhibited dramatic decreases in the abundance of Massilia and Alkanindiges.In contrast,the abundance of Sphingomonas and Acinetobacter showed a considerable decrease that was independent of disinfection.The abundance of Escherichia–Shigella dramatically increased after storage from 0.003% to 58.82%,0.01% to 55.34%,undetected to 50.71%,and 0.007% to 33.09% for the LA,acetic acid,acetic acid mixture,and control groups,respectively,indicating that acid disinfection methods can change the microbial composition and stimulate Escherichia–Shigella growth.These results provide evidence for using acid mixtures as an alternative to individual acids for fresh produce disinfection,and they improve our understanding of the relationship between disinfection and microbial compositional changes in fresh-cut produce.Based on the above result in which the disinfection efficacy of the acid mixture was consistent with that of LA against E.coli O157:H7,a proteomic technique was employed to understand the different mechanisms of action of LA and the acid mixture against E.coli O157:H7.Results showed that 1755 proteins(794 upregulated and 961downregulated)were differentially expressed after treatment with LA and that 1835(761upregulated and 1074 downregulated)proteins were differentially expressed after treatment with the acid mixture.Gene Ontology biological process annotations and Venn diagram analysis revealed that LA can cause unique changes in cellular localization,macromolecular localization,and protein unfolding;and the acid mixture can cause unique changes in taxis,response to stress,catabolic processes,and regulation of molecular function.Protein network analysis indicated that key proteins in cellular and macromolecule localization pathways were responsible for the transfer and insertion of nascent proteins into cell membranes,and that key proteins in the taxis pathway were responsible for the flagellar movement and chemical stress sensing.These results suggest that LA may uniquely influence cell membrane integrity and that the acid mixture may uniquely stimulate chemotaxis in response to chemical stress. |
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