Microbial Stress Adaptation On Produce

Information in published literature regarding microbial stress adaptation on produce is scarce. However, greater processing resistance of natural microbial contaminants on produce surfaces compared with those inoculated onto these products may support the hypothesis that most produce microbiota are adapted to stresses encountered in the field and throughout the production chain. Readers are cautioned that apparent processing resistance of produce microbiota also could be attributed to their inaccessibility to treatments, or the inability of the analyst to recover these microorganisms using common sample preparation and processing techniques. Association of microbial contaminants with pores, stem scars, wounds, and other surface irregularities could protect microorganisms and make them appear resistant to processing.

Attachment of microorganisms to fruit and vegetable surfaces could initiate stress adaptive response against physical and chemical treatments. Gawande and Bhagwat [92] reported that Salmonella spp. attached to apple, tomato, or cucumber had enhanced surface contact-mediated acid tolerance and increased survival, by 4 to 5 log, to acid stress induced by exposure to sodium citrate (50 mM, pH 3) for 2 hours when compared to cell suspensions treated under the same conditions. When these investigators inoculated Salmonella Typhimurium on the surface of fresh-cut apples, and stored them at 4°C for 2 hours, the tolerance of the pathogen to acid stress increased. Han et al. [93] treated green pepper, contaminated on the surface with E. coli O157:H7, using chlorine dioxide gas (0.2 to 1.2mg/l), and reported that attachment of the microorganism to injured pepper surfaces protected the cell against the gas when compared to cells attached to uninjured surfaces.

Francis and O'Beirne [94] inoculated acid-adapted L. monocytogenes, previously exposed to pH 5.5 for 60 minutes, on lettuce, swedes, dry coleslaw mix, and bean-sprouts, which were packed under modified atmosphere and subsequently stored at 8°C for 14 days. The researchers observed that the stress-adapted microorganism had enhanced survival compared to unstressed controls, under relatively high (25 to 30%) carbon dioxide atmospheres. Hsin-Yi and Chou [95] stressed E. coli O157:H7 in acidified medium (pH 5 for 4 hours) and inoculated the microorganism in acidic mango or asparagus juice with subsequent storage at 7°C. The investigators reported that acid adaptation and low temperature increased microbial survival in both fruit juices.

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