Spoilage Organisms and Commodity Shelf Life

The spoilage microorganisms present on produce in MAP storage systems will be influenced by the particular commodity and by the atmospheres and temperatures employed. Initially, Gram-negative bacteria predominate in the microflora of typically low-acid vegetables while LAB, molds, and yeasts predominate on high-acid fruits. Indigenous microflora on vegetables that cause spoilage include a majority of Gram-negative bacteria, predominantly Pseudomonas spp., Enterobacter spp., and Erwinia spp. as well as Flavobacte-rium spp. and Xanthomonas spp. and Gram-positive LAB such as Leuconostoc mesenteroides and Lactobacillus spp. Indigenous yeasts and molds that cause spoilage include Cryptococcus spp., Candida spp., Rhodotorula spp., Fusarium spp., Rhizopus spp., Cryptococcus spp., Botrytis spp., Mucor spp., and Penicillium spp., among others.

Zagory [38] reported that for a majority of fresh vegetables, Pseudomonas spp. comprised 50% or more of the total initial spoilage microflora in MAP stored product. Jacxsens and others [42] reported that MAP spoilage of leafy greens and cucumber was primarily due to growth of members of the Enterobacteriacea family while spoilage of celeriac and green bell peppers was due to LAB and yeasts. The diversity of spoilage organisms initially found on MAP produce upon packaging may dynamically change during the course of shelf life and establishment of EMA. Bennik and others [43] found lowest counts of pseudomonads under 0% O2 compared to 21% O2 atmospheres, irrespective of CO2 levels. Pseudomonads were predominant at 21% O2, while enterics were more predominant under 0% O2. Differences in sensitivities to modified atmospheres among strains, availability of nutrients, nutrient requirements, and/or physiological state of the produce can result in shifts in microbial populations during storage. Bennik and others [44] examined the microbial composition of MP mung bean sprouts and chicory endive stored under MAP (atmospheres of 1.5 or 21% O2 with 0, 5, 20 or 50% CO2) at 8°C. On mung bean sprouts, the predominant species before and after storage were Enterobacter cloacae, Pantoea agglomerans, Pseudomonas fluorescens, Ps. viridilivida, and Ps. corrugata. Predominant species on chicory endive before storage were Rahnella aquaatilis and several Pseudomonas spp.; after storage, E. vulneris and Ps. fluorescens predominated.

Generally, MAP utilizing mixed atmospheres of O2/CO2/N2 is most inhibitory towards aerobic bacteria and molds and may not inhibit or only minimally inhibit many spoilage yeasts and LAB. Exceptions have been discovered using specific MAP atmospheres and produce commodities. Martinez-Ferrer and others [45] found that MAP atmospheres of 4% O2,

10% CO2, balance N2 significantly reduced total yeast populations on prepared mango and pineapple stored for up to 30 days at 5°C, as compared with storage under vacuum or air. Piga and others [46] found that in cactus pear fruit stored for 9 days at 4°C under atmospheres of 17%CO2, < 1% O2, balance N2, fungal mycelia were not visible on produce surfaces, but these conditions did not inhibit mold growth; molds increased in number from 20 to 5 x 102 CFU/g.

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