Psychrotrophic Pathogens

A potential consumer safety risk may occur due to MAP inhibition of the aerobic microorganisms that usually warn consumers of spoilage, resulting in reduction in growth competition, and creation of an altered environment, allowing enhanced or unrestricted growth of anaerobic or facultative anaerobic pathogens capable of growing under MAP conditions at refrigeration temperatures. These include L. monocytogenes, C. botulinum, Yersinia enterocolitica, and Aeromonas hydrophila.

L. monocytogenes, ubiquitous in the environment, is naturally found on many fruits and vegetables. A facultative anaerobe capable of growing under temperatures as low as — 1.5°C and under CO2-enriched environments, this pathogen can feasibly grow on MAP refrigerated produce. Beuchat [50] reported little inhibitory effect of MAP at 4 to 15°C on growth of L. monocytogenes on broccoli, cauliflower, and asparagus. Bennik and others [37] found that the extent of growth of L. monocytogenes on chicory endive was not influenced by MAP atmospheres; the initial inoculum level, cultivar of chicory endive, and population of competitive spoilage microorganisms were primary growth influences. L. monocytogenes grew better on chicory disinfected with chlorine than on chicory left untreated prior to MAP storage, most likely due to reduction of competitive indigenous microflora after treatment. Francis and O'Beirne [33] found that survival and growth of L. innocua (as a model of L. monocytogenes) was affected by the indigenous microflora; Enterobacter cloacae and LAB reduced growth of L. innocua while pseudomonads had little effect. Thus MAP treatments have the potential to change the dynamics of microbial populations and alter product safety. Berrang and others [51] increased shelf life and reduced spoilage microorganisms of cut asparagus, broccoli, and cauliflower by MAP; however, in later studies they found that the growth of L. monocytogenes and A. hydrophila was unaffected. Thus shelf life extension feasibly allowed a longer time period for the pathogens to grow by removing competitive microorganisms [52,53].

Aeromonas spp. generally grow at temperatures between 1 and 45°C, and under low O2 atmospheres. Aeromonas spp. can grow at low temperatures under vacuum but are inhibited by high concentrations of CO2. Researchers have found A. hydrophila to be present on 100% of 12 different produce items surveyed, recovering the pathogen from green salad, coleslaw, salad samples, and mixed salad greens [9]. A. hydrophila was found to survive but not grow on vegetable salads stored under MAP at 4°C, but rapidly grew at 15° C. Others [54] have found that A. hydrophila would grow on cucumber slices but not on mixed lettuce under MAP conditions at 2°C. Bennik and others [44] found that growth of A. hydrophila was the same under MAP conditions of 1.5 or 21% O2; mmax decreased with increasing CO2 concentrations; however, Nmax was not affected until CO2 levels were above 50%.

At 4°C, Y. enterocolitica has been found to grow in air, under vacuum, and under MAP and atmospheres containing 40 to 50% CO2 [9]. Yersinia can grow at temperatures as low as 1°C with a 40-hour doubling time. Farber [1] reported that 10% CO2 stimulated growth of Y. enterocolitica, but 40% CO2 increased x, and 100% CO2 increased X and decreased mmax.

C. botulinum poses a significant food safety risk in MAP produce, as previously discussed in this chapter. MAP conditions that extend product shelf life may create an organoleptically acceptable consumer product, but may pose a food safety hazard not immediately visible to the consumer. Macura and others [55] found that anaerobic conditions developed under a range of MAP atmospheres and temperatures employed for storage of ginseng roots; at 10°C, C. botulinum toxin was detected in roots while overall product quality was still acceptable. Nonproteolytic strains of C. botulinum have a growth potential between 3.3 and 45°C and are minimally affected by CO2 concentrations <50% [15]. Toxigenesis has been detectable at 8 and 5°C [9] and at O2 concentrations up to 10%; however, it has been reported that toxin production is dependent as well on the produce commodity [56]. Of all vegetables tested (butternut, onion, mixed greens, lettuce, rutabaga), at 5°C nonproteolytic strains could only produce toxin on butternut squash; proteolytic strains could produce toxin on all vegetables tested at temperatures >15°C. While acidic environments such as those produced by high CO2 packaging atmospheres, acid treatments, and/or low pH produce can inhibit growth of C. botulinum, microbial diversity and dynamics may increase product pH. Growth of mold on tomato may increase typical product pH from about 4 to 5-9 [57], creating microenvironments suitable for growth of C. botulinum. Thus the effects of atmospheres, temperatures employed, temperature abuse, influences of other organisms, as well as commodity type should be assessed when designing MAP systems to reduce risk of food poisoning due to C. botulinum.

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