The Reactions Of Spoilage Floras And Pathogenic Bacteria To Modified Atmospheres

When lack of oxygen is not a factor, meat spoilage floras are inevitably dominated by strictly aerobic pseudomo-nads, at least at temperatures of 20°C or below. Those organisms will ultimately cause putrid spoilage of the meat. The presence of C02 in the atmosphere slows the rates of growth of both pseudomonads and competing species so that the pseudomonads retain the growth rate advantage that insures their predominance in the spoilage flora. Consequently, C02 in an aerobic atmosphere extends the time required for the spoilage flora to reach spoilage numbers but does not significantly alter the flora composition or the type of spoilage that finally occurs.

The inhibition of pseudomonads by C02 increases with decreasing temperature and with increasing C02 concentration. However, the increased inhibition is small for C02 levels >20%. Thus, provided the C02 concentration does not fall below that level, changes in C02 concentration will have an insignificant effect on the storage life of meat packaged in a high-oxygen MAP pack (18). At chiller temperatures, the growth rate of pseudomonads is approximately halved by 20% C02 in the atmosphere. Therefore, high-oxygen MAP packaging will at best double the time before microbial spoilage of meat develops. Microaerobic or anaerobic conditions severely inhibit or suppress the growth of pseudomonads. Under essentially anaerobic conditions, low-pH (<5.8) meat will develop a flora composed solely of anaerobic but aerotolerant lactobacilli. Lactobacilli produce acid or dairy spoilage flavors in meat, but only substantially after the bacteria have achieved their maximum numbers (19).

Under the same conditions, high-pH muscle tissue and meat with extensive fat cover, which is of neutral pH, will develop a flora containing facultatively anaerobic organisms in addition to the lactobacilli. Species of Enterobac-teriaceae will inevitably be present and cause putrid spoilage as they approach their maximum numbers. Two other species, Brochothrix thermosphacta and Alteromonas pu-trefaciens, may also be present when processing hygiene is poor. When those organisms are present in a spoilage flora, they can cause early spoilage when their numbers are relatively low; B. thermosphacta produces sour-aromatic odors and flavors, and A. putrefaciens causes putrid spoilage with much H2S.

High concentrations of C02 impose significant lags on the lactobacilli and B. thermosphacta before they can commence growth and thus halve the growth rates of these organisms. In addition, the minimum temperature for growth of B. thermosphacta is increased from about — 3°C to above 0°C. The effect of high C02 concentrations on the enterobacteria is even more pronounced, because their lag phase is extended to very long times, and they finally commence only very slow growth. The effect of high C02 concentrations of A. putrefaciens is less certain, but available evidence suggests that its growth is effectively suppressed (20).

Increasing concentrations of 02 in a nominally anaerobic CO2 atmosphere will tend to accelerate the weak growth of the facultative anaerobes. When the 02 concentration is sufficiently high, slow growth of pseudomonads will occur. Thus low-oxygen MAP atmospheres, with a range of 02 concentrations that usually increase during storage, tend to allow growth of mixed spoilage flora in which facultative anaerobes are a substantial fraction and in which even pseudomonads may occur. In contrast, ultra-low-oxygen-packaged meat will develop a flora of lactobacilli, irrespective of the meat pH. Poor microbiological condition before packaging has minimal effect on the type of flora that develops when the product is stored at 0°C or below. However, at higher storage temperatures, B. thermosphacta will grow on ultra-low-oxygen-packaged meat that was heavily contaminated with that organism during processing, at a rate similar to that of the lactobacilli. The presence of B. thermosphacta in the flora will cause early spoilage of the product. On ultra-low-oxygen-packaged meat, the ubiquitous enterobacteria will finally initiate growth to cause putrid spoilage when they approach the relatively low numbers of 106/cm2. Such putrid spoilage may, however, be preempted by acid-dairy spoilage due to the lactobacilli that attain maximum numbers of about 108/cm2 well before the enterobacteria start to grow.

In addition to spoilage organisms, cold-tolerant pathogenic species may grow on chilled meat, and mesophilic pathogens may grow when the chilled meat experiences temperature abuse. As with the spoilage organism, pathogen growth is generally slowed by substantial concentrations of C02. However, the response of bacteria to C02 is highly varied, and under some circumstances, a C02-containing atmosphere may confer a growth-rate advantage on an individual pathogen over competing spoilage types. In that situation, the pathogen might reach hazardous numbers before the meat is rejected because of spoilage. Such a condition can develop with vacuum-packaged meat and might be expected in high- and low-oxygen MAP packs. However, in ultra-low-oxygen-packs, the combination of anoxia and high C02 concentration tends to raise the minimum growth temperature and extend the lag phase of pathogens so that their growth is disadvantaged relative to that of spoilage organisms (21).

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