Effect of modified atmosphere packaging on growth of Listeria monocytogenes and nonproteolytic Clostridium botulinum in cooked turkey

The growth of Listeria monocytogenes and nonproteolytic Clostridium botulinum type B spores in cooked, uncured turkey was investigated separately under varying conditions of modified atmosphere packaging (MAP), refrigerated and temperature-abuse storage, and lactic acid bacteria (LAB) competition. L. monocytogenes (LM) growth was suppressed when initially outnumbered 3.5 logs:1 or 2.1 logs:1 by naturally-occurring LAB, but not when the initial LAB:LM population ratio was equivalent. Lowering storage temperature from 10 degrees to 4 degrees C enhanced the inhibitory effect of CO₂ in the packaging atmosphere, and extended the period of product olfactory acceptability. When the LAB:LM population ratio was equivalent, objectionable odors were not detected in most of the samples, despite LAB counts in excess of 10E⁸/g. This raises concerns with respect to public health, since high levels of L. monocytogenes can be present in MAP meat and poultry products without accompanying signs of overt spoilage.

Cellular fatty acid (CFA) analysis was a valuable tool for distinguishing between phenotypically distinct isolates of LM inoculated into MAP turkey. Fatty acid composition of foodborne outbreak-associated (serotype 4) and non-outbreak-associated (serotype 1) strains of LM correlated with antigenic type (4 or 1) and agglutination reaction (granular or flocculent). Strain ATCC 43256 (serotype 4b) produced a consistently unique CFA profile, making it the easiest of the four test strains to be differentiated. Analysis of additional LM serotypes, as well as examination of existing clinical and environmental CFA databases for correlations between fatty acid profiles and diagnostic characteristics of LM, is necessary before CFA analysis can be effectively applied as an epidemiological tool for tracking the distribution of LM strains in food products and throughout the farm-to-table food chain.

Reduced storage temperature significantly delayed botulinal toxin production and extended the period of olfactory acceptability of cooked turkey, but even strict refrigeration did not prevent growth and toxigenesis by nonproteolytic C. botulinum type B. Toxin was detected on day 7 for product stored at 15 degrees C and on day 14 for product stored at 10 degrees C, irrespective of packaging atmosphere. At 4 degrees C, toxin was detected on day 14 for product packaged without carbon dioxide, and on day 28 for product packaged with 30% carbon dioxide. At all three storage temperatures, toxin detection preceded or coincided with olfactory unacceptability, demonstrating the potential for consumption of toxic product when spoilage-signalling sensory cues are absent.