Sorbate (sorbic acid) generally is an effective inhibitor of most molds and yeasts and some bacteria. Environmental factors such as pH, water activity, temperature, atmosphere, microbial load, microbial flora and certain food components can influence the effectiveness of sorbate. Strains of microorganisms resistant to sorbate exist and therefore are common causes of food spoilage. Some molds and bacteria are able to degrade sorbate. This paper reviews the factors that affect the antimicrobial effectiveness of sorbate in foods.

A bacteriocin-producing Pediococcus species inhibitory to Listeria monocytogenes was used to manufacture fermented semidry sausage. Separate 13.6 kg batches of a commercial summer sausage formulation were inoculated to contain an initial level of 106 cells/g of Listeria monocytogenes Scott A. In each of two independent studies, an ca. 2 log10 CFU/g reduction of L. monocytogenes occurred over the fermentation period, as compared to a less than 1 log10 CFU/g reduction in sausage fermented with a non-inhibitory Pediococcus strain. Inactivation of L. monocytogenes was also observed in one study where adequate acid production did not occur (pH>5.5), indicating that bacteriocin production occurred independently of carbohydrate fermentation. Following heating to an internal temperature of 64.4°C and storage up to 2 weeks, 9 of 90 sausages sampled were positive for Listeria. Recovery was intermittent and did not indicate that the bacteriocin was effective in eliminating L. monocytogenes that had survived the heating process.

The antimicrobial effects of two commonly used dairy plant sanitizers on Listeria monocytogenes ATCC 7644 were studied. The two sanitizers used were commercial sodium hypochlorite and quaternary ammonium compound (QAC). The effects were studied on L. monocytogenes in vitro and on stainless steel chips inoculated with the organism. Cells were exposed to concentrations of 0, 50, 100, 200, 400, and 800 ppm chlorine and QAC for 1, 2, and 5 minutes, and neutralized with tryptic soy broth. Decreases in cell numbers ranged from 3-logs to >4-logs in vitro, whereas with the stainless steel, it ranged from 1-log to >4-logs. Scanning electron microscopic (SEM) studies were done to evaluate the attachment characteristics of L. monocytogenes as compared to those of Escherichia coli on stainless steel. L. monocytogenes was found to produce a fibrous-like material similar in appearance to acidic polysaccharide fibrils produced by Pseudomonas sp., which appeared to be removed by the sanitizer solutions.

The ability of Listeria monocytogenes to survive and grow on head lettuce obtained from a retail outlet over a period of 10 months was determined. Lettuce was torn into bite sized pieces, inoculated with L. monocytogenes ATCC 7644, placed into plastic bags, and held under a variety of storage conditions. Samples stored at 5°C and 12°C were subjected to aerobic plate count analysis, and levels of L. monocytogenes were determined immediately after inoculation and after 7 and 14 d of storage. Samples stored at 25°C were sampled after inoculation and after 4 and 8 h storage. Lettuce juice was inoculated, stored at 5°C and sampled as described for head lettuce. Aerobic plate counts on lettuce stored at 5°C and 12°C increased greatly during the 14 d of storage. Behavior of L. monocytogenes was variable. In most trials, numbers increased by several log cycles during 14 d of storage, but in several trials growth never occurred or did not persist for 14 d. The same general growth patterns were observed in lettuce held at 25°C. Aerobic plate counts increased by 1 or 2 log cycles and L. monocytogenes increased by 1 log cycle, except for occasional trials where the organisms did not grow or survive. Lettuce juice held at 5°C was also able to support growth of L. monocytogenes. L. monocytogenes serotype 1 was isolated from some uninoculated samples indicating that the organism was naturally present on some of the lettuce heads purchased from retail outlets.