The Fate of Listeria Monocytogenes in Fermented Sausages and in Vacuum-Packaged Frankfurters

Abstract

In inoculated fermented sausages, the initial “most probable number” (MPN) of Listeria monocytogenes decreased from 8 × 106/g to 1.1 × 104/g during the production process. When the initial MPN was 6.5 × 102/g, L. monocytogenes were detected in finished sausages only when 25-g samples were used. The MPN of L. monocytogenes in finished experimental sausages remained constant during 20 d of storage at room temperature. The average number of lactobacilli in the experimental sausages was initially 1.2 × 107/g, reached 8 × 107/g after ripening, and decreased to 2.2 × 107/g in finished sausages. Sterilized filtrates of 12 Lactobacillus plantarum broth cultures produced zones of inhibition of L. monocytogenes growth on solid medium, when the pH of the filtrates ranged from 3.47 to 4.52. When the pH of the same filtrates was adjusted to 7.0, only slight zones of inhibition were registered with filtrates of 7 out of the 12 L. plantarum strains examined. In vacuum packages of surface-contaminated frankfurters, the MPN of L. monocytogenes increased from 5 × 102 to 2.1 × 105 per package during 20 d of storage at 4°C. In the same packages, the number of lactobacilli increased from 2.4 × 105 to 6.6 × 109 per package.