Survival and growth of alkali-stressed Listeria monocytogenes on beef frankfurters and thermotolerance in frankfurter exudates

Abstract

Cells of Listeria monocytogenes exposed at 4°C to 1% solutions of two alkaline cleaners or alkali-adapted in tryptose phosphate broth (pH 10.0) at 37°C for 45 min, followed by 4°C for 48 h, were inoculated onto beef frankfurters containing high fat (16 g) and high sodium (550 mg) or low fat (8 g) and low sodium (250 mg) per 57-g serving. Frankfurters were surface inoculated (2.0 log10 CFU/g), vacuum packaged, stored at -20, 4, or 12°C, and analyzed for populations of L. monocytogenes at 2-day to 2-week intervals. Populations did not change significantly on frankfurters stored at -20°C for up to 12 weeks. After storage at 4°C for 6 weeks (1 week before the end of shelf life), populations of control cells and cells exposed to alkaline cleaners were ca. 6.0 log10 CFU/g of low fat, low sodium (LFLS) frankfurters and ca. 3.5 log10 CFU/g of high fat, high sodium (HFHS) frankfurters. Growth of alkali-adapted cells on both types of frankfurters was retarded at 4°C. Growth of L. monocytogenes on frankfurters stored at 12°C was more rapid than at 4°C, but a delay in growth of alkali-adapted cells on HFHS and LFLS frankfurters was evident during the first 9 and 6 days, respectively. Alkali-adapted cells had a significantly (P ≤ 0.05) lower logistic D59°C-value (decimal reduction time) than alkaline cleaner-exposed cells, but the D59°C-value was not different from that of control cells. Cells exposed to a nonbutyl alkaline cleaner, and then heated in LFLS frankfurter exudates, had a significantly lower D62°C-value than cells that had been exposed to some of the other treatments. Growth characteristics of L. monocytogenes inoculated onto the surface of frankfurters may be altered by previous exposure to alkaline environments. Differences in growth characteristics of L. monocytogenes on HFHS versus LFLS beef frankfurters stored at refrigeration temperatures indicate that composition influences the behavior of both alkaline-stressed and control cells.