Acid adaptation does not promote survival or growth of Listeria monocytogenes on fresh beef following acid and nonacid decontamination treatments

The objective of this study was to evaluate the survival and growth of acid-adapted and nonadapted Listeria monocytogenes inoculated onto fresh beef subsequently treated with acid or nonacid solutions. Beef slices (2.5 by 5 by 1 cm) from top rounds were inoculated with acid-adapted or nonadapted L. monocytogenes (4.6 to 5.0 log CFU/cm(2)) and either left untreated (control) or dipped for 30 s in water at 55degreesC, water at 75degreesC, 2% lactic acid at 55degreesC, or 2% acetic acid at 55degreesC. The beef slices were vacuum packaged and stored at 4 or 10degreesC and were analyzed after 0, 7, 14, 21, and 28 days of storage. Dipping in 75degreesC water, lactic acid, and acetic acid resulted in immediate pathogen reductions of 1.4 to 2.0, 1.8 to 2.6, and 1.4 to 2.4 log CFU/cm(2), respectively. After storage at 10degreesC for 28 days, populations of L. monocytogenes on meat treated with 55degreesC water increased by ca. 1.6 to 1.8 log CFU/cm(2). The pathogen remained at low population levels (1.6 to 2.8 log CFU/cm(2)) on acid-treated meat, whereas populations on meat treated with 75degreesC water increased rapidly, reaching levels of 3.6 to 4.6 log CFU/cm(2) by day 14. During storage at 4degreesC, there was no growth of the pathogen for at least 21 days in samples treated with 55 and 75degreesC water, and periods of no growth were longer for acid-treated samples. There were no differences between acid-adapted and nonadapted organisms across treatments with respect to survival or growth. In conclusion, the dipping of meat inoculated with L monocytogenes into acid solutions reduced and then inhibited the growth of the pathogen during storage at 4 and 10degreesC, while dipping in hot water allowed growth despite initial reductions in pathogen contamination. The results of this study indicate a residual activity of acid-based decontamination treatments compared with water-based treatments for refrigerated (4degreesC) or temperature-abused (10degreesC) lean beef tissue in vacuum packages, and these results also indicate that this activity may not be counteracted by prior acid adaptation of L. monocytogenes.