Inhibitory effects of raw carrots on Listeria monocytogenes

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Abstract
The survival and growth of two strains of Listeria monocytogenes on raw and cooked carrots stored at 5 and 15.degree.C and in carrot juice media at 30.degree.C were investigated. The influence of shredding, chlorine treatment, and packaging under an atmosphere containing 3% O2 and 97% N2 on the behavior of L. monocytogenes and naturally occurring microflora was determined. Populations of viable L. monocytogenes decreased upon contact with whole and shredded raw carrots but not cooked carrots. Viable populations also decreased in cell suspensions in which raw carrots were dipped. Small populations of L. monocytogenes detected on whole carrots immediately after dipping were essentially nondetectable after 7 days of storage at 5 or 15.degree.C. After a lag of 7 days at 5.degree.C, significant (P .ltoreq. 0.05) increases in populations were detected on shredded carrots after 24 days of storage. Carrots stored at 5 or 15.degree.C spoiled before L. monocytogenes grew. Populations of mesophilic aerobes, psychrophiles, and yeasts and molds increased throughout storage at 5 and 15.degree.C. Cutting treatment (whole or shredded carrots), chlorine treatment, and modified-atmosphere packaging had no effect on the survival or growth of L. monocytogenes or naturally occurring microflora. The presence of raw carrot juice in tryptic phosphate broth at a concentration as low as 1% substantially reduced the maximum population of L. monocytogenes reached after 24 h at 30.degree.C. The anti-Listeria effect of carrots was essentially eliminated when the carrots were cooked. Additional research is needed to characterize, on a molecular level, the component(s) in raw carrots which is toxic to L. monocytogenes.