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Behavior of Listeria monocytogenes During Manufacture and Ripening of Brick Cheese

Department of Food Science and The Food Research Institute, University of Wisconsin-Madison, Madison 53706

ABSTRACT

Brick cheese was made by the washedcurd procedure from pasteurized whole milk inoculated to contain ca. 1 x 10² to 1 x 10³ Listeria monocytogenes [strain Scott A, Ohio, V7, or California]/ml. Cheeses were ripened (15°C/95% relative humidity) with a surface smear for 2, 3, or 4 wk to simulate production of mild, aged, or "Limburger-like" brick cheese, respectively, and then stored an additional 20 to 22 wk at 10°C. Populations of strains Scott A, Ohio, V7, and California increased 1.89, 1.72, .83, and .86 orders of magnitude, respectively, following completion of brining ca. 32 h after the start of cheese making. All four L. monocytogenes strains leached from cheese into brine during 24 h and survived in brine at 10°C at least 5 d after removal of cheese. Strains Scott A and Ohio grew rapidly during the initial 2 wk of smear development and attained maximum populations of ca. 6.6 and 6.2, 7.0 and 6.9, and 5.6 and 5.1 log₁₀/g in 4-wk-old slice (pH 6.0 to 6.5), surface (pH 6.5 to 6.9), and interior (pH 5.6 to 6.2) samples of cheese, respectively. Numbers of strains Scott A and Ohio generally decreased 1- to 7-fold during 20 to 22 wk at 10°C. Strains V7 and California failed to grow appreciably in any cheese during or after smear development, despite pH of 6.8 to 7.4 in fully ripened cheese; the strains were never isolated from 2- and 3-wk-old cheese and with direct plating were detected sporadically at levels generally 4.0 log₁₀/g in cheese aged 4 wk. Cold enrichment of slice, surface, and interior samples of cheeses aged 4 wk generally yielded positive results for L. monocytogenes; strains V7 and California were detected in all cheeses after 20 to 22 wk at 10°C. At 10 ppm, methyl sulfide, dimethyl disulfide, or methyl disulfide (compounds commonly produced during ripening of brick and Limburger cheese) failed to inhibit appreciably growth of L. monocytogenes.