Papageorgiou Demetrios K, Marth Elmer H
Department of Food Science and the Food Research Institute, University of Wisconsin-Madison, Madison, Wisconsin 53706.
J Food Prot. 1989 Sep;52(9):625-630. doi: 10.4315/0362-028X-52.9.625.
Autoclaved samples of skim milk and deproteinated whey were fortified with 6 or 12% NaCl, inoculated with Listeria monocytogenes strains Scott A or California (CA), to contain ca. 1.0 × 10 cfu/ml (in the products with 6% salt) or ca. 5.0 × 10 cfu/ml (in the products with 12% salt) and incubated at 4 and 22°C. The pH values of the 6% salted whey, 6% salted skim milk, 12% salted whey, and 12% salted skim milk were 5.65, 6.20, 5.50, and 6.00 respectively. These values remained relatively constant during the entire experiment. Listeria counts were obtained by surface-plating appropriate dilutions and/or undiluted samples on Trypticase Agar (TA). Samples in which L. monocytogenes was not detected, were re-examined after 2, 4, 6 and 8 weeks of cold-enrichment. Generation times of L. monocytogenes in 6% salted whey at 22°C (3.67 h and 3.56 h for strains Scott A and CA, respectively) were significantly shorter than those in 6% salted skim milk at 22°C (4.31 and 4.42 h for the two strains, respectively). Generation times in 6% salted products at 4°C ranged between 37.49 h and 49.43 h. Maximum populations reached at 22 and 4°C ranged from 7.58 to 8.10 Log cfu/ml, and were significantly higher in 6% salted whey than in 6% salted skim milk. In 12% salted whey and skim milk incubated at 22°C, L. monocytogenes gradually decreased in numbers. Strain CA was inactivated within 85 d in 12% salted skim milk or within 110 d in 12% salted whey, and was significantly less salt tolerant than strain Scott A which survived for more than 130 d under the same conditions. Loss of viability by both strains was similar in 12% salted whey and skim milk after 130 d of storage at 4°C, and the decreases in population were less than 0.7 order of magnitude.
将脱脂牛奶和脱蛋白乳清的高压灭菌样品分别添加6%或12%的氯化钠，接种产单核细胞李斯特菌菌株Scott A或加利福尼亚（CA）菌株，使其在含6%盐的产品中约含1.0×10 cfu/ml，在含12%盐的产品中约含5.0×10 cfu/ml，并在4℃和22℃下培养。含6%盐的乳清、含6%盐的脱脂牛奶、含12%盐的乳清和含12%盐的脱脂牛奶的pH值分别为5.65、6.20、5.50和6.00。在整个实验过程中，这些值保持相对稳定。通过将适当稀释和/或未稀释的样品表面接种于胰蛋白酶琼脂（TA）上获得李斯特菌计数。未检测到产单核细胞李斯特菌的样品，在冷增菌2、4、6和8周后重新检测。产单核细胞李斯特菌在22℃的6%盐渍乳清中的代时（菌株Scott A和CA分别为3.67小时和3.56小时）显著短于在22℃的6%盐渍脱脂牛奶中的代时（两种菌株分别为4.31小时和4.42小时）。在4℃下，6%盐渍产品中的代时在37.49小时至49.43小时之间。在22℃和4℃下达到的最大菌数范围为7.58至8.10 Log cfu/ml，且6%盐渍乳清中的菌数显著高于6%盐渍脱脂牛奶中的菌数。在22℃下培养的12%盐渍乳清和脱脂牛奶中，产单核细胞李斯特菌数量逐渐减少。菌株CA在12%盐渍脱脂牛奶中85天内失活，在12%盐渍乳清中110天内失活，且其耐盐性显著低于菌株Scott A，后者在相同条件下存活超过130天。在4℃储存130天后，两种菌株在12%盐渍乳清和脱脂牛奶中的活力丧失相似，菌数减少小于0.7个对数级。
