Department of Food Science and Human Nutrition, University of Illinois Urbana-Champaign, Urbana 61801.
Department of Food Science and Human Nutrition, University of Illinois Urbana-Champaign, Urbana 61801.
J Dairy Sci. 2018 Sep;101(9):7736-7745. doi: 10.3168/jds.2017-14037. Epub 2018 Jul 13.
Queso Fresco (QF), a fresh Hispanic-style cheese, is often associated with Listeria monocytogenes outbreaks and recalls. Queso Fresco's susceptibility to bacterial contamination is partially due to its high pH and moisture content as well as Listeria's tolerance for the salt content typical for QF. Nine different brands of US QF, 2 packages from 4 different lots (to account for temporal variability), were sampled. The pH, salt content, and moisture content were analyzed in addition to microbial testing including yeasts and molds, coliforms, lactic acid bacteria enumeration, and L. monocytogenes counts. The cheeses were also inoculated with a cocktail of 5 food and human isolates of food-borne outbreak-associated Listeria monocytogenes strains to evaluate how the differences between brands influenced Listeria growth. Three of the cheeses underwent additional genus-level microbial analysis using extracted 16S rDNA, allowing for phylogenetic analysis between bacterial taxa including diversity and relative abundance. We found little variation between the sampled QF pH (range = 6.62-6.86), salt content (1.53-2.01%), and moisture content (43.90-54.50%). Yeasts and molds were below the detection limit of enumeration in all of the cheeses and coliforms were below the detection limit across the first 3 lots, but were detected at varying levels in the fourth lot (>3.0 most probable number/g) for 3 of the brands. Listeria monocytogenes was not isolated after enrichment in any of the samples. All cheeses tested positive for the presence of lactic acid bacteria, with only 1 of the cheeses being labeled as produced with added cultures having substantial counts. Fourteen days after inoculation with L. monocytogenes, at least 2.5 log10 cfu/g of growth was found for all QF brands stored at 4°C. Microbial genus analysis showed that, among the 3 brands, the microbial community was more similar within brand than when compared with the other 2 brands. Thermus, Anoxybacillus, and Streptococcus accounted for the dominant genera of brands A, B, and C, respectively. These variations within the microbial community may account for sensory differences and help manufacturers determine quality control consistency more readily than culture-based methods.
鲜奶酪（QF）是一种新鲜的西班牙风格奶酪，常与李斯特菌单核细胞增生李斯特菌爆发和召回有关。鲜奶酪易受细菌污染的部分原因是其高 pH 值和水分含量，以及李斯特菌对 QF 典型盐含量的耐受性。从美国的 9 个不同品牌的 QF 中，抽样了 4 个不同批次的 2 个包装（以说明时间变异性）。除了微生物测试（包括酵母和霉菌、大肠菌群、乳酸菌计数和单核细胞增生李斯特菌计数）外，还分析了 pH 值、盐含量和水分含量。还将 5 种食源性病原体相关李斯特菌单胞菌的食物和人体分离株的混合物接种到奶酪中，以评估品牌之间的差异如何影响李斯特菌的生长。其中 3 种奶酪还进行了额外的属水平微生物分析，使用提取的 16S rDNA 进行分析，以便在包括多样性和相对丰度在内的细菌分类群之间进行系统发育分析。我们发现，采样的 QF 的 pH 值（范围=6.62-6.86）、盐含量（1.53-2.01%）和水分含量（43.90-54.50%）之间几乎没有差异。所有奶酪中的酵母和霉菌计数均低于可计数的检测限，在前 3 批中，大肠菌群的含量也低于检测限，但在第 4 批中，3 个品牌的含量在不同水平上有所检测（>3.0 最可能数/g）。在任何样本中，经过富集后都没有分离出单核细胞增生李斯特菌。所有奶酪都检测到存在乳酸菌，只有 1 种奶酪被标记为添加了大量培养物。在 4°C 下储存 14 天后，所有 QF 品牌的李斯特菌的生长量至少为 2.5 log10 cfu/g。微生物属分析表明，在 3 个品牌中，品牌内的微生物群落比与其他 2 个品牌相比更为相似。嗜热菌、嗜冷芽孢杆菌和链球菌分别占品牌 A、B 和 C 的主要菌群。这些微生物群落内的变化可能解释了感官差异，并帮助制造商比基于培养的方法更容易确定质量控制的一致性。
