Milk Quality Improvement Program, Department of Food Science, Cornell University, Ithaca, NY 14853.
Milk Quality Improvement Program, Department of Food Science, Cornell University, Ithaca, NY 14853.
J Dairy Sci. 2020 Jun;103(6):5043-5046. doi: 10.3168/jds.2019-17792. Epub 2020 Apr 16.
Postpasteurization contamination (PPC) of fluid milk remains a challenge for some dairy processors. Pseudomonas is the most common contaminant of fluid milk after pasteurization, and therefore methods to detect PPC should be inclusive of Pseudomonas and other gram-negative contaminants (e.g., coliform bacteria). Our objective was to compare the ability of 3M (St. Paul, MN) coliform and Enterobacteriaceae (EB) Petrifilm to detect total gram-negative bacteria with that of the standard method, crystal violet tetrazolium agar. To that end, we evaluated coliform Petrifilm, EB Petrifilm, and crystal violet tetrazolium agar to detect gram-negative bacteria in naturally contaminated samples of fluid milk. A total of 92 observations derived from shelf-life testing of 33 milk samples from 5 different processing facilities were evaluated for (1) presence of coliforms on coliform Petrifilm at both 24 and 48 h of incubation; (2) presence of any growth, regardless of gas production, on coliform Petrifilm at both 24 and 48 h of incubation; (3) presence of EB on EB Petrifilm at both 24 and 48 h of incubation; (4) presence of any growth, regardless of gas or acid production, on EB Petrifilm at both 24 and 48 h of incubation; and (5) presence of gram-negative bacteria on crystal violet tetrazolium agar after 48 h of incubation. Sensitivity and specificity analysis of results indicated that compared with the standard method (i.e., crystal violet tetrazolium agar), the method that performed the best, based on balanced accuracy (i.e., the average of sensitivity and specificity), was coliform Petrifilm evaluated for the presence of any growth after 48 h of incubation (sensitivity = 0.787; specificity = 0.839). This method can be easily adopted by the dairy industry as many processing facilities already test for coliforms using coliform Petrifilm. Improving the ability of processors to detect PPC will improve the quality of the fluid milk supply.
巴氏消毒后污染（PPC）仍然是一些乳制品加工商面临的挑战。在巴氏消毒后，假单胞菌是液体牛奶中最常见的污染物，因此用于检测 PPC 的方法应包括假单胞菌和其他革兰氏阴性污染物（例如，大肠菌群细菌）。我们的目标是比较 3M（明尼苏达州圣保罗）大肠菌群和肠杆菌科（EB）Petrifilm 检测总革兰氏阴性菌的能力与标准方法结晶紫四唑琼脂的能力。为此，我们评估了大肠菌群 Petrifilm、EB Petrifilm 和结晶紫四唑琼脂在检测来自 5 个不同加工设施的 33 个牛奶样品的自然污染样本中的革兰氏阴性菌的能力。共有 92 个观察结果来自对来自 33 个不同牛奶样本的 33 个货架寿命测试的评估，这些观察结果用于：（1）在 24 和 48 小时孵育时，大肠菌群 Petrifilm 上是否存在大肠菌群；（2）在 24 和 48 小时孵育时，大肠菌群 Petrifilm 上是否存在任何生长，无论是否产气；（3）在 24 和 48 小时孵育时，EB Petrifilm 上是否存在 EB；（4）在 24 和 48 小时孵育时，EB Petrifilm 上是否存在任何生长，无论是否产气或产酸；（5）在 48 小时孵育后，结晶紫四唑琼脂上是否存在革兰氏阴性菌。基于灵敏度和特异性分析的结果表明，与标准方法（即结晶紫四唑琼脂）相比，基于平衡准确性（即灵敏度和特异性的平均值）表现最佳的方法是在 48 小时孵育后评估大肠菌群 Petrifilm 上任何生长的存在（灵敏度=0.787；特异性=0.839）。这种方法可以很容易地被乳制品行业采用，因为许多加工设施已经使用大肠菌群 Petrifilm 测试大肠菌群。提高加工商检测 PPC 的能力将提高液体牛奶供应的质量。
