Postpasteurization contamination (PPC) of high temperature, short time-pasteurized fluid milk by gram-negative (GN) bacteria continues to be an issue for processors. To improve PPC control, a better understanding of PPC patterns in dairy processing facilities over time and across equipment is needed. We thus collected samples from 10 fluid milk processing facilities to (1) detect and characterize PPC patterns over time, (2) determine the efficacy of different media to detect PPC, and (3) characterize sensory defects associated with PPC. Specifically, we collected 280 samples of high temperature, short time-pasteurized milk representing different products (2%, skim, and chocolate) and different fillers over 4 samplings performed over 11 mo at each of the 10 facilities. Standard plate count (SPC) as well as total GN, coliform, and Enterobacteriaceae (EB) counts were performed upon receipt and after 7, 10, 14, 17, and 21 d of storage at 6°C. We used 16S rDNA sequencing to characterize representative bacterial isolates from (1) test days with SPC >20,000 cfu/mL and (2) all samples with presumptive GN, coliforms, or EB. Day-21 samples were also evaluated by a trained defect judging panel. By d 21, 226 samples had SPC >20,000 cfu/mL on at least 1 d of shelf life; GN bacteria were found in 132 of these 226 samples, indicating PPC. Crystal violet tetrazolium agar detected PPC with the greatest sensitivity. Spoilage due to PPC was predominantly associated with Pseudomonas (isolated from 101 of the 132 samples with PPC); coliforms and EB were found in 27 and 37 samples with spoilage due to PPC, respectively. Detection of Pseudomonas and Acinetobacter was associated with lower flavor scores; coagulated, fruity fermented, and unclean defects were more prevalent in d-21 samples with PPC. Repeat isolation of Pseudomonas fluorescens group strains with identical partial 16S rDNA sequence types was observed in 8 facilities. In several facilities, specific lines, products, or processing days were linked to repeat product contamination with Pseudomonas with identical sequence types. Our data show that PPC due to Pseudomonas remains a major challenge for fluid milk processors; the inability of coliform and EB tests to detect Pseudomonas may contribute to this. Our data also provide important initial insights into PPC patterns (e.g., line-specific contamination), supporting the importance of molecular subtyping methods for identification of PPC sources.

高温，短时间革兰氏阴性（GN）细菌对巴氏杀菌后的液态奶进行巴氏杀菌后污染（PPC）仍然是加工商面临的问题。为了改善PPC控制，需要更好地了解随着时间的推移以及跨设备的乳品加工厂中的PPC模式。因此，我们从10个液态奶加工设施中收集了样本，以（1）随时间推移检测和表征PPC模式，（2）确定不同介质检测PPC的功效，以及（3）表征与PPC相关的感觉缺陷。具体来说，我们在10个工厂中的11个月进行的4次采样中，收集了280个高温，短时间巴氏杀菌牛奶样品，分别代表不同的产品（2％，脱脂和巧克力）和不同的填充剂。标准板数（SPC）以及总GN，大肠菌群和肠杆菌科（EB）的计数在收到时以及在6°C下储存7、10、14、17和21天后进行。我们使用16S rDNA测序来表征（1）SPC> 20,000 cfu / mL的测试日和（2）所有假定的GN，大肠菌或EB样品的代表性细菌分离株。还由训练有素的缺陷判断小组对21天的样品进行了评估。到第21天，在至少1天的保质期内，有226个样品的SPC> 20,000 cfu / mL。在这226个样本中的132个中发现了GN细菌，表明存在PPC。结晶紫四唑琼脂检测PPC的灵敏度最高。PPC导致的腐败主要与假单胞菌有关（从132个PPC样本中分离出101个）；分别在27和37个样本中发现大肠菌群和EB因PPC变质。检测假单胞菌和不动杆菌与较低的风味得分相关；在带有PPC的d-21样品中，凝固，果味发酵和不清洁的缺陷更为普遍。在8个设施中观察到重复分离的荧光假单胞菌群菌株具有相同的16S rDNA部分序列类型。在一些设施中，将特定的生产线，产品或加工天数链接在一起，以重复的产品假单胞菌污染，且序列类型相同。我们的数据表明，假单胞菌引起的PPC仍然是液态奶加工者面临的主要挑战。大肠菌群和EB测试无法检测假单胞菌可能对此有所贡献。我们的数据还提供了对PPC模式（例如，特定于行的污染）的重要初步见解，支持了分子亚型分析方法对PPC来源鉴定的重要性。