Quantifying changes in spore-forming bacteria contamination along the milk production chain from farm to packaged pasteurized milk using systematic review and meta-analysis

Approximately one-third of the fluid milk produced in the United States is lost annually. One important factor contributing to the loss is the contamination with spore-forming bacteria, which can not only survive the pasteurization process but also grow under refrigeration conditions, resulting in subsequent spoilage. The objective of this study was to describe the population dynamics of spore-forming bacteria in milk, from the farm to the packaged product, through a systematic review approach. A database search was conducted to identify, appraise, and summarize primary research studies that described the prevalence and/or concentration of spore-forming bacteria throughout the fluid milk supply chain. Due to variations in sampling points in reported studies, the pasteurized milk supply chain was standardized to include the following steps: “milking machine” “raw milk” “bulk tank” “transportation” “silo” “pasteurized milk” and “packaged milk”. A literature search retrieved 9,778 citations, from which 46 relevant citations were selected for data extraction and subsequent meta-analysis. Concentration data were more abundant with 758 data points, as compared to prevalence data with 74 data points. In general, great heterogeneity was observed among studies on the contamination in milk samples with spore-forming bacteria. The findings showed that the concentration of spore-forming bacteria in milk samples increased within the range of 0.58–2.41 logs CFU/mL from raw milk to pasteurized milk according to the weighted mean estimates. Similarly, the prevalence of contaminated samples with spore-forming bacteria increased from 23% on farm, to up to 58% at the step of “pasteurized milk”. Meta-regression analysis indicates that the variables, season and location, of the study, accounted for 56.35% of the between-study heterogeneity. Although considerable primary research has been conducted on this topic, there are limited studies that comprehensively describe the dynamic changes of spore-forming bacteria under the current milk production system. In summary, the analyses based on comprehensively collected evidence show that the contamination of spore-forming bacteria originating from the farm remains stable with steady increases as the milk moves downstream. These findings indicate that in addition to on-farm interventions, special attention should be paid to introducing effective mitigation measures at the processing stage to further lower spore-forming bacteria levels introduced by the raw milk, and to prevent post-pasteurization contamination from raw ingredients and processing environments.