Gram-negative postpasteurization contamination patterns of single-serve fluid milk produced in 4 different processing facilities

Abstract

An analysis of historic data on high temperature, short time (HTST) fluid milk quality showed higher total bacterial counts and lower sensory defect judging scores at d 14 post-processing for milk packaged in single-serve containers as compared with milk packaged in half-gallon containers from the same processing facilities. As post-pasteurization contamination with gram-negative bacteria is likely a major contributor to an increased spoilage risk associated with milk packaged in single-serve containers, we performed a comprehensive assessment of the microbial quality and shelf-life of 265 commingled single-serve HTST fluid milk samples (including white (unflavored) skim, white (unflavored) 1%, chocolate skim, and chocolate 1%) collected over 2 visits to 4 commercial fluid milk processing facilities. Over 2 initial sampling visits, the frequency of gram-negative spoilage ranged from 14 to 79% of the product collected from the 4 facilities, with significant differences of gram-negative spoilage frequency between sampling visits, facilities (sampling visit 1, sampling visit 2, and both sampling visits combined), milk types (sampling visit 2), and filler lanes (sampling visit 2). There were no significant differences in the frequency of gram-negative spoilage between sampling time points (e.g., beginning, middle, and end of production run). Across facilities, single-serve containers of milk with gram-negative contamination showed significantly higher bacterial counts on d 7 and 14 and significantly lower sensory scores as compared with those without gram-negative contamination. Follow-up investigations, based on in-facility surveys that identified carton forming mandrels as filler components that frequently failed quality assurance ATP swab checks, found that bacterial genera, including Pseudomonas and Bacillus, isolated from single-serve milk samples were also frequently isolated from mandrels. While interventions aimed at improving cleaning and sanitation of mandrels did not lead to significant reduction of gram-negative spoilage frequency in a comparison of 398 control and 400 intervention samples, our data still suggest that the unhygienic design of single-serve fillers is likely a root-cause of gram-negative contamination of single-serve milk.