Population dynamics of coliforms in a commercial Cheddar cheese production facility

The detection of coliforms in young cheese is a potential indication of undesirable microbial growth within the processing environment. The aim of this study was to investigate sources and conditions that lead to the intermittent detection of coliforms (1-3 log cfu/g) in young Cheddar cheese at a single commercial facility. Analysis of historical production data, in combination with iterative investigative sampling events, was performed to determine coliform levels in milk, whey, curd, and surfaces at the beginning, middle, and end of the production day. After sanitation, conveyor belt pieces from the draining and matting conveyor (DMC) were collected and evaluated for bacterial survivors using culture-based methods and scanning electron microscopy. Production data analysis indicated that cheese produced later in the production day (≥16 h) was significantly more likely to test positive for coliforms than cheese made earlier in the production day (<12 h). Enumeration of coliforms in raw and heat-treated milk demonstrated that the subpasteurization thermal treatment (67-70°C, 26-28 s) was effective at reducing, but not eliminating, coliforms. Repeated sampling identified the DMC, particularly the drain belt and belt 1, as a critical area that supported coliform growth during the production day. Coliform levels in whey entering the weir maintained a level of <1 cfu/mL throughout production; however, coliform levels in whey below the drain belt increased from <1 cfu/mL at midday (8 h) to 5.04 log cfu/mL by the end of the production day (~18 h). Routine sanitation inside the DMC resulted in undetectable coliform levels on easily accessible surfaces. However, enrichment and scanning electron microscopy of belt sections revealed pockets of viable coliforms and other bacteria in cracks and defects in conveyor belts, indicating that sanitation did not eliminate all viable bacteria. Low levels of coliforms are present in heat-treated milk and survive sanitation in the DMC and could serve as the initial seed for high levels of coliforms at the end of the production day.