- The production of Cheddar cheese is a complex process with multiple potential sources of undesirable bacteria, including those that have negative impacts on product quality (spoilage organisms) and those that are used to evaluate sanitary conditions in the production environment (indicator organisms). The dairy industry commonly uses coliform bacteria as indicators of potential fecal contamination or unsanitary conditions.
The aim of this study is to identify potential sources and conditions in a commercial Cheddar cheese production facility that contribute to intermittent coliform detection in finished cheese. To approach this goal, we worked closely with an industry partner to analyze product and surfaces of the commercial processing facility to identify locations and conditions that promoted the growth of coliforms during Cheddar production. Historical commercial production data was analyzed to formulate initial hypotheses for potential coliform sources. A series of investigative sampling events were completed to confirm sources and locations as conditions that promote coliform growth. A second lab-scale study was conducted as a “proof-of-concept” to isolate the contribution of coliforms from raw product that lead to finished product contamination.
Historical production data indicated a higher likelihood of coliform detection in finished cheeses that were produced later in the production day. Samples of raw milk, heat treated milk, whey, curd, and surface swabs were collected at different points in the production cycle and enumerated for coliforms using MacConkey agar incubated at 37°C for 24 hrs. Results confirmed the efficacy of heat treatment to reduce (to < 1 CFU/ml), but not eliminate, coliforms in milk. Coliform levels remained low in the vat and upon entry via the weir of the draining and matting conveyor (DMC). At the beginning of the production day, coliform levels remained low in all parts of the cheese production process. However, as the production day extended (18 hrs since sanitation), coliform counts in the DMC increased to 5.04 Log CFU/mL in whey collected below the drain belt and 2.20 Log CFU/g in curd collected just before the mill. Surface swabs of belts inside the DMC indicated that coliform subpopulations were increasing in each section of the DMC throughout the production day; however, the largest increased were found on the drain belt (5.53 ± 0.10 log CFU/swab) and belt 1 (5.46 ± 0.63 log CFU/swab). Pre- and post-sanitation swab results suggested that low levels of coliforms may be surviving on DMC belt surfaces. During the annual replacement of the draining belt, belt sections were collected and evaluated for post-sanitation survival using traditional cultural (enrichment-isolation-identification) and visualization (scanning electron microscopy; SEM) methods. Cultural methods confirmed that sanitized belt pieces (7/32; 22%) harbored low levels of Enterobacter sp., Escherichia fergusonii, Klebsiella pneumoniae, and K. variicola. SEM provided evidence of large clusters of bacteria within belt cracks after sanitation. Taken collectively, in-plant sampling demonstrated two sources coliforms (low levels surviving heat treatment and harborage sites in belt pieces in the DMC) that serve as the seeds for coliform growth in early stages of the DMC during the production day. A lab-scale single-pass continuous flow system was designed to model the beginning section of the DMC to evaluate the potential growth of low levels of coliforms entering the DMC from naturally contaminated whey. Cheddar whey was sourced from the OSU Arbuthnot Dairy and tempered to 35°C for flow through a CDC bioreactor containing stainless steel and polypropylene coupons. Whey and coupons were enumerated for various subpopulations (coliforms, lactic acid bacteria, and pseudomonads) of bacteria after 0, 12, and 18 h of continuous flow and select isolates were identified by 16S rDNA sequencing. Non-starter bacteria present in the whey at 0 h included coliforms (Enterobacter), Pseudomonas, and Acinetobacter (0.80, 2.55, 2.32 log CFU/mL respectively), with each increasing significantly in whey (6.18, 7.00. 5.89 log CFU/mL) and on coupons (5.20, 6.85, 5.29 log CFU/cm2, respectively) after 18 hrs of flow. Results from the lab-scale study demonstrated that naturally low levels of coliforms entering the DMC in the whey could replicate within the conditions of the draining section of the DMC to the levels found in the commercial production environment.
Continuous environmental conditions (pH, temperature, moisture and nutrients) within the DMC support the growth of various subpopulations of non-starter bacteria, including coliforms. Food contact surfaces, including conveyor belts can harbor bacteria in cracks and defects that survive routine sanitation. Our lab-scale model system demonstrated that low levels of coliforms in incoming product entering the DMC can increase on surfaces exposed to a continuous flow of whey. Low level coliforms in incoming product as well as surviving bacteria on belt surfaces could serve as the seed for high levels of coliforms in the draining section of the DMC and could lead to finished production contamination. Production schedule, sanitation frequency, and the age or condition of conveyor belts are factors that contribute to intermittent coliform contamination in Cheddar cheese.