Preliminary Incubation Method for Detection of Coliforms in Commercially Processed Milk Samples

Two surveys of 80 commercially processed milk samples revealed that 10% were positive for coliforms by the violet red bile agar method when tested within 24h of processing; however, 60% were positive after 10 days storage of the samples at 6.7°C. A preliminary incubation procedure that involved incubating samples for 6h at 37°C prior to plating on violet red bile agar, and a large plate (150mm d) method where 10ml of milk was plated directly on violet red bile agar were developed to detect coliforms. These two methods were compared with the standard violet red bile method and the most probable numbers procedure. Analysis of samples from 48 commercial processing plants after storage for 1, 7, 10, and 14 days at 6.7°C by the violet red bile agar method indicated coliforms in 13, 58, 60, and 75% of the samples, respectively. The large plate, preliminary incubation, and most probable numbers method on 1-day samples indicated coliforms in 58, 75, and 67% of the samples, respectively. All samples that were positive for coliforms after 14-days storage at 6.7°C by the standard violet red bile agar method were positive by the preliminary incubation method after 1 day at 6.7°C.