Variations in preenrichment pH of poultry feed and feed ingredients after incubation periods up to 48 hours

Human salmonellosis outbreaks have been linked to contamination of animal feeds. Thus, it is crucial to employ sensitive Salmonella detection methods for animal feeds. Based on a review of the literature, Salmonella sustains acid injury at about pH 4.0 to 5.0. Low pH can also alter the metabolism of Salmonella, hampering the identification process. Because most Salmonella bacteria present in feed and feed ingredients are already stressed by desiccation and heat during conditioning, pelleting, or both, exposure to low pH may kill or severely injure these cells and result in contamination going undetected. Five grams of 7 different feed ingredients and 5 types of feed were added to 45 mL of buffered peptone water (BPW), lactose broth (LB), minimal salts medium (M-9), or universal preenrichment (UP). Media were incubated at 37 degrees C for 18, 24, and 48 h, and the pH of the media was determined. In the case of cereal grains, in general, the initial pH of the preenrichment media ranged from 6.1 to 7.2. Distillers dried grains with solubles in lactose broth was the exception to this trend (pH 4.5). After 18 h of incubation, the pH of the media had decreased to a pH range of 4.0 to 6.1. The preenrichment media behaved in a similar fashion when testing oilseed meals, canola, and soy (i.e., initial pH range was 6.2 to 7.2 and decreased to pH 4.2 to 5.4 for soy and pH 4.4 to 6.0 for canola). With these ingredients, the buffering capacity of BPW, LB, and M-9 was similar. There was more variation in the pH of the preenrichment media among the different poultry feed types. The media from broiler and layer feeds had an initial pH of 6.2 to 7.0, whereas turkey feed was 6.4 to 7.1. After 18 to 48 h incubation in broiler and in layer feed, the pH was 3.9 to 5.6, compared with pH values of 4.5 to 6.8 for turkey feed. For feed, the buffering capacities of BPW, LB, and M-9 were similar. After incubation in commonly used preenrichment media, mixed feeds and feed ingredients can reach a pH that may kill or injure Salmonella. In light of these data, detection methods for Salmonella in feed and feed ingredients may need to be re-evaluated.