Characterization of Psychrotrophic Bacterial Contamination in Commercial Buttermilk

Abstract

The extent to which fresh and stored commercial buttermilk was contaminated with psychrotrophic bacteria was determined with pour plates of crystal violet tetrazolium agar incubated at 7°C for 10 days. Fresh buttermilk samples were obtained from six dairy plants on 5 days during 3 mo. For these samples, psychrotrophic counts ranged from less than 10 to 86,000/ml. After samples were refrigerated for 14 days, counts ranged from less than 10 to 1,200/ml. Microbial isolates from the medium were identified, and their ability to reduce diacetyl was determined. The diacetyl content of fresh buttermilk ranged from undetectable to 8.6 µ/ml and declined after 7 and again after 14 days of refrigerated storage. The concentration of diacetyl in refrigerated buttermilk may be influenced by psychrotrophic contamination, but no consistent pattern was observed. Species of Pseudomonas, Enterobacter, Acineto-bacter, Escherichia, and Actinobacillus genera were isolated from the buttermilk. Isolates of these genera reduced diacetyl at 21°C in 24h 30 to 40%, 25 to 75%, 26%, 45%, and 48%. Yeast isolated from the buttermilk reduced 58% of the diacetyl under these conditions. Microbial contaminants of buttermilk have the potential to affect adversely flavor quality.