Purpose : Assess the overall ecology of psychrotolerant coliforms in pasteurized fluid milk and identify common psychrotolerant coliform genera associated with refrigerated, pasteurized fluid milk.
Methods : Coliform bacteria were isolated from milk samples submitted over the course of one year to the Voluntary Shelf-life (VSL) Program at the Milk Quality Improvement Laboratory at Cornell University. Packaged pasteurized products collected were representative of products processed at each facility, including whole fat, reduced fat, lowfat, and nonfat milk in quart, half gallon, or gallon containers. Samples were plated in duplicate on Petrifilm Coliform Count plates according to manufacturer’s directions on initial day of sample arrival and days 7, 14, and 21 of refrigerated shelf-life (6ºC). Coliform-positive isolates were selected from each duplicate Petrifilm plate and subsequently characterized using 16s rDNA PCR and Sanger sequencing.
Results : A total of 467 coliform isolates were collected. Preliminary molecular characterization of 96 isolates identified 9 different coliform genera in the samples: Raoultella spp., Serratia spp., Citrobacter spp., Klebsiella spp., Kluyvera spp., Pantoea spp., Enterobacter spp., Hafnia spp., and Buttiauxella spp. Of these genera, the most frequently isolated were Raoultella spp., which accounted for 26% (25/96) of the isolates; Hafnia spp. which accounted for 16% (15/96); Serratia spp. which accounted for 15% (14/96); Enterobacter spp. which accounted for 13% (12/96); and Citrobacter spp. which accounted for 9% (9/96) of the isolates.
Significance : The coliform genera isolated from the milk samples are associated with sensory defects in fluid milk and in other dairy products including cheese, cottage cheese, butter, and ice cream. Identification of specific organisms responsible for product spoilage is an essential first step toward development of targeted strategies for control and elimination of key spoilage organisms.