Abstract
Cold-tolerant bacteria, also known as psychrotrophic bacteria, are notorious contaminants of milk in the refrigerated dairy food chain. These organisms, especially the pseudomonads, may produce heat-resistant enzymes that are responsible for the breakdown of proteins and lipids in milk and dairy products. Such reactions result in a variety of defects in the raw or unprocessed milk that may affect the suitability of such milk for further processing. The enzymes produced may cause defects in long-life dairy products such as cheese, butter and long-life milk. In the present study, a range of 18 yellow pigmented psychrotrophic bacteria, collectively known as flavobacteria, were isolated from local dairy products. One aim of this study was to identify these bacteria to species level using molecular techniques. A second aim was to determine the spoilage potential of these organisms based on profiles generated by the BIOLOG system (that may relate to hydrolytic enzymes produced). Of the 18 isolates, 14 belonged to the genus Chryseobacterium while 4 were identified as Empedobacter isolates. The most active spoilage organisms in this group were shown to be C. bovis, C. shigense and E. brevis. These findings illustrate that enzymatically catalysed defects in dairy products should not be attributed solely to acknowledged psychrotrophic bacteria such as the pseudomonads, but that flavobacterial species may also be actively involved.
Highlights
Extended cold storage times of raw milk at 2–6 °C have a significant influence on the natural microbial population
The results of the 16S rRNA sequencing indicated that 14 of the 18 dairy isolates belonged to the Chryseobacterium genus with sequence similarities between 94% and 99%, while four isolates belonged to the Empedobacter genus with sequence similarities between 96% and 98%
Isolate Hb8b was closely associated with the type strain of C. aquaticum which was originally isolated from a water reservoir.[26]
Summary
Extended cold storage times of raw milk at 2–6 °C have a significant influence on the natural microbial population. At these temperatures, the predominantly Gram-positive mesophilic aerobic bacteria are gradually replaced by Gram-negative and Gram-positive psychrotrophic bacteria.[1,2] Psychotrophic bacteria are able to grow at 7 °C or less regardless of their optimal growth temperature.[3] In addition to this ability, psychrotrophic bacteria have the ability to produce heat stable extracellular and/or intracellular hydrolytic enzymes which may retain their activity even after the conventional heat treatment of milk.[4]. These flavobacteria produce proteolytic enzymes that may survive pasteurisation[7,8,9] and cause bitter flavours, gelation of long-life heat treated milk[10] and the production of off-odours.[11,12,13] Jooste and Britz[8] found that the practical importance of dairy flavobacteria lies as much in their psychrotrophic growth and consequent proteinase production in refrigerated milk as in their contamination of milk via poorly sanitised pipelines and equipment
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