Discipline: contamination; Key words: identification, spoilage, Chryseobacterium, Empedobacter, dairy
Long term cold storage of raw milk at 2 to 6 °C has 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. Psychotrophic bacteria are able to grow at 7 °C or less even though their optimal growth temperature is higher. As such, they are notorious contaminants of milk in the refrigerated dairy food chain. In addition to this ability, psychrotrophic bacteria also have the ability to produce heat stable enzymes which may retain their activity even after the normal heat treatment of milk. They, especially the pseudomonad group, are then 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 study of the authors cited below, a range of 18 yellow pigmented psychrotrophic bacteria, collectively known as flavobacteria, were isolated from local dairy products. One aim of the 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 based on the relation to hydrolytic enzymes produced (the so-called BIOLOG system).
Of the 18 isolates, 14 belonged to the genus Chryseobacterium while 4 were identified as Empedobacter isolates. These flavobacterial species are believed to have the potential to cause spoilage defects in dairy products because they were able to utilise a wide range of compounds in the BIOLOG system. In this regard C. bovis, C. shigense and E. brevis were the most active and could utilise 16 of the 22 carbon sources. This was followed by C. aquaticum (10/22), C. indoltheticum and C. indologenes (9/22).
These results 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.
L. I. Tsôeu, P. J. Jooste, G. Charimba & C. J. Hugo, 2016. Spoilage potential of a novel group of bacteria isolated from dairy products. S.A. Journal of Science 112 (1/2), 140-147.