Die voorkoms en betekenis van Acinetobacter in roumelk

MSc (Microbiologie), North-West University, Potchefstroom Campus

The practice of cold storage of large volumes of milk before processing for relatively long periods, poses potential quality problems for the dairy industry. Serious sensory defects and spoilage of milk and dairy products can mainly be related to the metabolic activity of different psychrotrophic genera at low storage temperatures. Acineto-bacter is a common contaminant in milk and can represent a dominant part of the Gram-negative psychrotrophic population in raw milk. The incidence and the potential spoilage ability of Acinetobacter in raw milk were investigated as well as the ability to attach to stainless steel milk contact surfaces under specific conditions. In this investigation 59 55 colonies were isolated on five isolation media from twenty raw milk samples. After primary and secondary phenotypic tests had been performed only thirty-eight isolates (0,6 %) conformed to the required characteristics of Acinetobacter. The ability of fifty-six Acinetobacter cultures to spoil milk (thirty-eight isolates and eighteen reference cultures) was determined. The ability of these cultures to grow at low temperatures was determined. Only ten isolates could not grow at 10°c, while thirty-six isolates grew at 7°C and ten at 4°C. Two organisms were killed by laboratory pasteurisation, while a significant decrease occurred in the numbers of the remaining fifty-four Acinetobacter cultures. From these results it can be concluded that these organisms are reduced by pasteurisation, but a certain percentage can survive it. The growth of Acinetobacter in milk did not cause any change in the pH of the milk. Although proteolysis was observed in some Acinetobacter cultures the proteolytic activity was consistently low. At 30°C only eight of the fifty-six cultures showed proteolysis, three at 10°C and one organism could hydrolyse casein at 4°C and 7°C. None of the cultures showed any defect which could be ascribed to lypolysis. Several cultures reduced litmus milk, but no acid or alkaline was produced. The latter observation is an indication that the organisms are obligate aerobic to such an extent that they are inclined to die off after short periods in liquid media. The presence of biogenic amines in various foods ( dairy products included) poses a health risk to the consumer. In this investigation thirty-five of the thirty-eight Acineto-bacter cultures isolated from raw milk produced tyramine, while nineteen cultures could produce histamine at 30°C. Acinetobacter could play an important role especially in the production of tyramine but also of histamine. The production of certain amines by Acinetobacter could make an important contribution to the sensory defects in milk. The production of extra-cellular polymeric substances and the ability of selected Acinetobacter cultures to attach to stainless steel surfaces were investigated, employing scanning electron microscopy. Alcianblue staining showed no production of extra-cellular polymeric substances. After the pre-conditioned stainless steel discs had been incubated in inoculated milk at 30°C and 7°C no attachment was observed. Except for the production of amines, the metabolic activities of Acinetobacter cultures affecting the quality of raw milk were negligible in this investigation.

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