Generally, spoilage of pasteurised milk is a quality failure not a food safety issue. As such whether the milk is suitable for consumption, is generally determined immediately prior to consumption by the consumer using visual and/or odour cues. Microbial spoilage of milk in New Zealand and Australia is typically due to gram negative bacteria like Pseudomonads and occurs as a result of post pasteurisation contamination.

Our objective was to the understand relationship between microbial numbers and off-odour generation, including the types of microorganisms present; the types and concentration of volatile compounds (odour) microorganism generate; and how this related to consumers’ perception of product quality. This was achieved by measuring volatile organic compounds (VOCs) by proton transfer reaction mass spectrometry (PTR-MS) in milk stored at 4°C with and without inoculation of bacteria isolated from spoiled milk. Consumer perception of milk quality was determined using rejection threshold methodology.

In milk stored at 4°C a significant increase in VOC was not observed until a threshold number of microorganisms of about 1x10⁷ CFU/mL were present. Consumers determined that the quality of the stored milk was inferior to freshly produced milk when microbial numbers were about 2.7x10⁷CFU/mL. In addition, it was found two Pseudomonas fluorescensstrains produced different levels of VOCs at the same microbial numbers. Therefore, spoilage microorganisms differ in the concentration and composition of the VOC they produce, with some microorganisms capable to causing the rejection of the food by consumers at lower total bacterial numbers than others. A better understanding of the progression of spoilage and the factors that cause consumers to reject food products and the associated microorganisms responsible for the spoilage will allow food manufacturers to optimise product shelf life.

Phil Bremer

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