Microbially induced changes in the volatile constituents of fresh chilled pasteurised milk during storage

• Relationship between microbial numbers and volatile organic compounds is not linear.
• VOC concentrations change once a threshold number of bacteria are exceeded.
• Bacterial numbers in milk may not accurately reflect consumer (odour) perception.
• VOC detection plus bacterial numbers data improves understanding of shelf-life.

Off-odours caused by volatile organic compounds (VOCs) are often the first indicators consumers have of milk spoilage. In this study the VOCs associated with three types (trim, 0.25–0.40% fat; lite, 1.40–1.50% fat; and full-cream, 3.18–3.28% fat) of fresh chilled pasteurised milk (FCPM), held for up to 17 days at 4.5 ± 0.5 °C, were measured using proton-transfer-reaction mass spectrometry (PTR-MS). The chemical identification of VOCs in the headspace of the milk was supported by SPME–GC–MS analysis. Bacterial numbers (aerobic plate count at 25 °C) in the milk were also estimated. Replicate sets of milk types treated with sodium azide (NaN3) to inhibit microbial activity were investigated. The relationship between microbial numbers and VOCs was not linear; rather the concentrations of VOCs only started to change after a threshold number of bacteria ranging from 106–108 CFU mL−1 was reached. This combined approach provided new insights on the effect of microbial growth on FCPM shelf-life.