AbstractsBiology & Animal Science

The stated shelf life of fresh chilled pasteurized milk (FCPM) in New Zealand is 14 days, which although considered long enough to meet the needs of the domestic market, limits its export potential. The shelf life of FCPM is usually limited by the growth of microorganisms including Gram-positive spore-forming bacteria that survive the pasteurization process and psychrotrophic Gram-negative bacteria (particularly Pseudomonas sp.) that can re-contaminate the milk during bottling. During growth, microorganisms produce different combinations and concentrations of volatile organic compounds (VOCs) in FCPM, some of which are perceived as off-flavours and influence the sensory quality and shelf life of FCPM. The total number of microorganisms in milk is used as a broad indicator of quality by the dairy industry. However, FCPM containing high microbial numbers can be palatable, while FCPM with lower microbial numbers can be rejected by consumer depending on the activity of the contaminating microbes. Therefore, to ensure the production of FCPM with an export potential (i.e., shelf life > 14 days), a better understanding of the effect of microorganisms (types and numbers) on the sensory quality and the shelf life of FCPM is required. The present study was comprised of 5 experiments that covered three broad tasks. The first task focused on studying changes in the microbial population and the VOC composition (using proton transfer reaction – mass spectrometry (PTR-MS)) in three types of FCPM, namely whole fresh chilled pasteurized milk (WFCPM), high protein – calcium fortified milk (HP-Ca), and skim (Trim) milk during extended storage for up to 26 days (Chapter 4). This experiment was followed by two sensory studies which examined the relationship between changes in the microbial quality, the VOC composition, and the sensory quality of FCPM during storage using firstly a consumer based method (Chapter 5) and then a trained descriptive panel (Chapter 6). The experimental work was concluded by two inoculation studies (Chapter 7 and 8), which investigated the spoilage potential of a number of microorganisms, isolated from aged FCPM (3-21 days) and inoculated into FCPM as either pure or mixed cultures of microorganisms. It was found that post pasteurization contamination (PPC) with Gram-negative bacteria is what primarily limits the shelf life of FCPM in New Zealand. In the absence of PPC, the milk maintained a good quality and had a shelf life of beyond 14 days. The initial microbial population in FCPM was mainly comprised of spore-forming bacteria, however, Gram-negative psychrotrophic microorganisms dominated the microbial population as the milk aged. The different species and numbers of microorganisms entering the milk via PPC caused batch-to-batch and within batch variations in the shelf life of FCPM, due to differences in their initial numbers at the time of contamination, growth rates, and spoilage potential. Owing to this variation in the species and numbers of microorganisms in the milk, it was determined…