Abstract
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 that storage time correlated poorly with VOC and the end of shelf life. Rather, microbial numbers correlated well with increases in VOC concentrations and the end of shelf life which was determined at an estimated rejection threshold (RjT) by consumers occurring when microbial numbers exceeded > 107 CFU.mL-1. The RjT (end of shelf life) was defined as when a unanimous decrease in consumers’ preference occurred due to changes in the sensory characteristics of FCPM. At the same RjT, significant changes in the signal intensities for a number of spoilage-related volatile mass ions occurred in the headspace of FCPM (p≤0.05), including m/z 33, 45, 47, 61, 63, 71 and 89, which were tentatively identified as being methanol, acetaldehyde, ethanol, acetic acid, dimethyl sulfide, butanoic acid or 2-pentanol respectively.
Extended storage of WFCPM (26 days) resulted in significant changes in a number of the key sensory characteristics of the milk determined by descriptive sensory analysis namely cheesy odour, cultured odour, barny odour, cheesy flavour, cultured flavour, creamy flavour, sweet taste, mouth coating, sweet after swallow, and creamy after swallow (p≤0.05). These changes had a weak relationship with the microbial quality due to the low number of microorganisms detected in WFCPM throughout the storage period. However, a better relationship between changes in the sensory characteristics and the VOC composition of WFCPM was found suggesting that the contaminating microorganisms had strong spoilage potential despite their occurrence at low numbers.
Inoculating FCPM with microorganisms isolated from aged FCPM showed that microorganisms varied in their ability to grow in FCMP at 4.5 °C ± 0.5 and to cause changes in its VOC composition (i.e., had differing spoilage potential). In addition, inoculating milk with different combinations of mixed bacterial cultures of two strains of Pseudomonas fluorescens and Chryseobacterium sp. resulted in VOC composition that is similar but not equal to that inoculated with pure cultures of the dominating bacteria. This could be ascribed to the interaction (competition and/or synergism) that occurs between bacteria during growth.
In conclusion, in the absence of post pasteurization contamination FCPM had a shelf life (time before RjT), which exceeded 14 days. However, depending on the spoilage potential of the contaminating microorganisms, PPC could result in changes in the VOC composition and the sensory quality of the FCPM that result in its rejection by consumers if microbial numbers exceeded approximately 107 CFU.mL-1 regardless of the storage time. Preliminary trials carried out in the bottling plant identified a number of potential sources of PPC and it is suggested that eliminating these potential sources will enable the production of milk, which consistently has a shelf life exceeding 14 days.