Effect of cold chain interruptions on the shelf-life of fluid pasteurised skim milk at the consumer stage

Abstract This study aimed to verify the effect of time and temperature abuse on bacterial numbers in fluid pasteurized skim milk by simulating the real-life scenario, which usually occurs when cold chain is interrupted by consumers prior to consumption that affect the shelf-life of milk. Total three trials were carried out in this study. Thermal abuse was simulated with temperature fluctuations from 5 °C. In the first trial, the information about holding the milk samples for 8 hours at three different temperatures of 15 °C, 20 °C and 25 °C was obtained using a data logger to predict the effect of temperature abuse on the milk microbial quality. Further, in the second and third trial, the effect of temperature abuse on bacterial numbers was examined by holding milk at 5 °C and then shifts temperature to 25 °C for 8 h and 6 h. The pH was monitored during storage. The total bacterial count was examined by the Standard Plate Count (SPC). The mean air temperature had the greatest impact on milk temperature. It took 3.0 h, 3.9 h and 4.2 h to warm up when exposed to the temperatures of 15 °C, 20 °C and 25 °C, respectively. The holding time of 8 h at 25 °C showed that bacterial numbers (1 x 105 CFU mL-1) were higher after 14 days of storage, but control samples at 5 °C (< 1 x 104 CFU mL-1) were still within the acceptable level (5 x 104 CFU mL-1). A holding time of 6 h at 25 oC showed much higher bacterial numbers (1 x 109 CFU mL-1) compared to control samples (1 x 107 CFU mL-1) which were held at 5 °C for 11 days. The pH of the milk decreased with increasing bacterial growth during the extended storage time. The results show that temperature abuse has a significant effect on milk microbial stability and shelf life. It is important to maintain the milk temperature at 5 °C or less as the bacterial growth directly depend on increasing temperature and holding time, which pose the potential risk of microbial hazards leading to foodborne illness. Thus, consumers must understand the factors associated with the safe handling of milk to keep it safe to use before the expiry date.

摘要
本研究旨在通过模拟消费者在饮用前中断冷链这一常见现实场景，验证时间与温度滥用对巴氏杀菌液态脱脂奶中细菌数量的影响，该场景会影响牛奶的货架期。本研究共开展3组试验。
试验以5℃为基准模拟温度滥用场景：第一组试验中，使用数据记录仪记录牛奶样品在15℃、20℃、25℃三种不同温度下放置8小时的相关数据，以探究温度滥用对牛奶微生物品质的影响。后续第二、第三组试验中，将牛奶先置于5℃环境，再升温至25℃分别放置8小时、6小时，以此考察温度滥用对细菌数量的影响。试验过程中监测储存期间的pH值变化，总细菌计数采用标准平板计数法（Standard Plate Count, SPC）进行测定。
环境平均气温对牛奶温度的影响最为显著：当分别暴露于15℃、20℃、25℃环境中时，牛奶分别耗时3.0小时、3.9小时、4.2小时完成升温。
在25℃下放置8小时的样品，在储存14天后细菌数量可达1×10^5 CFU·mL⁻¹，而置于5℃的对照组样品（<1×10^4 CFU·mL⁻¹）仍处于可接受范围内（≤5×10^4 CFU·mL⁻¹）；在25℃下放置6小时的样品，其细菌数量（1×10^9 CFU·mL⁻¹）远高于储存11天的5℃对照组样品（1×10^7 CFU·mL⁻¹）。
随着储存时间延长，细菌繁殖量增加，牛奶的pH值随之降低。
研究结果表明，温度滥用对牛奶的微生物稳定性与货架期具有显著影响。由于细菌繁殖直接受环境温度与放置时长的影响，可能引发微生物安全风险并导致食源性疾病，因此将牛奶温度维持在5℃及以下至关重要。
因此，消费者应当了解牛奶安全储存的相关注意事项，以确保在保质期前饮用时牛奶仍安全可靠。