Food Chemistry and Technology Department, Teagasc Food Research Centre, Moorepark, Fermoy, County Cork, P61 C996, Ireland.
Department of Biological Sciences, School of Natural Sciences, Faculty of Science and Engineering, University of Limerick, Limerick, V94 T9PX, Ireland.
J Dairy Sci. 2021 Sep;104(9):9627-9644. doi: 10.3168/jds.2020-19875. Epub 2021 Jun 12.
Heat treatment is one of the most widely used processing technologies in the dairy industry. Its primary purpose is to destroy microorganisms, both pathogenic and spoilage, to ensure the product is safe and has a reasonable shelf life. In this study microwave volumetric heating (MVH) was compared with a conventional tubular heat exchanger (THE), in terms of the effects of each at a range of temperatures (75°C, 85°C, 95°C, 105°C, 115°C, and 125°C) on indigenous microflora viability and the germination of inoculated Bacillus licheniformis endospores in reconstituted skim milk. To assess the heat treatment-related effects on microbial viability, classical agar-based tests were applied to obtain the counts of 4 various microbiological groups including total bacterial, thermophilic bacterial, mesophilic aerobic bacterial endospore, and thermophilic aerobic bacterial endospore counts, and additional novel insights into cell permeability and spore germination profiles post-heat treatment were obtained using real-time flow cytometry (FC) methods. No significant differences in the plate counts of the indigenous microorganisms tested, the plate counts of the inoculated B. licheniformis, or the relative percentage of germinating endospores were observed between MVH- and THE-treated samples, at equal temperatures in the range specified above, indicating that both methods inactivated inoculated endospores to a similar degree (up to 70% as measured by FC and 5 log reduction as measured by plate counting for some treatments of inoculated endospores). Furthermore, increased cell permeability of indigenous microflora was observed by FC after MVH compared with THE treatment of uninoculated skim milk, which was reflected in lower total bacterial count at a treatment temperature of 105°C. This work demonstrates the utility of FC as a rapid method for assessing cell viability and spore inactivation for postthermal processing in dairy products and overall provides evidence that MVH is at least as effective at eliminating native microflora and inoculated B. licheniformis endospores as THE.
热处理是乳制品行业中应用最广泛的加工技术之一。其主要目的是破坏微生物，包括致病菌和腐败菌，以确保产品安全并具有合理的保质期。在这项研究中，比较了微波容积加热 (MVH) 和传统管式热交换器 (THE) 在不同温度（75°C、85°C、95°C、105°C、115°C 和 125°C）下对原生微生物群落活力和接种的地衣芽孢杆菌内生孢子在再制脱脂乳中发芽的影响。为了评估与热处理相关的对微生物活力的影响，应用经典的基于琼脂的测试来获得 4 个不同微生物群的计数，包括总细菌、嗜热细菌、中温需氧细菌内生孢子和嗜热需氧细菌内生孢子计数，并且使用实时流式细胞术 (FC) 方法获得了处理后细胞通透性和孢子发芽谱的新见解。在上述指定温度范围内，MVH 和 THE 处理的样品中，未接种的地衣芽孢杆菌内生孢子的平板计数、接种的地衣芽孢杆菌的平板计数或发芽内生孢子的相对百分比没有显著差异，表明这两种方法以相似的程度使接种的内生孢子失活（FC 测量高达 70%，一些接种内生孢子的处理通过平板计数测量为 5 对数减少）。此外，与 THE 处理未接种脱脂乳相比，MVH 后 FC 观察到原生微生物群落的细胞通透性增加，这反映在处理温度为 105°C 时总细菌计数较低。这项工作证明了 FC 作为一种快速评估乳制品加工后细胞活力和孢子失活的方法的实用性，并且总体上提供了证据表明 MVH 至少与 THE 一样有效地消除原生微生物群落和接种的地衣芽孢杆菌内生孢子。
