Department of Food Science, Cornell University, Ithaca, NY 14853.
Department of Food Science, Cornell University, Ithaca, NY 14853.
J Dairy Sci. 2021 Mar;104(3):2758-2772. doi: 10.3168/jds.2020-19452. Epub 2020 Dec 25.
Shelf-stable milk is consumed worldwide, and this market is expected to continue growing. One quality challenge for UHT milk is age gelation during shelf life, which is in part caused by bacterial heat-stable proteases (HSP) synthesized during the raw milk storage period before heat processing. Some Pseudomonas spp. are HSP producers, and their ability to grow well at refrigeration temperature make them important spoilage organisms for UHT processors to control. Previous studies have shown that lactose oxidase (LO), a natural and commercially available enzyme that produces hydrogen peroxide and lactobionic acid from lactose, can control bacterial growth in raw milk. In this research, we investigated the ability of LO to control HSP producer outgrowth, and thus delay age gelation in UHT milk. Six strains of Pseudomonas spp. were selected based on their ability to synthesize HSP and used as a cocktail to inoculate both raw and sterile (UHT) milk at a level of 1 × 10 cfu/mL. Groups were treated with and without LO, stored for 4 d at 6°C, and monitored for cell count and pH. Additionally, a sample from each was tested for HSP activity via particle size analysis (average effective diameter at 90° angle and 658 nm wavelength) and visual inspection on each day of the storage period. The HSP activity results were contrasted using Tukey's HSD test, which showed that in UHT milk, a LO treatment (0.12 g/L) effectively prevented gelation as compared with the control. In raw milk, however, a concentration of 0.24 g/L of LO was needed to obtain a similar effect. This test was scaled up to 19-L pilot plant batches of raw milk where they were challenged with Pseudomonas cocktail, treated with LO for 3 d, and then UHT processed. Resulting UHT milk bottles were monitored for gelation. Significant differences in particle size between the LO-treated samples and the control were observed as early as 1 mo after processing, and gelation was not detected in the LO-treated samples through 6 mo of storage. These results demonstrated that LO can be used to delay age gelation in UHT milk induced by HSP-producing Pseudomonas spp., representing an opportunity to improve quality and reduce postproduction losses in the shelf-stable milk market sector.
货架稳定奶在全球范围内都有消费，并且预计这个市场将继续增长。超高温（UHT）奶面临的一个质量挑战是在货架期内发生老化凝胶，这在一定程度上是由于在热加工前的原料奶储存期间合成的细菌热稳定蛋白酶（HSP）引起的。某些假单胞菌是 HSP 的产生者，它们在冷藏温度下良好生长的能力使它们成为 UHT 加工商需要控制的重要变质生物。先前的研究表明，乳糖氧化酶（LO）是一种天然的、商业上可用的酶，它可以从乳糖中产生过氧化氢和乳寡糖，从而控制原料奶中的细菌生长。在这项研究中，我们研究了 LO 控制 HSP 产生菌生长的能力，从而延缓 UHT 奶的老化凝胶。根据合成 HSP 的能力，选择了 6 株假单胞菌作为研究对象，并将其作为混合物接种到原料奶和无菌（UHT）奶中，接种量为 1×10 cfu/mL。将这些组分别用和不用 LO 处理，在 6°C 下储存 4 d，并监测细胞计数和 pH 值。此外，在储存期间的每一天，从每个样本中提取 HSP 活性进行测试，通过粒度分析（90°角和 658nm 波长下的平均有效直径）和视觉检查进行测试。使用 Tukey 的 HSD 检验对比 HSP 活性结果，结果表明在 UHT 奶中，与对照相比，浓度为 0.12g/L 的 LO 处理有效地防止了凝胶形成。然而，在原料奶中，需要 0.24g/L 的 LO 浓度才能获得类似的效果。该测试扩展到了 19 升的原料奶中，在其中添加了假单胞菌混合物，用 LO 处理 3d，然后进行 UHT 加工。随后监测 UHT 奶瓶的凝胶形成情况。在加工后 1 个月左右，就可以观察到 LO 处理的样品与对照样品之间的粒径有显著差异，并且在 6 个月的储存过程中没有检测到 LO 处理的样品发生凝胶。这些结果表明，LO 可以用于延缓由 HSP 产生的假单胞菌引起的 UHT 奶的老化凝胶，这为改善货架稳定奶市场部门的质量和减少产后损失提供了机会。
