Dairy and Functional Foods Research Unit, USDA, ARS, Eastern Regional Research Center, Wyndmoor, PA 19038, USA.
J Dairy Sci. 2011 Sep;94(9):4277-91. doi: 10.3168/jds.2010-3879.
High-temperature, short-time pasteurization of milk is ineffective against spore-forming bacteria such as Bacillus anthracis (BA), but is lethal to its vegetative cells. Crossflow microfiltration (MF) using ceramic membranes with a pore size of 1.4 μm has been shown to reject most microorganisms from skim milk; and, in combination with pasteurization, has been shown to extend its shelf life. The objectives of this study were to evaluate MF for its efficiency in removing spores of the attenuated Sterne strain of BA from milk; to evaluate the combined efficiency of MF using a 0.8-μm ceramic membrane, followed by pasteurization (72°C, 18.6s); and to monitor any residual BA in the permeates when stored at temperatures of 4, 10, and 25°C for up to 28 d. In each trial, 95 L of raw skim milk was inoculated with about 6.5 log(10) BA spores/mL of milk. It was then microfiltered in total recycle mode at 50°C using ceramic membranes with pore sizes of either 0.8 μm or 1.4 μm, at crossflow velocity of 6.2 m/s and transmembrane pressure of 127.6 kPa, conditions selected to exploit the selectivity of the membrane. Microfiltration using the 0.8-μm membrane removed 5.91±0.05 log(10) BA spores/mL of milk and the 1.4-μm membrane removed 4.50±0.35 log(10) BA spores/mL of milk. The 0.8-μm membrane showed efficient removal of the native microflora and both membranes showed near complete transmission of the casein proteins. Spore germination was evident in the permeates obtained at 10, 30, and 120 min of MF time (0.8-μm membrane) but when stored at 4 or 10°C, spore levels were decreased to below detection levels (≤0.3 log(10) spores/mL) by d 7 or 3 of storage, respectively. Permeates stored at 25°C showed coagulation and were not evaluated further. Pasteurization of the permeate samples immediately after MF resulted in additional spore germination that was related to the length of MF time. Pasteurized permeates obtained at 10 min of MF and stored at 4 or 10°C showed no growth of BA by d 7 and 3, respectively. Pasteurization of permeates obtained at 30 and 120 min of MF resulted in spore germination of up to 2.42 log(10) BA spores/mL. Spore levels decreased over the length of the storage period at 4 or 10°C for the samples obtained at 30 min of MF but not for the samples obtained at 120 min of MF. This study confirms that MF using a 0.8-μm membrane before high-temperature, short-time pasteurization may improve the safety and quality of the fluid milk supply; however, the duration of MF should be limited to prevent spore germination following pasteurization.
高温短时间巴氏杀菌对炭疽杆菌（BA）等形成孢子的细菌无效，但对其营养细胞是致命的。使用孔径为 1.4 μm 的陶瓷膜进行错流微滤（MF）已被证明可以从脱脂乳中去除大部分微生物；并且与巴氏杀菌相结合，可以延长其保质期。本研究的目的是评估 MF 从牛奶中去除减毒 Sterne 株 BA 孢子的效率；评估使用 0.8-μm 陶瓷膜进行 MF 的联合效率，然后进行巴氏杀菌（72°C，18.6s）；并监测在 4、10 和 25°C 下储存时渗透物中任何残留的 BA 的情况，最长可达 28 天。在每次试验中，95 L 的生脱脂牛奶接种约 6.5 log(10) BA 孢子/mL 的牛奶。然后在 50°C 下以总循环模式使用孔径为 0.8 μm 或 1.4 μm 的陶瓷膜进行微滤，错流速度为 6.2 m/s，跨膜压力为 127.6 kPa，选择这些条件是为了利用膜的选择性。使用 0.8-μm 膜微滤可去除 5.91±0.05 log(10) BA 孢子/mL 的牛奶，而 1.4-μm 膜可去除 4.50±0.35 log(10) BA 孢子/mL 的牛奶。0.8-μm 膜对原生微生物群具有高效去除作用，两种膜均对酪蛋白表现出几乎完全的传递。在 MF 时间为 10、30 和 120 min（0.8-μm 膜）时，在渗透物中可明显看到孢子发芽，但在 4 或 10°C 下储存时，孢子水平分别在第 7 或 3 天下降到检测水平以下（≤0.3 log(10) 孢子/mL）。在 25°C 下储存的渗透物表现出凝结，因此未进一步评估。MF 后立即对渗透物样品进行巴氏杀菌会导致额外的孢子发芽，这与 MF 时间的长短有关。在 MF 时间为 10 min 并在 4 或 10°C 下储存的巴氏杀菌渗透物在第 7 和第 3 天分别未显示 BA 的生长。在 MF 时间为 30 和 120 min 时获得的巴氏杀菌渗透物会导致多达 2.42 log(10) BA 孢子/mL 的孢子发芽。在 4 或 10°C 下储存时，30 min MF 获得的样品中的孢子水平在整个储存期内下降，但 120 min MF 获得的样品中则没有下降。本研究证实，在高温短时间巴氏杀菌之前使用 0.8-μm 膜进行 MF 可能会提高液体牛奶供应的安全性和质量；然而，MF 的持续时间应限制在巴氏杀菌后防止孢子发芽。
