Proulx J, Sullivan G, Marostegan L F, VanWees S, Hsu L C, Moraru C I
Department of Food Science, Cornell University, Ithaca, New York 14853.
Department of Food Science, Cornell University, Ithaca, New York 14853.
J Dairy Sci. 2017 Mar;100(3):1664-1673. doi: 10.3168/jds.2016-11582. Epub 2017 Jan 18.
Postprocessing cross-contamination of cheese can lead to both food safety issues and significant losses due to spoilage. Pulsed light (PL) treatment, consisting of short, high-energy, broad-spectrum light pulses, has been proven effective in reducing the microbial load on cheese surface. As PL treatment effectiveness is limited by light-cheese interactions, the possibility to improve its effectiveness by combining it with the antimicrobial nisin was explored. The effect of natamycin, which is added to cheeses as an antifungal agent, on PL effectiveness was also investigated. Pseudomonas fluorescens, Escherichia coli ATCC 25922, and Listeria innocua were used as challenge microorganisms. Bacterial cultures in stationary growth phase were diluted to initial inoculum levels of 5 or 7 log cfu per cheese slice. Slices of sharp white Cheddar cheese and white American singles were cut in rectangles of 2.5 × 5 cm. For cheese slices receiving antimicrobial treatment before PL, slices were dipped in natamycin or nisin, spot inoculated with 100 μL of bacterial suspension, and then treated with PL. Cheese slices receiving PL treatment before antimicrobials were spot inoculated, treated with PL, and then treated with antimicrobials. The PL fluence levels from 1.02 to 12.29 J/cm were used. Survivors were enumerated by standard plate counting or the most probable number technique, as appropriate. All treatments were performed in triplicate, and the data were analyzed using a general linear model. Treatment with nisin or natamycin before PL decreased the effectiveness of PL for all bacteria tested. For instance, PL reduced P. fluorescens on Cheddar cheese by 2.19 ± 0.27 log after 6.14 J/cm, whereas combination treatments at the same PL fluence yielded barely 1 log reduction. Inactivation of L. innocua on Cheddar was only 0.78 ± 0.01 log when using PL after nisin, compared with a 1.30 ± 0.76 log reduction by nisin alone. This was attributed to the absorption of UV light by the 2 antimicrobials, which diminished the UV fluence received by the bacteria. Increased inactivation was obtained when antimicrobials were applied after PL. On process cheese, a maximum reduction of 3.73 ± 0.96 log of L. innocua was obtained at 9.22 J/cm for PL followed by nisin, compared with 3.01 ± 0.48 by PL alone. This study demonstrates that antimicrobials may increase the antimicrobial effectiveness of PL on cheese surface, but the order of treatments is critical.
奶酪的后处理交叉污染会导致食品安全问题以及因变质造成的重大损失。脉冲光（PL）处理由短时间、高能量、广谱光脉冲组成，已被证明能有效降低奶酪表面的微生物负荷。由于PL处理的效果受光与奶酪相互作用的限制，因此探索了将其与抗菌剂乳链菌肽结合以提高其效果的可能性。还研究了作为抗真菌剂添加到奶酪中的纳他霉素对PL效果的影响。荧光假单胞菌、大肠杆菌ATCC 25922和无害李斯特菌被用作挑战微生物。将处于稳定生长期的细菌培养物稀释至每片奶酪初始接种水平为5或7 log cfu。将尖锐的白色切达干酪片和白色美式单片奶酪切成2.5×5厘米的矩形。对于在PL处理前接受抗菌处理的奶酪片，将其浸入纳他霉素或乳链菌肽中，用100μL细菌悬液进行点接种，然后进行PL处理。在抗菌处理前接受PL处理的奶酪片先进行点接种，再进行PL处理，然后进行抗菌处理。使用的PL通量水平为1.02至12.29 J/cm²。根据情况，通过标准平板计数或最可能数技术对存活菌进行计数。所有处理均重复三次，并使用一般线性模型对数据进行分析。在PL处理前用乳链菌肽或纳他霉素处理会降低PL对所有测试细菌的效果。例如，在6.14 J/cm²的PL处理后，切达干酪上的荧光假单胞菌减少了2.19±0.27 log，而在相同PL通量下的联合处理仅减少了1 log。在使用乳链菌肽后再进行PL处理时，切达干酪上无害李斯特菌的灭活率仅为0.78±0.01 log，而单独使用乳链菌肽时减少了1.30±0.76 log。这归因于两种抗菌剂对紫外线的吸收，从而减少了细菌接收到的紫外线通量。在PL处理后使用抗菌剂可提高灭活效果。在再制干酪上，先进行PL处理（9.22 J/cm²）然后使用乳链菌肽，无害李斯特菌的最大减少量为3.73±0.96 log，而单独使用PL时为3.01±0.48 log。这项研究表明，抗菌剂可能会提高PL对奶酪表面的抗菌效果，但处理顺序至关重要。
