Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824, USA.
J Food Prot. 2012 Jul;75(7):1184-97. doi: 10.4315/0362-028X.JFP-11-489.
Postharvest contamination and subsequent spread of Escherichia coli O157:H7 can occur during shredding, conveying, fluming, and dewatering of fresh-cut leafy greens. This study quantified E. coli O157:H7 transfer from leafy greens to equipment surfaces during simulated small-scale commercial processing. Three to five batches (22.7 kg) of baby spinach, iceberg lettuce, and romaine lettuce were dip inoculated with a four-strain cocktail of avirulent, green fluorescent protein-labeled, ampicillinresistant E. coli O157:H7 to contain ∼10(6), 10(4), and 10(2) CFU/g, and then were processed after 1 h of draining at ∼23°C or 24 h of storage at 4°C. Lettuce was shredded using an Urschel TransSlicer at two different blade and belt speeds to obtain normal (5 by 5 cm) and more finely shredded (0.5 by 5 cm) lettuce. Thereafter, the lettuce was step conveyed to a flume tank and was washed and then dried using a shaker table and centrifugal dryer. Product (25-g) and water (40-ml) samples were collected at various points during processing. After processing, product contact surfaces (100 cm(2)) on the shredder (n = 14), conveyer (n = 8), flume tank (n = 11), shaker table (n = 9), and centrifugal dryer (n = 8) were sampled using one-ply composite tissues. Sample homogenates diluted in phosphate or neutralizing buffer were plated, with or without prior 0.45- m m membrane filtration, on Trypticase soy agar containing 0.6% yeast extract supplemented with 100 ppm of ampicillin to quantify green fluorescent protein-labeled E. coli O157:H7 under UV light. During leafy green processing, ∼90% of the E. coli O157:H7 inoculum transferred to the wash water. After processing, E. coli O157:H7 populations were highest on the conveyor and shredder (P<0.05), followed by the centrifugal dryer, flume tank, and shaker table, with ∼29% of the remaining product inoculum lost during centrifugal drying. Overall, less (P<0.05) of the inoculum remained on the product after centrifugally drying iceberg lettuce that was held for 1 h (8.13%) as opposed to 24 h (42.18%) before processing, with shred size not affecting the rate of E. coli O157:H7 transfer.
在切碎、输送、喷淋和脱水过程中，新鲜叶菜类的食源性病原菌大肠杆菌 O157:H7 可能会受到污染并进一步传播。本研究量化了模拟商业小规模处理过程中大肠杆菌 O157:H7 从叶菜类转移到设备表面的情况。将三到五批（22.7 公斤）菠菜、冰山生菜和罗马生菜用含有四株无毒、绿色荧光蛋白标记、氨苄青霉素抗性的大肠杆菌 O157:H7 的鸡尾酒浸泡接种，使每克含有的菌数分别约为 10(6)、10(4)和 10(2)CFU/g，然后在 23°C 下沥干 1 小时或在 4°C 下储存 24 小时后进行加工。使用 Urschel TransSlicer 将生菜切成不同的大小，刀片和皮带速度分别为 5×5cm 和 0.5×5cm，获得正常（5×5cm）和更细的切碎生菜。然后，将生菜通过输送器输送到喷淋槽，并用喷淋槽和离心干燥机进行清洗和干燥。在处理过程中的不同时间收集 25 克产品和 40-毫升水的样品。处理后，使用一次性复合纸巾从切碎机（n=14）、输送器（n=8）、喷淋槽（n=11）、摇床（n=9）和离心干燥器（n=8）上的产品接触表面（100cm(2)）采集样品。将样品的匀浆在磷酸盐或中和缓冲液中稀释，然后在含有 0.6%酵母提取物的胰蛋白酶大豆琼脂上进行平板划线，平板中添加 100ppm 的氨苄青霉素，并在紫外光下对含有绿色荧光蛋白标记的大肠杆菌 O157:H7 的平板进行计数。在叶菜类加工过程中，约 90%的大肠杆菌 O157:H7 接种物转移到洗涤水中。处理后，在输送机和切碎机上发现的大肠杆菌 O157:H7 数量最高（P<0.05），其次是离心干燥机、喷淋槽和摇床，在离心干燥过程中，约 29%的剩余产品接种物丢失。总体而言，离心干燥冰生菜时，接种物残留量较少（P<0.05），在处理前 1 小时（8.13%）而不是 24 小时（42.18%），且切碎尺寸不会影响大肠杆菌 O157:H7 的转移速度。
