Yousef A E, Ryser E T, Marth E H
Department of Food Science, University of Wisconsin-Madison 53706.
Appl Environ Microbiol. 1988 Nov;54(11):2643-9. doi: 10.1128/aem.54.11.2643-2649.1988.
Method of homogenization (Waring blender versus stomacher), type of diluent (tryptose broth [TB] versus aqueous 2% trisodium citrate), and temperature of diluent (20 versus 40 degrees C) were compared for recovery of Listeria monocytogenes from freshly made and ripened Colby cheese. By using direct plating on McBride listeria agar, significantly higher numbers of L. monocytogenes were recovered when cheese samples were (i) homogenized for 2 min with the blender rather than the stomacher (P less than 0.01), (ii) diluted in trisodium citrate rather than TB (P less than 0.01), and (iii) diluted in diluents at 40 rather than 20 degrees C (P less than 0.05). Based on these results, a new diluent/enrichment medium was developed by adding 2% trisodium citrate to TB (TBC). Despite superior results with the blender, biosafety concerns led to use of the stomacher for homogenization of cheese samples; hence, the stomaching time was increased to 3 min. Results obtained by direct plating indicated that recovery of L. monocytogenes from Colby cheese and from curd samples taken during manufacture of brick cheese increased when samples were diluted 1:10 in TBC at 45 degrees C and stomached for 3 min, as compared with similarly treated samples diluted in TB at 25 degrees C. A similar comparison of both diluents for recovery of L. monocytogenes from cold-pack cheese food yielded bacterial counts which were not significantly different. Recovery of L. monocytogenes from cold-enriched (at 4 degrees C for up to 8 weeks) samples of Colby cheese and cold-pack cheese food was generally similar for samples homogenized in TBC or TB.
比较了均质化方法（沃林搅拌器与均质器）、稀释剂类型（胰蛋白胨肉汤[TB]与2%柠檬酸三钠水溶液）以及稀释剂温度（20℃与40℃）对从新鲜制作和成熟的科尔比奶酪中回收单核细胞增生李斯特菌的影响。通过在麦克布赖德李斯特菌琼脂上直接平板计数，当奶酪样品：(i) 用搅拌器均质2分钟而非均质器时（P<0.01），(ii) 用柠檬酸三钠而非TB稀释时（P<0.01），以及(iii) 在40℃而非20℃的稀释剂中稀释时（P<0.05），回收的单核细胞增生李斯特菌数量显著更高。基于这些结果，通过向TB中添加2%柠檬酸三钠（TBC）开发了一种新的稀释剂/富集培养基。尽管搅拌器的效果更佳，但出于生物安全考虑，仍使用均质器对奶酪样品进行均质化处理；因此，将均质时间增加到3分钟。直接平板计数结果表明，与在25℃下用TB稀释并同样处理的样品相比，当样品在45℃下用TBC按1:10稀释并均质3分钟时，从科尔比奶酪以及砖形奶酪制作过程中采集的凝乳样品中回收的单核细胞增生李斯特菌数量增加。对两种稀释剂从冷包装奶酪食品中回收单核细胞增生李斯特菌的情况进行类似比较，得到的细菌计数没有显著差异。对于在TBC或TB中均质的样品，从冷藏富集（4℃下长达8周）的科尔比奶酪和冷包装奶酪食品样品中回收单核细胞增生李斯特菌的情况通常相似。
