Department of Food Science and Technology, University of California, Davis, One Shields Avenue, Davis, California 95616-5270.
Western Center for Food Safety, University of California, Davis, One Shields Avenue, Davis, California 95618.
J Food Prot. 2022 Jun 1;85(6):961-972. doi: 10.4315/JFP-21-394.
The impact of water application method on bacterial survival at or after the final irrigation was evaluated in bulb onions during commercially relevant field drying (curing). A three-strain rifampin-resistant cocktail of Escherichia coli was introduced to onions via a single overhead spray application in two separate trials (5.22 [trial 1] or 2.40 [trial 2] log CFU per onion) 2 to 3 days after the final irrigation. Onions were lifted from the soil 8 days after spray inoculation and, in some cases, foliage was removed (topping); onions remained in the field for an additional ca. 2 weeks (total ca. 3 weeks of curing). E. coli populations declined on the onions in the first 4 h after spray inoculation. E. coli was recovered from 38 (48%) or 28 (35%) of 80 whole-onion enrichments at the end of curing in trials 1 or 2, respectively. Topping did not significantly impact the percentage of E. coli-positive onions detected at the end of curing. From 8 h to 21 days, E. coli populations on positive onions ranged from 1 CFU per onion to 7 log CFU per onion in both trials, representing a potential risk of E. coli growth with overhead application of contaminated water at the end of onion production. In trial 2, additional rows of onions were inoculated via a 22-cm subsurface or surface drip irrigation line (1.94 log CFU/mL for 2.5 h). E. coli was detected in 0 (subsurface) and 4 (surface) of 50 whole-onion enrichments 3 h after the initiation of drip irrigation. Positive onions were detected at days 1 (4 of 50) and 7 (1 of 50) with subsurface drip inoculation, and at days 1 (7 of 50), 7 (2 of 50), and 14 (2 of 50) with surface drip inoculation. E. coli was not detected in whole-onion enrichments at the end of curing when inoculated by subsurface (0 of 50) or surface (0 of 50) drip irrigation. Application of contaminated water through drip irrigation, when coupled with field curing, results in low rates of contamination of bulb onions at the time of harvest.
本研究评估了商业相关田间干燥（晾晒）过程中不同浇水方法对最终浇水后洋葱中细菌存活的影响。在两次独立试验中（试验 1 中每颗洋葱接种 5.22 [试验 1] 或 2.40 [试验 2] 对数 CFU 的三株利福平抗性混合大肠杆菌；试验 2 中在最后一次浇水后 2 至 3 天通过单一顶部喷雾接种），将含有三株利福平抗性大肠杆菌的混合物通过单一顶部喷雾接种到洋葱上。在喷雾接种后 8 天从土壤中拔出洋葱，在某些情况下会去除叶片（打顶）；洋葱在田间再放置大约 2 周（总共晾晒大约 3 周）。在喷雾接种后 4 小时内，洋葱上的大肠杆菌数量开始下降。在试验 1 和 2 中，在晾晒结束时，分别从 80 个全洋葱富集物中的 38 个（48%）或 28 个（35%）中回收了大肠杆菌。打顶对晾晒结束时检测到的大肠杆菌阳性洋葱的百分比没有显著影响。在 8 小时至 21 天期间，两个试验中阳性洋葱上的大肠杆菌数量范围为每颗洋葱 1 个 CFU 至每颗洋葱 7 个对数 CFU，这表明在洋葱生产结束时，通过顶部浇水应用污染水可能会导致大肠杆菌生长。在试验 2 中，通过 22 厘米的地下或地表滴灌线（每 2.5 小时 1.94 log CFU/mL）对额外的几行洋葱进行接种。在开始滴灌后 3 小时，在 50 个全洋葱富集物中检测到 0（地下）和 4（地表）个 E. coli。在地下滴灌接种后第 1 天（50 个中的 4 个）和第 7 天（50 个中的 1 个）检测到阳性洋葱，在地表滴灌接种后第 1 天（50 个中的 7 个）、第 7 天（50 个中的 2 个）和第 14 天（50 个中的 2 个）检测到阳性洋葱。当通过地下（50 个中均未检出）或地表（50 个中均未检出）滴灌进行接种时，在晾晒结束时，全洋葱富集物中未检出大肠杆菌。当结合田间晾晒时，通过滴灌应用污染水会导致收获时鳞茎洋葱的污染率很低。
