Swanson Sara, Fu Tong-Jen
Institute for Food Safety and Health, Illinois Institute of Technology, 6502 South Archer Road, Bedford Park, Illinois 60501.
U.S. Food and Drug Administration, Division of Food Processing Science and Technology, 6502 South Archer Road, Bedford Park, Illinois 60501, USA.
J Food Prot. 2017 Mar;80(3):497-501. doi: 10.4315/0362-028X.JFP-16-112.
This study examined how the hardness of water affected the efficacy of sodium hypochlorite in inactivating Escherichia coli O157:H7 in water. Water was prepared at different degrees of total hardness (0, 50, 100, 200, 500, 1,000, 2,000, and 5,000 mg/liter CaCO). Inactivation was assessed at different levels of free chlorine (0, 0.2, 0.5, and 1.0 ppm) at 2 to 4°C and pH 6.5. Thirty milliliters of chlorinated water was inoculated with 6 log CFU/ml of E. coli O157:H7 and allowed to mix for 3, 10, 20, or 30 s. In the absence of sodium hypochlorite, no reduction in counts of E. coli O157:H7 was observed regardless of the degree of water hardness. However, in the presence of hard water, under certain chlorine concentrations and exposure times, the reduction of E. coli O157:H7 in chlorinated hard water was significantly less than the reduction observed in chlorinated deionized water. For example, after exposure to 0.5 ppm of free chlorine for 10 s, E. coli O157:H7 counts were reduced by 4.8 ± 1.4, 2.0 ± 1.3, 1.6 ± 0.7, 0.5 ± 0.7, and 0.0 ± 0.1 log CFU/ml in water containing 0, 100, 1,000, 2,000, and 5,000 mg/liter CaCO, respectively. With the exception of 5,000 mg/liter CaCO, the effect of water hardness was no longer visible after 20 s of exposure to 0.5 ppm of free chlorine. Also, hard water significantly lowered the efficacy of sodium hypochlorite at 3 s of exposure to 1.0 ppm of free chlorine. But after 20 s of exposure to 1.0 ppm of free chlorine, the impact of water hardness was no longer observed. This study demonstrated that water hardness can affect the germicidal efficacy of sodium hypochlorite, and such an impact may or may not be apparent depending on the condition of the solution and the treatment time at which the observation is made. Under the conditions typically seen in commercial produce washing operations, the impact of water hardness on chlorine efficacy is likely to be insignificant compared with that of organic load.
本研究考察了水的硬度如何影响次氯酸钠对水中大肠杆菌O157:H7的灭活效果。制备了不同总硬度（0、50、100、200、500、1000、2000和5000mg/L碳酸钙）的水。在2至4°C和pH值为6.5的条件下，评估了不同游离氯水平（0、0.2、0.5和1.0ppm）下的灭活情况。将30毫升氯化水接种6log CFU/ml的大肠杆菌O157:H7，并使其混合3、10、20或30秒。在没有次氯酸钠的情况下，无论水的硬度如何，均未观察到大肠杆菌O157:H7数量的减少。然而，在硬水存在的情况下，在一定的氯浓度和暴露时间下，氯化硬水中大肠杆菌O157:H7的减少量明显低于氯化去离子水中观察到的减少量。例如，在暴露于0.5ppm游离氯10秒后，含有0、100、1000、2000和5000mg/L碳酸钙的水中大肠杆菌O157:H7的数量分别减少了4.8±1.4、2.0±1.3、1.6±0.7、0.5±0.7和0.0±0.1log CFU/ml。除了5000mg/L碳酸钙外，在暴露于0.5ppm游离氯20秒后，水硬度的影响不再明显。此外，硬水在暴露于1.0ppm游离氯3秒时显著降低了次氯酸钠的效果。但在暴露于1.0ppm游离氯20秒后，未再观察到水硬度的影响。本研究表明，水的硬度会影响次氯酸钠的杀菌效果，这种影响可能因溶液条件和观察时的处理时间而明显或不明显。在商业农产品清洗操作中常见的条件下，与有机负荷相比，水硬度对氯效果的影响可能微不足道。
