Microbiological Control of Cucumber Hydrocooling Water with Chlorine Dioxide ^†

The time required to cool size 2B (3.43 to 3.75-cm-diameter) pickling cucumbers by a commercial spray-type hydrocooler to less than 9°C was about 18 min at typical initial fruit temperatures of 25 to 29°C. During this period, the fruit was exposed to the recycled water, which reached relatively high populations of bacteria (10⁶ to 10⁷ colony forming unites [CFU]/g total aerobes and 10⁵ to 10⁶ CFU/g total Enterobacteriaceae) during a typical day's operation. These numbers exceeded those present on the unwashed fruit, depending upon the volume of fruit previously cooled. Residual chlorine dioxide at 1.3 ppm was found to optimally control (2 to 6 log-cycles reduction) the numbers of bacteria. At 0.95 ppm chlorine dioxide, the numbers of bacteria in the water were relatively static, while at 2.8 and 5.1 ppm the odor of chlorine dioxide became excessive. The bacterial populations in/on the cucumbers were not greatly influenced by chlorine dioxide, even at 5.1 ppm. Apparently, microorganisms on or in the fruit were protected from the chlorine dioxide. Thus, the use of chlorine dioxide in hydrocooling water of cucumbers seems to be an effective means of controlling microbial build-up in the water, but has little effect upon microorganisms on or in the fruit.