Food Protection Trends - May 2009 - 276

FIGURE 3. Bacterial counts and temperatures in dishwater during hand dishwashing. Dishware was soiled with soil 1 (Experiment A) or soils 2 and 3 (Experiment B). An amount of 3% tryptic soy and 0.5 ml/liter detergent (Dizzy) was added to the dishwater. The dotted line at 0.7 CFU/ml indicates the detection level. FIGURE 4. Bacterial counts and temperatures in rinse water during the two hand dishwashing experiments. The dotted line at 0.7 CFU/ml indicates the detection level. around 7, with or without antibacterial substances (3, 9). For machine dishwashing, the normal pH is about 10–12. The combination of high temperature (55–65°C) and high pH in machine dishwashing reduces the bacterial number more effictively than hand dishwashing (10, 14, 29). Machines with granules to assist in cleaning do not show any increased levels of bacteria in the dishwater or among granules (25). From the start of the hand dishwashing experiments, the numbers of bacteria were below the detection limit in the rinse water. When the hand dishwashing was finished after 40 minutes, a slight increase of the bacterial content of the rinse water was seen. After another 40 minutes, the content of bacteria was below the detection limit. The reason for this behavior is the lack of nutrients remained in the rinse water. After completion of dishwashing, a small amount of nutrients remained in the water and the temperature had decreased; both of these are favorable factors for growth of microorganisms. The amount of nutrition measured as COD was less than 300 mg/liter in the rinse water, in contrast to the amount in the dishwater, in which all the soil from the dishware was collected. In dishwashers, values from 2000 up to 6000 COD mg/liter have been reported (8, 28). Furthermore, the procedure in a dishwasher is different from the rinse procedure in hand dishwashing. In a machine, rinse water is sprayed over the dishware after the washing cycle, and the rinse water is reused as dishwater. Contaminating the rinse water from the dishware is therefore not possible. The number of CFU/item is considerable higher for hand dishwashing than for machine dishwashing, because the concentration of bacteria in the dishwater was considerably higher for hand dishwashing than for machine dishwashing, and the plates were not properly cleaned. To compare hand dishwashing to machine dishwashing, plates were used as dishware. However, a dishwasher can be used for both trays and plates; for this reason, experiments were conducted for trays also. The results (CFU/item) were the same for plates and for trays in the dishwasher. Cross-contamination from dishwater to plates has been found in a traditional dishwasher and in hand dishwashing, but only on those plates or bowls that contained visible soil after dishwashing (26). There are, of course, other processes and items in the kitchen in which different surfaces act as vectors for microbial transfer, i.e., plastic gloves, plastic aprons, plastic salad crates, stainless steel surfaces and conveyor belts (11, 17). Because of higher temperatures of the plates after the machine dishwashing cycle and the addition of drying agent in the rinse water, drying of the dishware is much faster for machine dishwashing than for hand dishwashing. Trays, utensils and plates should be as dry as possible after the dishwashing cycle to minimize the risk of bacterial growth on the surfaces. Machine dishwashing is a more reproducible process than hand dishwashing, since it is controlled by the specifications of the machine. The temperature of the dishwater is kept constant at 65°C, in contrast to the dishwater temperature for 276 FOOD PROTECTION TRENDS | MAY 2009

Food Protection Trends - May 2009

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