Hygiene Efficacy of Short Cycles in Domestic Dishwashers

Abstract

This study investigated how factors associated with Sinner's principle-namely temperature, time, and the chemical composition of detergents-affected the antimicrobial efficacy of domestic dishwashers, particularly during short cycles. These are of particular interest, because many consumers refrain from using long cycles while it is still unclear if short cycles can provide a sufficient level of hygiene. Thus, we chose a range of bacterial strains, including standard test strains such as Micrococcus luteus and Enterococcus faecium, as well as important foodborne pathogens such as Escherichia coli, Staphylococcus aureus, and Salmonella enterica. To account for the complexity of dishwasher cycles, we correlated hygiene efficacy with area under the curve (AUC) measurements derived from the respective cycle profiles. Our findings revealed that the reductions in M. luteus and E. faecium were minimally affected by the reference detergent. In contrast, a high-tier market detergent demonstrated a significant decrease in bacterial counts. Notably, both strains exhibited reduced efficacy at a main cycle temperature of 45 °C, suggesting that temperatures below 50 °C might represent a critical threshold at which the hygiene efficacy of domestic dishwashing processes declines. However, since food-related pathogens were more susceptible to the dishwashing process, even lower temperatures might deliver a sufficient level of hygiene. Plotting the logarithmic reduction/AUC ratio against the AUC indicated that the main cycle contributed approximately 10-fold more to microbial reduction than the rinse cycle. Furthermore, the antimicrobial impact of detergents was greatest at the lowest AUC values (i.e., during short cycles). Taken together, our results suggest that the applied chemistry may help to enhance antimicrobial performance especially in short dishwashing cycles.

Keywords: detergent; dishwasher; foodborne pathogens; household; hygiene; short cycle.

Figure 1

The typical sequence of each cycle, highlighting the distinct phases: phase A—initial draining combined with the influx of freshwater; phase B—simultaneous heating of the machine and activation of the spray arms with detergent, followed by the drainage of heated water; phase C—supply of fresh soft water without heating or detergent, ending with the drainage of remaining water; and phase D—influx of new water with the addition of rinse aid, if necessary, accompanied by the operation of the spray arms and final drainage of the remaining water inside the dishwasher.

Figure 2

Areas under the curves (AUC) illustrated for an exemplary program. All temperatures ≥ 25 °C (highlighted area) in the main wash and in the final rinse were considered.

Figure 3

Reduction in Micrococcus luteus in different programs with water (blue), IEC standard detergent (green), and market detergent (red). Parameter combinations for the indicated program codes are described in Table 1. The dotted line represents the maximum possible reduction relative to the initial cell count. The letters above each bar indicate a different significance level through a statistical analysis of one-sided ANOVA with Tukey’s multiple comparison test. Groups with the same letter are not statistically different, while groups with different letters have statistically different means.

Figure 4

Reduction in Enterococcus faecium (A), Escherichia coli (B), Salmonella enterica (C), and Staphylococcus aureus (D) across selected programs with water (blue), IEC standard detergent (green), and market detergent (red). Parameter combinations for the indicated program codes are described in Table 1. The dotted line represents the maximum possible reduction relative to the initial cell count. The letters above each bar indicate a different significance level through a statistical analysis of one-sided ANOVA with Tukey’s multiple comparison test. Groups with the same letter are not statistically different, while groups with different letters have statistically different means. The stars above each letter indicate that bacteria were completely absent on the bio-monitors.

Figure 5

Log reduction for Micrococcus luteus across the investigated codes with water (blue), IEC standard detergent (green), and market detergent (red) in relation to the AUC. Dotted lines indicate a possible mathematical dependency, including the corresponding formula and determination coefficients.

Figure 6

LR/AUC ratio for Micrococcus luteus across the investigated codes with water (blue), IEC standard detergent (green), and market detergent (red) in relation to the AUC. The LR/AUC ratio was calculated for the AUC in the rinse phase (A), the main wash (B), or the total cycle (C) and plotted against the corresponding AUC.

Figure 6

LR/AUC ratio for Micrococcus luteus across the investigated codes with water (blue), IEC standard detergent (green), and market detergent (red) in relation to the AUC. The LR/AUC ratio was calculated for the AUC in the rinse phase (A), the main wash (B), or the total cycle (C) and plotted against the corresponding AUC.