Human norovirus outbreaks are common in many crowded locations such as cruise ships (Wikswo et al., 2011), nursing homes (Calderon-Margalit, 2004), schools (Yu et al., 2011), and even hospitals (Johnston et al., 2007). While the report written by Wilkswo and coauthors (2011) was not able to directly link poor dishwashing conditions with the HuNoV outbreak, the authors did note that a dishwasher utilized to clean buffet dishes was operating below the required minimum temperature during the sanitizing rinse, and that items soiled with food debris (due to inadequate washing) were being stored with clean dishes and used to service the buffet. Crowded places such as those listed, are prime locations for the implementation of 3DFP processes. Therefore, having reliable recommendations for the cleaning of 3DFP food ink capsules will be important in preventing or mitigating future HuNoV outbreaks.
In the present study, manufacturer cleaning recommendations for the removal/inactivation of TuV as a surrogate for HuNoV were evaluated. When comparing the efficacy of the three washing protocols examined (DSC, DHC, MW), only the DSC in combination with the pure butter soil was found to be statistically unique at P = 0.05, and the interaction between the DSC and pure butter was found to be a significant predictor of estimated mean log reduction (P = 0.0067). In Figs. 3 and 4, it can be observed that the DHC always performed the best at removing/inactivating TuV (complete log reductions were always measured). Manual washing procedures performed second-best (with the exception of the sugar solution soil), but it was not statistically different from the DHC in any instance. The DSC performed the worst (with the exception of the sugar solution soil), but it was not significantly different from MW in any instance and was only significantly different from DHC when pure butter soil was utilized. This general pattern of efficacy was also found in a similar study by the authors that examined Salmonella Typhimurium and L. monocytogenes removal from soiled 3DFP capsules (Hamilton & Gibson, 2022). However, in Hamilton and Gibson (2022), it was the complex soil mixture, not pure butter, that had a significant interaction effect with the DSC. This may indicate differences between bacteria and viruses insofar as the type of shielding provided by soils with different macromolecule compositions.
To the authors’ knowledge no studies have explicitly examined the potential relationship between the macromolecule composition of a food and thermal protection in HuNoVs. However, in a study of Hepatitis A virus (HAV), Deboosere and coauthors (2004) varied the sucrose concentration in strawberry mashes at 85°C and found D-values of 0.96, 2.37, and 4.98 min at 28°Brix, 40°Brix, and 52°Brix, respectively. Bidawid and coauthors (2000) conducted a similar experiment with HAV to examine the relationship of milkfat content on thermal inactivation. Those authors reported a time for each of the first five log10 reductions for three different fat concentrations (1% fat skim milk, 3.5% fat homogenized milk, and 18% cream) and eight different temperatures (65°C, 67°C, 69°C, 71°C, 73°C, 75°C, 80°C, 85°C). Overall, the protective nature of increased fat content was demonstrated in every condition (except at 85°C) (Bidawid et al., 2000). The thermal inactivation of poliovirus in ground beef at three different fat contents (3% fat, 27% fat, and 47% fat) and four different temperatures (50°C, 60°C, 70°C, and 80°C), demonstrated the protective nature of fat in meat. Unfortunately, the authors did not investigate the role of protein content (Filppi & Banwart, 1974). To the authors’ knowledge, no studies examining the protective role of protein in the thermal inactivation of viruses have been published.
While poliovirus and HAV are less than ideal surrogates for HuNoV, the data provided in the studies by Deboosere and coauthors (2004), Bidawid and coauthors (2000), and Filppi and Banwart (1974) can likely be used to show general patterns of protection as the macromolecule concentration increases, but not estimated D-values or log10 reductions. Also, only Filppi and Banwart (1974) studied temperature ranges relevant to the DSC. Studies on the protective nature of fat, carbohydrate, and protein during thermal treatment of viruses under various dishwashing conditions should be completed with suitable HuNoV surrogates such as MNV or TuV. In the present experiment, it is not possible to conclude whether TuV is being removed or thermally/chemically inactivated, but future studies could be completed using methods that assess viral capsid integrity such as the use of intercalating dyes combined with polymerase chain reaction (Manual et al., 2018). When considering capsule position during DSC conditions (55°C for 45 min) (Fig. 6), the protectiveness of the matrices ranked from best to worst are (1) pure butter, (2) complex soil matrix, (3) protein powder solution, (4) sugar solution, and (5) unsoiled. While no authors have compared the protective nature of different macromolecules with one another, there is evidence that fat and sugar can play a protective role when viruses are exposed to challenging thermal conditions (Filppi & Banwart, 1974; Bidawid et al., 2000; Deboosere et al., 2004). In addition, the present study also supports that protein may play a similar protective role.
Lucassen and coauthors (2021) investigated MNV as a surrogate for HuNoV and found that a combination of a bleach-containing dishwasher detergent, a cleaning temperature of 45°C for 45 min, and a rinsing temperature of 50°C led to 4.2 log10 reductions, however, only limited conditions were tested (i.e., other dishwasher cycle times and manual washing were not tested). Rinsing temperature was tested at 30°C, 50°C, and 70°C, and in agreement with the results of the present study, increased temperature led to a significant increase in viral log reduction (Lucassen et al, 2021).
To the authors’ knowledge, no prior studies have examined washing efficacy with TuV as a HuNoV surrogate. Feliciano and others (2012) conducted a study with MNV to examine the sanitization efficacy of manual and mechanical washing protocols used in restaurants. The authors used ceramic plates, drinking glasses, and stainless steel forks as food contact surfaces and contaminated them with cream cheese or reduced-fat milk soils. The mean reductions of MNV on the plates, forks, and drinking glasses after the washing treatment without sanitization were, respectively, 2.6, 1.3, and 0.7 log10 PFU/mL for mechanical washing and 2.8, 1.1, and 1 log10 PFU/mL for manual washing. The mean reductions achieved after mechanical washing with chlorine sanitization and washing with quaternary ammonium (QAC) sanitization were not statistically different (P = 0.0001) from those achieved by the control for either manual or mechanical washing. Fortunately, the viral counts detected on the different surfaces after washing and sanitization were statistically different (P = 0.0001) from the initial viral counts prior to the washing, indicating that washing does have a statistically positive effect, while sanitization had none. Unfortunately, the authors did not directly compare manual washing with mechanical washing (Feliciano et al., 2012).
Individual capsule placement was investigated based on within-group variability and prior findings by the authors (Hamilton & Gibson, 2022). Moreover, even visual inspection suggested that the first capsule position (see Fig. 2) was being cleaned to a lesser extent than other positions during the DSC (see Figs. 5 and 6). All positions were cleaned equally (P > 0.05) in the DHC, and positions 2 and 3 in the DSC were not statistically different from any position in the DHC. Moreover, unsoiled capsules were not statistically different from the prior two groupings. When capsules were soiled, TuV in capsule position 1 underwent fewer log reductions than in any other position in the DSC at P = 0.05. This difference was especially pronounced for the complex soil mixture and pure butter, which were both significantly different from the sugar solution, but not from the protein solution. All soiled conditions in capsule position 1 were significantly different from the unsoiled conditions, DHC, and positions 2 and 3 in the DSC. The observed inconsistencies in cleanliness across the capsule positions in the dishwasher have been previously explained by Hamilton and Gibson (2022). Briefly, the authors have observed that the dishwasher arm is not able to fully access the corners of the dishwasher due to geometric limitations and thus, resulting in inconsistent cleaning. No previous studies on dishwasher placement and pathogen reduction are known to the authors.
Based on these data, the authors continue to recommend that 3DFP capsules be washed on the highest available dishwasher setting and suggest that capsules be placed away from the corners where the dishwasher arm may be less effective. This research seeks to continue to develop the field of AM hygiene as it relates to foodborne illness risk and hygienic recommendations. Additionally, the authors seek to develop the foundation of understanding hygienic efficacy in regard to foodborne pathogen reduction in dishwashers and by manual washing of 3DFP food ink capsules. As was previously recorded by the authors (Hamilton & Gibson, 2022), these results elucidate weaknesses in manufacturer recommendations for cleaning of 3DFP food ink capsules that should be addressed in the manuals which are distributed with the machines.
In the future, this study should be repeated using other foodborne viruses, HuNoV surrogates, and HuNoV strains when their cultivation becomes more feasible. More ratios and sources of macronutrients should be explored. For example, would pea protein powder and cricket protein powder yield similar results as the beef-based protein used in the present study? More dishwasher settings, detergents, and models should be tested, as should the presence of other soiled objects in the dishwasher. Different MW conditions should be tested, such as colder water and shorter washing times. Finally, contamination location on the food ink capsules should be varied. For example, the threads and rubber O-rings present niches not available in the main body of the stainless steel capsule.