Submitted by: Amy Simonne, Ph.D., Associate Professor, Food Safety and Quality

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A recent research publication in the December 2006 issue of Journal of Environmental Health regarding the use of microwave to sterilize cellulose sponges contaminated with raw sewage had brought up a lot of attention to consumers (1). However, it is imperative that consumers acquire additional interpretation and understanding of the experiment before applying the study to a real life situation in their own kitchen.

Researchers (2, 3, 4, 5, 6, 7, 8, 9) have definitely shown that kitchen cloths, sponges and other kitchen food contact surfaces are important factors contributing to cross-contamination of potentially harmful microbes to foods in the home kitchen and restaurants. Many methods (chemical, physical [e.g. heat]) can be used for disinfecting or sanitizing the harmful microbes from those surfaces, but in relation to the use of microwave one study was already conducted by Park and Cliver in 1997 (4)

Park and Cliver (4) clearly demonstrated that household microwave can be use for sanitizing cellulose sponges and cotton cloths. The researchers contaminated sponge and disk cloths with two strains of Escherichia coli and Staphylococcusaureus, two of the typical foodborne bacteria at levels between five hundred thousand to ten million cells and then heated the materials with microwave oven at highest setting (800W at 2450MHz). They found that after 30 seconds, no living bacterial cells were found in full size sponges and dishcloths, however, after wetting additional time required for both were 60 and 180 seconds, respectively. The result suggested that microwave heating is affective for decontamination of cellulose sponge and cotton dishcloths.

In comparison of this study (http://www.ees.ufl.edu/) and the previous work by Park and Cliver (4) in 1997 the conclusions are remarkably similar. Most bacteria are killed within two minutes of microwave heating.

What is the catch? Most scientific studies were conducted under specific conditions, and thus the results may not be necessary applicable to every situation. In both studies, cellulose sponge was the only sponge tested, and thus the effects on other type of sponges many not be the same. Furthermore, because the killing effects on the microbes in both studies come from heat generated by the vibration of water molecules by the microwave, rather than the microwave energy itself (10), changes in the mass of the material and the amount of water may affect the heating rate.

Let’s look at the definitions

Sterilization is a process to eliminate all form of microbial agents such as bacteria, fungi, viruses from a surface, food or biological culture medium. Sterilization process needs some special tools or equipments (autoclave, high dose irradiation, or some form of chemical treatment). Once sterilized, the objects can be re-contaminated again if not protected.

Disinfecting or sanitizing is to reduce the number of microorganism or to partially remove some microbes from a clean surface or clean object to a safe level.

Cleaning is a process of removing food and other type of soil from a surface or objects.

Take home message

Is it necessary to sterilize sponges and dishcloths in your kitchen?

No, per definitions, it is not necessary to sterilize sponges or dishcloths in the home kitchen, but regular cleaning (after each use) and sanitizing (every other day) is definitely necessary. Cleaning remove source of nutrient for microbial growth, and sanitizing reduce the number of bacteria and other germs so that they remain at a low number.

Steps to sanitizing or disinfecting household sponges (cellulose sponges) or cotton dishcloths

In order for sanitizing process to be effective, the sponges or cotton dishcloths must be cleaned first.

Sanitizing process can be done with household chemicals or with heat. For chemical sanitizers to work properly they must be used in a proper manner; this means that a proper combination of concentration and time must be observed. Using chemical sanitizing in home setting can be difficult for many consumers. Further, if the chemicals are not properly used it may cause some harm to the users. Therefore, using heat especially with the microwave can present a simple solution at household level.

In order to meet a minimum requirement for killing live bacterial cells, a temperature of 171ºF or 77 ºC for at least 30 seconds is necessary. This could be done by either heating cellulose sponges in hot water at 171ºF or 77 ºC for at least 30 seconds or using the microwave heating as an alternative. Things to remember: for microwave heating, cellulose sponges must be wet with water.

In the case of microwave heating, both studies (UF study and Park and Cliver) had recommended 2-3 minutes respectively; this allow enough time for the materials to reach a minimum sanitizing temperature.

Use caution when heating sponges or dishcloths in hot water or microwave! Some newer dishwashers may also have sanitizing options for operations.

References

1. D.K. Park, G. Bitton, and R. Melker. 2006. Microbial inactivation by microwave radiation in the home environment. J. Environ. Health. 69(5):17-24.
2. C. E. Enriquez, V.E. Enriquez, and C.P. Gerba. 1997. Reduction of bacterial contamination in the household kitchen environment through the use of self disinfecting sponge. Dairy, Food and Environmental Sanitation. 17(9): 550-554.
3. D. Worsfold and C. Griffith. 1997. Keeping it clean – A study of the domestic kitchen. Food-Science -&- Technology Today. 11(1):28-35.
4. P.K. Park and D. O. Cliver. 1997. Disinfection of kitchen sponges and dishcloths by microwave oven. Dairy, Food and Environmental-Sanitation. 17(3): 146-149.
5. U. Mori, T. Nakano, K. Harada, and T. Ohnishi. 1996. Various antiseptic techniques in the kitchen against Escherichia coli and Vibrio parahemolyticus J. Antibacterial and Antifungal Agents- Japan. 24(2):115-118.
6. M.S. Yepiz-Gomez, K.R. Bright, and C.P. Gerba. 2006. Identity and numbers of bacteria present on table tops and in dishcloths used to wipe down table tops in public restaurants and bars. Food Protection Trends. 26(11):786-792.
7. C. Chaidez and C.P. Gerba. 2000. Bacteriological analyses of cellulose sponges and loofahs in domestic kitchens from developing country. Dairy, Food and Environmental Sanitation. 20(11):834-837.
8. M.P. Doyle, K.L. Ruoff, M. Pierson, W. Weinberg, B. Soule and B.S. Michaels. 2000. Reducing transmission of infectious agents in the home. Control points. Dairy, Food and Environmental Sanitation. 20(6):418-425.
9. P. Rusin, P. Orosz-Coughlin, and C. Gerba. 1998. Reduction of faecal coliform and heterotrophic plate count bacteria in the household kitchen and bathroom by disinfection with hypochlorite cleaners. J. Applied Microbiology. 85(5): 819-828.
10. Anonymous. 2007. News: Microwave oven can sterilize sponges, scrub pads. Food Protection Trends. 27(3):193.