As people live lifestyles where they work longer hours and are unable to focus on kitchen hygiene it has enabled the incidence of foodborne illness to increase. Communal kitchens, (such as those in student accommodation and hostels) have also allowed the prevalence of food poisoning to increase due to there being a lack of responsibility or knowledge for kitchen hygiene between those who are sharing the kitchen, (Ojima et al., 2002; Sharp and Walker, 2003). In the year 2000, in England and Wales, there were 86,528 reported cases of foodborne illness, (Sharp and Walker, 2003) however there could have been more as many cases of foodborne illness are not reported. It has been estimated that between 50-80% of these cases originate from the home, (Redmon and Griffith, 2005; Sharp and Walker, 2003) due to incorrect practices when handling, preparing and consuming food, (Mattick et al., 2003; Sharma et al., 2009). Places within kitchens that are frequently touched with human hands or areas that are moist are found to be the most contaminated with ‘high numbers of faecal coliforms, coliforms and heterotrophic bacteria than other areas in the kitchen', (Sharma et al., 2009). As various foodborne pathogens such as E. coli and Salmonella spp. can survive on hands, surfaces, utensils and kitchen cloths or sponges for hours upon initial contact there is potential for foodborne disease, (Kusumaningrum et al., 2003; Rusin et al., 2002). If a contaminated object or contaminated hands come into contact with a person's mouth they can get food poisoning so in order to prevent foodborne illness improved hygienic methods in the kitchen should be employed, (Carrasco, et al., 2008; Rusin et al., 2002).
Foodborne illness can be caused by consuming food or drink which is contaminated with microorganisms or toxins produced by them. Escherichia coli is a relatively harmless microorganism as it is present in the intestinal tract of humans and is involved in our metabolism by synthesising vitamins such as vitamin K, (Farthing, 2004). However, pathogenic strains of E. coli, such as E. coli 0157:H7, can produce enterotoxins which are exotoxins that are secreted from the cells. These enterotoxins act on the small intestines where they cause them to secret large amounts of fluid into the intestinal lumen which results in vomiting and diarrhoea. (Madigan, et al., 2005). The genus Salmonella, like E. coli, includes Gram-negative, rod-shaped bacteria which are also involved in foodborne illness. In contrast to E. coli, Salmonella spp. do not produce toxins but large amounts of Salmonella which have been ingested colonise the large and small intestines which results in common foodborne illness symptoms such as vomiting and diarrhoea. (Madigan, et al., 2005). Even though Staphylococcus aureus is a microorganism which usually colonises the skin, it can cause foodborne illness where it produces enterotoxins like E. coli. However some enterotoxins produced by S. aureus act as superantigens where they induce large numbers of lymphocytes which leads to intestinal and systemic inflammation, Madigan, et al., 2005). Other microorganisms which cause foodborne illness include Campylobacter jejuni, Clostridium perfringens, Listeria monocytogenes.
Kitchen sponges are commonly used during washing-up, to wipe sinks and to wipe kitchen surfaces. They can act as reservoirs for foodborne pathogens as they remain wet, enabling the microorganisms to survive. As air drying isn't sufficient enough to kill these pathogens then other means of disinfection are needed to do so to prevent the spread of these microorganisms when the sponges are in use, (Sharma et al., 2009). A study by Mattick et al., (2003) showed that kitchen sponges which were used during the washing up process had transferred E. coli 0157:H7 to surfaces as well as Salmonella spp. but the former was transferred more frequently. Josephson, Rubino and Pepper, (1997), showed that 33 % of kitchen sponges from ten kitchens in the US were contaminated with E. coli and 67 % were contaminated with faecal coliforms. Other studies involving kitchens from the US found that there were 15.4 % of sponges that were contaminated with Salmonella spp., (Enriquez, Enriquez-Gordillo and Gerba, 1997) and 4 % were contaminated with Staphylococcus aureus according to Hilton and Austin, (2000). They also demonstrated that kitchen sponges had higher bacterial counts when compared to dishcloths and this was confirmed by Josephson, Rubino and Pepper, (1997). All of these studies indicate that disinfection of kitchen sponges is needed following their use in order to prevent the spread of foodborne pathogens.
Studies have shown that the most effective ways to inactivate microorganisms in kitchen sponges is by using microwave irradiation or a dishwasher as opposed to chemical means, (Mattick et al., 2003; Sharma et al., 2009). One of the earliest studies by Olsen, (1965) used microwave energy to eliminate the presence of microorganisms in bread and found that it reduced the numbers of spores produced by Aspergillus niger, Penicillium spp., as well as Rhizopus nigricans following two minutes of heating at 5 KW.
Microwaves have been used for ‘many industrial applications such as tempering, thawing, blanching, cooking, dehydration, sterilisation and pasteurisation', (Yaghmaee and Durance, 2005). They have been employed in many settings such as in hospitals where they are used for the sterilisation of hospital waste and sterilising medical utensils, (Kim et al., 2009). Other uses include the decomposition of organic material as well as for the elimination of pathogens present in animal manures, food and soil, (Banik et al., 2003; Hong, et al., 2004; Kim et al., 2009; Tonuci et al., 2008).
Microwave heating has been known to kill many microorganisms within short exposure periods such as Bacillus cereus, Campylobacter jejuni, Clostridium perfringens, Escherichia coli, Enterococcus, Listeria monocytogenes, Proteus mirabilis, Pseudomonas aeruginose, Salmonella spp., Staphylococcus aureus, Streptococcus faecalis, and Listeria spp., (Woo et al., 2000; Yaghmaee and Durance, 2005). However for sterilisation of spore-forming microorganisms longer periods of exposure are needed (Woo et al., 2000), but no microorganism has been found to be microwave resistant, (Yaghmaee and Durance, 2005).
A microwave works by emitting non-ionising radiation and can cause different ‘biological effects depending upon field strength, frequencies, wave forms, modulation and duration of exposure', (Banik et al., 2003). Within the electromagnetic spectrum, microwaves are ‘between millimetre waves, (0.01 m) and radiowaves, (1 m), corresponding to frequencies between 30 and 0.3 GHz', (Hong et al., 2004). Microwave are able to disinfect materials by dielectric heating where heat is generated by polar molecules, (such as water and methanol) absorbing the high frequency radiation which causes them to vibrate and align with each other with the oscillating electrical field, (Banik et al., 2003; Hong et al., 2004; Kim et al., 2009; Tonuci et al., 2008). The agitation of the dipolar molecules causes surrounding molecules to vibrate too so the generated heat and spread throughout the non-metallic material, (Banik, et al., 2003; Tonuci et al., 2008)
But how does a microwave eliminate microorganisms? This isn't currently known as there is controversy between whether microbial death is due to thermal effects caused by the microwave or by non-thermal effects due to ‘difficulty in keeping the temperature constant during the microwave irradiation', (Banik et al., 2003). Thermal effects induce the causing ‘the denaturation of nucleic acids, proteins and membrane disruption' (Yaghmaee and Durance, 2005) and non-thermal effects result in DNA leakage, the release of proteins from the cells and cell surface damage, (supported by the elimination of certain microorganisms below their thermal destruction point), (Woo et al., 2000). Papadopoulou et al., (1995), was the first to report that the elimination of microorganisms could be due to differences between thermal and non-thermal effects of microwaves when observing the effects microwaves had on enterobacteria.
In this study the effects of a microwave oven at 455 W on four different species of bacteria which were inoculated onto pieces of kitchen sponge were observed. The microorganisms used were Bacillus subtilis, Escherichia coil, Salmonella typhimurium and Staphylococcus aureus and they were each subjected to different lengths of microwave heating, (15 seconds, 30 seconds and 60 seconds). These microorganisms were chosen as they are involved in foodborne illness apart from Bacillus subtilis which was chosen to act as ‘an optimum indicator bacterium for microwave sterilisation', (Kim et al., 2009). There were controls used for each species which were not heated within the microwave. This study was conducted to determine if microwave heating is sufficient enough to eliminate the majority of microorganisms. This would demonstrate whether or not using a microwave would provide a fast and effective method of disinfection which could be employed in domestic households to fit in with our busy lifestyles.