Bakteriereduktion i diskmaskiner
Studie av olika metoder för att reducera bakterier
Bacteria are a problem that occurs naturally in dishwashers when the dish stands there dirty for an extended period of time. To get rid of the unpleasant smell that the bacteria is causing, the most common solution is to run a regular washingcycle which can cause problems for the environmentally-conscious consumers since this causes unnecessary energy waste and emissions of chemicals in the washwater. To solve the problem in a more environmentally-friendly way by directly attacking the bacteria with an appropriate bacterialreductionmethod can therefore be an attractive feature in a dishwasher. There are many different ways to treat surfaces that have been exposed to the bacteria. Some high-profile methods are UV-radiation, microwaves and oxidant (ozone) that all three have had success in reducing bacteria in different contexts. There are also other solutions that are already used in dishwashers and maybe suitable for bacterial reduction in addition to their regular purpose. These are drying with hot air and shorter washingprograms in various temperatures. The aim of this study is to, after the tests, evaluate which of these methods will work best when it comes to bacterialreduction in a dishwasher.The method that by far gives the best result in reducing the simulated bacteria is rinsing with heated water without dishwasherdetergent. This method is also what most dishwasher-manufacturers already have and encourage the consumers to use if it begins to smell bad from a half-loaded machine.The tests are done on regular plates contaminated with a solution of yeast and sodiumchloride (NaCl) that has dried on the plates. The plates are after every contamination exposed to a bacteriareductionsystem and then tests with so called pressureplates are taken on the plates contaminated area. The pressureplates consist of an agarsolution that works as food to the yeastcells. The plates are incubated in a heatingcabinet for 24 hours in which each yeastcell that has been transferred from the plate to the pressureplate forms a white dot, thus an evaluation of the different bacterialreductionsystems can be made by comparing the pressureplates appearance after the incubation is complete. The results of the bacterialreductionmethods are compared against each other and against reference plates, which have only been contaminated with yeast cells and not subjected to any treatment.The methods UV-radiation and microwaves that work well for bacterialreduction in other areas are difficult to apply practical in a dishwasher. In the case of UV radiation the problem lies in reaching out to all areas that need to be disinfected without anything getting in the way to shade the radiation. The microwaves have difficulties to affect the yeast cells probably because of the absence of water in the dried solution. The oxidizing agent potassium permanganate which is used in the absence of ozone was found to oxidize with the dried yeast and cause it to be dissolved and flow of the plates. In actuality the ozone would function in a similar way with the difference that it is a gas, and thus also can reach more inaccessible areas which the floating potassium permanganate misses. The problem with ozone is that it is expensive and difficult to manage, if the dishwasher leaks it would be very dangerous and the gas can also damage both the dish and the machine components.Exposing the contaminated plates of a drying system with heated air gave some reduction of yeast cells but what proved to have the best impact on the yeast where washprograms that already exist in most dishwashers. Rinsing both with cold and hot water showed reduction of yeastcells without any dishwasherdetergent, although the heated water gave substantially better results. Instead of developing specific bacterial reduction methods and applying those in dishwashers, the company is recommended to examine the existing washprograms and its effect on the bacteria. This allows an optimal germicidalrinsing-program in terms of temperature and length to be developed. Then the challenge is to get consumers to actively use the program instead of a regular washcycle for half-full machines.