Dishwashers
by Chris Woodford. Last updated: March 3, 2022.
When you're feeling dirty, you jump in the shower. When your pots and pans are dirty, they take a shower too—in a magic machine called a dishwasher. Scrubbing old food off dirty dishes is a tedious chore many people love to hate. Thanks to an ingenious American called Josephine Cochran (1839–1913), who invented the first automatic dishwasher in 1886, jobs like this can be a thing of the past. In Cochran's machine, you simply loaded your dirty crockery into baskets and the machine showered it clean with jets of hot and soapy water. Dishwashers have changed very little in the century since then—but how exactly do they work? Have you ever stopped to think what goes on inside the machine after you close the door? Let's take a closer look!
Photo: Inside a dishwasher. When the door closes, pieces of rubber around the frame ensure a tight seal so no water can escape. Note the soap dispenser on the top of the open door.
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Contents
How a dishwasher differs from hand washing
When you wash your dishes by hand, the water and soap stays still in the bowl or sink and you move the dishes around as you scrub them with a cloth or a brush. In a dishwasher, the opposite happens: the dishes and cutlery stay still in plastic baskets while pressurized jets of hot water shoot all around them. That means a dishwasher has to be sealed shut all the time it's operating or the water would fire off in all directions—all over your kitchen floor!
A dishwasher starts its cycle by taking in cold water from a hose connected to the machine. Once a certain amount of water's sitting inside the bottom of the machine, a heating element starts to warm it up. The element is just a thick metal bar that gets hot when an electric current passes through it, and it gradually heats the water during the first part of the wash cycle. An electric pump takes the warming water and forces it up pipes in the side of the machine, which are connected to two spinning paddles. There's one paddle, made of metal, underneath the bottom rack of dishes and another one, made of plastic, under the top rack.
Photos: This is the plastic paddle underneath the top rack of the dishwasher. In the first photo, which is taken from the side, you can see where water enters the paddle through a large pipe. The water squirts up from small holes in the top of the paddle, which you can see in the second photo (looking down on the paddle from directly above).
When water enters the paddles, it makes them spin around much like garden sprinklers. As the paddles rotate, the water emerges from small holes in their upper surface. The paddles make lots of hot spinning jets of water that fire upward onto your dirty plates. (That's why it's best to arrange your crockery so the dirty surfaces are facing downward.) The bottom rack and the bottom paddle are nearer to the heating element so the water is much hotter in the lower part of the machine. (That's why you'll see some crockery items marked "Top rack dishwasher safe"—which means it's alright to put them in the upper, cooler part of the machine.) After the water hits your dishes and plates, it falls back to the bottom of the machine, where it's heated up once more by the element and pumped round again for another cycle. Water pumps around the machine for half an hour or so until all the dishes and plates are clean. A sieve at the bottom of the machine catches any large bits of debris (to stop the machine jamming up), while smaller bits (and food remains) simply flush down the drain.
Photo: 1) This is what the bottom of the machine looks like when you remove the lower rack. The curved dark gray loop on the left is the heating element. You can see the metal paddle in the center. The holes in its upper surface make very hot jets of water that fire up into the lower rack. The drain holes are just beneath the paddle on the upper right. Just beneath it is a compartment where you pour salt to make the machine wash properly (the white oval shape).
Photo: 2) The detergent dispenser in the door of the dishwasher, shown here in its open position. The spring catch at the bottom flips the door open when the machine starts. The soap tablet drops down into the floor of the machine and dissolves in the hot water.
What gets the dishes dry at the end of the cycle? The water that washes them is so hot that it simply evaporates away in the heat. Providing you don't overload your machine, the dishes should dry naturally with no need for you to wipe them afterward.
How hot does the water need to be?
Dishwashers aim to strike a balance between speed, economy, and effectiveness. The hotter the water and the longer the wash, the more electricity you'll use and the more expensive each wash will be. In terms of hygiene, the hotter the water, the shorter the time you'll need to kill off any pathogens (bacteria and other nasties) lingering on your pots and pans. According to the [PDF] World Health Organization (summarizing a number of different scientific studies), water at temperatures close to that used in pasteurization (~60°C or 140°F) kills pathogens rapidly, with "inactivation times" ranging from a few seconds up to 30 minutes (depending on the pathogen). In other words, if your dishwasher is running at 60°C for 30–60 minutes, you can be fairly confident it's getting your things hygienically clean. By the same token, you can see that typical handwashing—a dip and swizzle in lukewarm water, with bits of old food floating around in it, using a brush or a sponge you might not have cleaned for days or weeks—is really not very hygienic at all.
Why dishwashers need detergent
The other essential ingredient in a dishwasher is liquid or solid (tablet-form) detergent to help break down the grease and grime on your crockery.
You load up the detergent into a dispenser in the door and, sometime during the wash cycle, usually a while after the machine first switches on, the dispenser flips open, dropping or dripping the detergent into the hot water bath in the bottom of the machine. Dishwasher detergents work much the same as clothes detergents, which you can read all about in our article on detergents. Briefly, they consist of a mixture of ingredients for tackling the various different kinds of food deposits likely to be left on your cutlery and crockery, plus things like water softeners, anti-corrosion chemicals to stop the machine rusting, and attractive fragrances.
Chart: The ingredients in a typical dishwasher detergent liquid. Sodium tripolyphosphate (STPP) is an alkaline "builder" (water softener). Sodium silicate helps to prevent rust. Sodium carbonate breaks down acids and fats. Chlorine compounds disinfect and help to break down protein-based food remains. Sodium sulfate is an anionic surfactant; nonionic surfactants are also present. Other additives include enzymes and fragrances. Source: Based on the approximate figures quoted in Table 9.1, Liquid Detergents. Edited by Kuo-Yann Lai. CRC Press, 2005, p.321.
Apart from detergent, dishwashers also need occasional supplies of "salt" to reduce limescale and keep their built-in water filters working properly.
Dishwashers and kids don't really mix. If you have a dishwasher, be sure to keep the detergent tablets well out of reach of younger children; according to a recent scientific study, in the United States alone, over 62,000 children under the age of six were exposed to toxic laundry and dishwashing detergents in just one year. Another study by paediatricians in New Zealand confirms that dishwasher tabs can cause serious internal injuries: "Dishwasher detergents are highly corrosive substances that cause potentially life-threatening injuries and ongoing morbidity." It recommends limiting the strength of the chemicals in detergents and more use of child-safe packaging.
