A dishwasher is a mechanical device for cleaning dishes and eating utensils. They can be found in restaurants and private homes.
How dishwashers work
Unlike manual dishwashing
, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot (55–65 degrees Celsius
or 130–150 degrees Fahrenheit
on the dishes. First detergent-added water is used for cleaning purposes, then clean water to remove the detergent residue. Some dishwashers have multiple wash and rinse periods within the complete cycle. In some dishwashers, a rinsing aid can be added to the rinse cycle. As there is no human contact during the process, strong detergents
may be used which would be too alkaline
for habitual exposure to the skin
. Many dishwashers have a heating element to achieve fast drying and sanitation of the dishes. In some models, this element can also be used to heat cold water to the desired wash temperature
The word dishwasher may also refer to a person who washes dishes in a commercial setting. These employees rinse dishes, load them into a stainless steel dishwasher, unload them, and stack them into their respective dish holders. Pots and pans are also washed by hand by scrubbing them in a soap and water mix, immersing them in a rinse of plain water, and then immersing them in a water/sanitizer solution for a period. Silverware is washed by placing loose silverware in a tray, washing them several times like this, then sorting them into circular holders, and washing them again in the dishwasher.
The first reports of a mechanical dishwashing device are of an 1850 patent by Joel Houghton
of a hand-powered device.
Modern dishwashers are descended from the 1886 invention of Josephine Cochrane, also hand-powered, which she unveiled at the 1893 Chicago World's Fair. Cochrane was quite wealthy and was the granddaughter of John Fitch, the inventor of the steamboat. She never washed dishes herself and only invented the dishwasher as her servants were chipping her fine china.
Models installed with permanent plumbing arrived in the 1920s. In 1937, William Howard Livens invented a small dishwasher suitable for home. It had all the features of a modern dishwasher, including a front door for loading, a wire rack to hold crockery and a rotating sprayer. Electric drying elements were added in 1940.
Adoption was greatest at first in commercial environments, but by the 1970s dishwashers had become commonplace in domestic residences in the US.
The international standard for the capacity of a dishwasher is expressed as standard place settings
. Dishes or plates of irregular sizes may not fit properly in a dishwasher's cleaning compartment, so it is advisable to check for compatibility before buying a dishwasher.
Commercial dishwashers are rated as plates per hour. The rating is based on standard sized plates of the same size. The same can be said for commercial glass washers, they are based on standard glasses, normally pint glasses.
Dishwashers that are installed into standard kitchen cabinets have a standard width and depth of 60 cm
(Europe) or 24 inches
(US), and most dishwashers must be installed into a hole a minimum of 86 cm
(Europe) or 34 inches
(US) tall. Portable dishwashers exist in 45 and 60 cm
(Europe) 18 and 24 inch
(US) widths, with casters and attached countertops. Dishwashers may come in standard or tall tub designs; standard tub dishwashers have a service kickplate beneath the dishwasher door that allows for simpler maintenance and installation, but tall tub dishwashers have approximately 20% more capacity and better sound dampening
from having a continuous front door.
The inside of a dishwasher, called the tub, can be composed of plastic
or stainless steel
. Stainless steel tubs resist hard water
, provide better sound dampening
, and preserve heat to dry dishes faster. They also come at a price premium. Older models used a baked enamel
on steel and are prone to chipping and erosion; chips in the baked enamel finish must be cleaned of all dirt and corrosion then patched with a special compound or even a good quality two-part epoxy
Mid-to-higher end North American dishwashers often come with hard food disposal units, which behave like miniature garbage (waste) disposal units that eliminate large pieces of food waste from the wash water. One manufacturer that is known for omitting hard food disposals is Bosch, a German brand; however, Bosch does so in order to reduce noise. If the larger items of food waste are removed before placing in the dishwasher, pre-rinsing is not necessary even without integrated waste disposal units. Pre-rinsing under a running tap beforehand simply wastes water.
Many newer dishwashers feature microprocessor-controlled, sensor-assisted wash cycles that adjust the wash duration to the quantity of dirty dishes (sensed by changes in water temperature) or the amount of dirt in the rinse water (sensed chemically/optically). This can save water and energy if the user runs a partial load. In such dishwashers the electromechanical rotary switch often used to control the washing cycle is replaced by a microprocessor but most sensors and valves are still required to be present. However, pressure switches (some dishwashers use a pressure switch and flow meter) are not required in most microprocessor controlled dishwashers as they use the motor and sometimes a rotational position sensor to sense the resistance of water, when it senses there is no cavitation it knows it has the optimal amount of water.
Most dishwashers include a large cone or similar structure in the bottom dish rack to prevent placement of dishes in the center of the rack. The dishwasher directs water from the bottom of the dishwasher up through this structure to the upper wash arm to spray water on the top dish rack. Some dishwashers, including many models from Whirlpool and Kitchenaid, use a tube attached to the top rack that connects to a water source at the back of the dishwasher which allows full use of the bottom rack.
Some dishwashers include a child-lockout feature to prevent accidental starting or stopping of the wash cycle by children. A child lock can sometimes be included to prevent young children opening the door during a wash cycle. This prevents accidents with hot water and strong detergents used during the wash cycle.
Modern dishwashers are quieter than older models. Using blankets, panels, and sound-absorbing materials in various configurations, dishwashers can achieve sound damping levels down to 44 decibels
or so. Undampened, low-end dishwashers generally output noise levels of anywhere from 65–70 decibels
. Manufacturers generally use their own nomenclature with sound damping, i.e. QuietGuard (Kenmore
), QuietPartner (Whirlpool
), Whisper Package (Maytag
), etc. How this nomenclature translates into decibel level is different for every manufacturer.
Dishwashing detergent contains:
- Phosphates (30 %)
- Dissolves calcium and magnesium ions to prevent 'hard-water' type limescale deposits.
- Oxygen-based bleaching agents (older-style powders and liquids contain chlorine-based bleaching agents)(1-5 %)
- Breaks up and bleaches organic deposits.
- Non-ionic surfactants (5%)
- Lowers the surface tension of the water, emulsifies oil, lipid and fat food deposits, prevents droplet spotting on drying.
- Breaks up and dissolves protein-based food deposits, and possibly oil, lipid and fat deposits. Proteases do this by breaking down the proteins into smaller peptides that are more easily washed away.
- Anti-corrosion agents
Dishwashing detergent may also contain:
- Anti-foaming agents
- Used as foam decreases the effectiveness of the washing action.
- Additives to slow down the removal of glaze & patterns from glazed ceramics
- Anti-caking agents (in granular detergent)
- Starches (in tablet based detergents)
- Gelling agents (in liquid/gel based detergents)
- Sand (inexpensive powdered detergents)
Dishwasher detergents are strongly alkaline (basic).
Inexpensive powders may contain sand, which can be verified by dissolving the powder in boiling water and then passing the solution through a coffee filter. Such detergents may harm the dishes and the dishwasher. Powdered detergents are more likely to cause fading on china patterns .
Besides chemical detergents for dishwashers, biodegradable detergents also exist for dishwashers. These detergents may be more environmentally friendly than conventional detergents, one example of a biodegradable detergent is Seventh Generation Inc.
Hand-washing dish detergent (washing up liquid) should not be used in a dishwasher, as it will create a large foam of bubbles which will leak from the dishwasher. If hand-washing detergent is accidentally used, the foam may be removed by spraying with salt, and the dishwasher should be forced into a drain cycle to remove the soap and water.
(sometimes called rinse agent
) contains surfactants that prevent droplet formation by reducing the surface tension of the water, so that it drains from the surfaces in thin sheets, rather than forming droplets.
The benefits of using it are that it prevents "spotting" on glassware (caused by droplets of water drying and leaving behind dissolved limescale minerals), and can also improve drying performance as there is less water remaining to be dried.
Dishwasher salt is used to recharge the built-in ion-exchange water softener in dishwashers. Unlike salt used for culinary purposes, it does not have added iodide salts. However, it should have minimal iron and manganese salt content, as these mineral ions tend to form precipitates that clog the ion-exchange resin.
Some dishwasher detergents are marketed as not requiring the use of dishwasher salt. These instead use increased levels of phosphates to increase the solubility of hard water ions. In very hard water areas, the amount of phosphate may be insufficient, requiring the additional use of salt in any case. Some newer dishwashers have a setting for "all in one" tablets. Incorrect use of "all in one" tablets may not be covered under the warranty; it is advisable to check the instruction book when using these types of tablets.
Pouring table salt, detergent (or anything other than dishwasher salt) into the salt compartment will damage the water softener unit.
However, it is possible to use salt granules or tablets sold for whole house softening units and save a significant amount of money in the process. Softener salt in 25Kg bags will work out about one third of the cost and is easily available.
If the dishwasher has an exposed heating element at the bottom near the salt compartment, the heating element may still be very hot to touch after using the appliance.
Hazing of glassware, prohibition on dishwashing lead crystal
Glassware washed by dishwashing machines can develop a white haze
on the surface over time. This may be caused by any or all of the below processes, only one of which is reversible:
If the dishwasher has run out of the salt that recharges the ion exchange resin
that softens the water, and the water supply is "hard", limescale deposits can appear on all items, but are especially visible on glassware. It can be removed by cleaning with vinegar or lemon juice, or a proprietary limescale removal agent. The dishwasher should either be recharged with salt, adjusted appropriately for the hardness of the supply water—or possibly this is a symptom of failure of the ion exchange resin in the water softener (which is one of the more expensive components). The resin may have stopped working because it has been poisoned by iron or manganese salts in the supply water.
Silicate filming/etching/accelerated crack corrosion
This film starts as an iridescence or "oil-film" effect on glassware, and progresses into a "milky" or "cloudy" appearance (which is not a deposit) that cannot be polished off or removed like limescale. It is formed because the detergent is strongly alkaline (basic) and glass dissolves slowly in alkaline aqueous solution. It becomes more soluble in the presence of silicates in the water (added as anti-metal-corrosion agents in the dishwasher detergent). In certain cases, the etching will primarily be seen in areas that have microscopic surface cracks as a result of the items' manufacturing. Limitation of this undesirable reaction is possible by controlling water hardness, detergent load and temperature (see Maytag Web site
on spotting and filming on the dishes
). The type of glass is an important factor in determining if this effect is a problem. In hard-water areas more detergent is needed to help prevent etching, and some dishwashers can reduce this etching effect by automatically dispensing the correct amount of detergent throughout the wash cycle based on the level of water hardness programmed. GE Appliances website
Glassware placed such that it is physically touching can abrade and produce a milky surface.
Components found in dishwasher detergents can chemically scour the glass, causing tiny crystals
, which can precipitate further crystal growth that can turn entire glasses cloudy
Lead crystal should not be cleaned in a dishwasher as the corrosive effect of dishwasher detergent is high on such types of glass—that is, it will quickly go 'cloudy'. In addition, the lead in the crystal glass can be converted into a soluble form, which could endanger the health of subsequent users.
Items that should not be put in a dishwasher
Some items can be damaged if washed in a dishwasher because of the effects of the chemicals and hot water. Lead crystal will be irreversibly damaged if put in a dishwasher, while aluminium items will discolour. Saucepan manufacturers often recommend handwashing due to the harsh effects of the chemicals on the pan coatings. Valuable items—such as antiques—should be washed by hand as they may be dulled or damaged, and detergents will gradually fade the glazing and print. Sterling silver and pewter will oxidize and discolour from the heat. Furthermore, pewter has a low melting point and may warp in some dishwashers. Cast iron is likely to rust in a dishwasher.
Items soiled by wax, cigarette ash or anything which might contaminate the rest of the wash load (such as poisons or mineral oils) should not be put in a dishwasher. Objects contaminated by solvents may explode in a dishwasher. Glued items, such as some cutlery handles or wooden cutting boards, may be melted or softened if dishwashed, especially on a hot wash cycle when temperatures can reach 75 °C; these high temperatures can also damage plastic items which are labelled as not being dishwasher safe, however some plastic items can be distorted or melted if placed in the bottom rack too close to an exposed heating element, hence many dishwasher-safe plastic items advise placing in the top rack only (many newer dishwashers have a concealed heating element away from the bottom rack entirely). Squeezing plastic items into small spaces may cause the plastic to distort in shape.
Dishwashers should only be used to wash normal household items, like plates, cutlery, cups, mugs, kitchenware etc. Items such as paintbrushes, tools, furnace filters etc. should not be put into a dishwasher as this will cause the subsequent washes to become contaminated and may cause damage to the appliance.
How to load a dishwasher
When loading a dishwasher, ensure that all the utensils are facing either upwards or downwards. Keeps cups, bowls and pots upside down. By keeping these sideways, the water pressure may not reach the entire surface to clean it. This may not be visible as there may not be any food particles left on it. Plates may be kept standing as the surface is flat and allows the water pressure to clean it properly. Knives should be placed with the handles
facing upwards to avoid injury. Sharp kitchen knives must be place in a special knife holder, if available.
Do not overlap items and ensure water can reach all the surfaces of each item.
Loading items from the back in the top rack allows filling the rack without having to completely extend it to reach over items already loaded. It also prevents tipping an entire unsecured dishwasher forward when a front-filled drawer is fully extended to the forward position.
A counter-top machine does not give the "top-rack" space, so place the plastic items on top of the cutlery box or try putting it a little higher than the bottom of the machine away from any visible heating element.
Items coated with strong coloured foods and sauces may stain white or other light plastic items; to avoid this the items must be rinsed beforehand.
Pots and pans and other items with burnt-on food are best soaked for a few hours prior to dishwashing and washed on the most intensive cycle – however delicate glassware and other delicate items should NOT be washed on an intensive cycle to avoid damage.
The heat inside the dishwasher dries the contents after the final hot rinse. Rinse aid is used in the final rinse cycle to allow water to run off items and prevent water droplets forming. Plastic and non-stick items may not dry properly compared to china and glass, which hold the heat better. Some dishwashers incorporate a fan to improve drying.
Governmental agencies often recommend air-drying dishes by either disabling or stopping the drying cycle to save energy.
Level of sanitizing
Dishwashers do not sterilize
the utensils, as proper sterilization requires autoclaving
at 121 °C with pressurized wet steam for at least 15 minutes. Commercial dishwashers can use one of two types of sanitizing methods: hot water sanitizing (using final rinse water at a temperature of at least 83 °C (180 °F)), or chemical sanitizing (by injecting chlorine in the final rinse water). Not all dishwashers are capable of reaching the high temperature required for hot water sanitizing. Medical grade dishwashers and sanitizers are starting to use ultrasonic cleaners, which use a liquid bath treated with sonics to remove particles and sterilize instruments.
Most consumer dishwashers use a 75°C thermostat in the sanitizing process. During the final rinse cycle, the heating element and wash pump are turned on, and the cycle timer (electronic or electromechanical) is stopped until the thermostat is tripped. At this point, the cycle timer resumes and will generally trigger a drain cycle within a few timer increments.
Most consumer dishwashers use 75°C rather than 83°C for reasons of burn risk, energy consumption, total cycle time, and possible damage to plastic items placed inside the dishwasher. With new advances in detergents, lower water temperatures (50–55°C) are needed to prevent premature decay of the enzymes used to eat the grease and other build-ups on the dishes. This also saves energy and can allow the washer to be hooked directly to the hot water supply for the house.
In the US, residential dishwashers can be certified to a NSF International testing protocol which verifies the cleaning and sanitation performance of the unit.
Large heavy-duty dishwashers are available for use in commercial establishments (e.g. hotels, restaurants) where a large number of dishes must be cleaned. Commercial machines can wash a rack of dishes in just a few minutes. In the UK, the British Standards Institute
set standards for dishwashers. In the US, the NSF International
(an independent not-for-profit organization) sets the standards for wash and rinse time along with minimum water temperature for chemical or hot water sanitizing methods. There are many types of commercial dishwashers including under counter, single tank, conveyor, flight type, and carousel machines.
Dishwashers and the environment
Dishwashers versus washing dishes by hand
Comparing the efficiency of automatic dishwashers and hand-washing of dishes is difficult because hand-washing techniques vary drastically by individual. A 2004 peer-reviewed study concluded that the best automatic dishwashers available at the time, when fully loaded use less electricity, water, and detergent than the average European hand-washer. The most efficient hand-washers in that study, however, were far more energy efficient than the dishwashers. In particular, higher-end dishwashers that are capable of heating water internally do not lose heat during transport to a sink. The study does not address costs associated with the manufacture and disposal of dishwashers, the cost of possible accelerated wear of dishes from the chemical harshness of dishwasher detergent or the value of labour saved.
Dishwasher detergents and rinse aids
Most dishwasher detergent contains complex phosphates
, as they have several properties that aid in effective cleaning. However, the same chemicals have been removed from laundry detergents in many countries as a result of concerns raised about the increase in algal blooms
in waterways caused by increasing phosphate levels (see eutrophication
). The State of Maryland
is considering a bill to limit phosphates in dish detergent to help clean up the Chesapeake Bay.
In addition, rinse aids have contained nonylphenol and nonylphenol ethoxylates. These have been banned in the European Union by EU Directive 76/769/EEC.