A dishwasher is a mechanical device for cleaning dishes and eating utensils. Dishwashers can be found in restaurants and private homes.
Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, typically between 55 to 75 °C (130 to 170 °F) at the dishes, with lower temperatures used for delicate items. A mix of water and detergent is used for cleaning purposes, followed by 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 (also called rinse aid) can be added to the rinse cycle to improve drying and avoid water spots remaining on dry items.
The word 'dishwasher' (or abbreviated as simply "dish") may also refer to a person who washes dishes in a restaurant, hotel or other private or commercial entities. Pots and pans are also washed by hand by scrubbing them in a detergent 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. Colloquially, a dishwasher may be known as a "dish-pig" or a "pan-diver", from the French "plongeur", and made famous by George Orwell in Down and Out in Paris and London. Commonly used also is the term "KP" for Kitchen Porter or Kitchen Police, who would have a variety of other duties. The area where dishes are washed, particularly in foodservice is sometimes also called a "dish-pit".
The first reports of a mechanical dishwashing device are of an 1850 patent by Joel Houghton of a hand-powered device. This device was made of wood and was cranked by hand while water sprayed onto the dishes. This device was both slow and unreliable. Another patent was granted to L.A. Alexander in 1865 that was similar to the first but featured a hand-cranked rack system. Neither device was practical or widely accepted.[1]
Modern dishwashers are descended from the 1887 invention of Josephine Cochrane who invented a new advanced dishwasher, 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 because her servants were chipping her fine china.
Models installed with permanent plumbing arrived in the 1920s. In 1924, William Howard Livens invented a small dishwasher suitable for domestic use. It had many of the features of a modern dishwasher, including a front door for loading, a wire rack to hold crockery and a rotating sprayer.[2] Livens' invention was not, however, a commercial success. Electric drying elements were added in 1940.
Initially home appliances were standalone or portable devices in a kitchen, along with other sinks and the water heater, but with the development of the wall-to-wall countertop and standardized height cabinets, dishwashers evolved into standardized size and shape appliances first integrated with the sink, and then underneath the kitchen countertop as a modular unit.
Adoption was greatest at first in commercial environments, but by the 1970s dishwashers had become commonplace in domestic residences in the US. By 2005, 60 percent of US homes had dishwashers.[3]
Spray methods, the direction of spray, and the shape of the wash tub has evolved over time due to the difficulty of cleaning some types of dishes. Plates and cutlery tend to be the easiest to clean because of their fairly flat and open shape. Bowls, glasses, pitchers, vases, and other containers are more difficult to clean because of the recessed inner cavity and the need for liquid to drain out of the interior cavity.
The difficulty of cleaning a container in a dishwasher increases as the neck diameter decreases and the interior space width and depth enlarges. This has generally required glasses, pitchers, and other containers to be placed in the device with the opening pointed downward and exposed directly to the spray arm jets or impeller in the base of the unit, so that the liquid is propelled with sufficient force as to reach all the way into the back / top of containers with small mouth openings.
However, for a simple dishwasher with only a single impeller in the base of the unit, glasses and other deep containers both need direct exposure to the jet, and will block liquid from reaching objects behind them. For this reason, glasses usually are placed into an upper basket, and flat open-shaped plates and cutlery that the water can spray through are placed in lower baskets.
Although direct exposure to the impeller spray has the deepest cleaning ability for containers, providing every dish, glass, and pitcher with direct spray in early dishwashers that only had a single impeller in the base of the unit resulted in a fairly light packing of dishware into the device, which may require two or three separate loads to process all the dishware.
In time, dishwashers increased in mechanical complexity to permit more water jets striking dishes directly and unimpeded by other dishware blocking the spray, allowing for much denser packing of dishware into the device.
The first of these mechanical improvements was the top spray arm placed above all dishes and spraying downward, to wet the backsides of bowls and glasses in the top rack. Prior to this, the top rack had to be ring-shaped, to allow impeller water from the base of the unit to reach the top of the chamber unimpeded, and then fall back down onto the dishes from above. The top spray arm provided water to the back of the dishes, allowing the upper rack to enlarge and fill in the open center spray region.
The second was the mid-level spray system, using either a pop-up spray tower extending upward from the base using water pressure to extend the tower during washing, or a fixed tower spray arm supplying water to a secondary rotating arm suspended below the upper rack(s). This mid-level spray system allows for large spray-blocking bowls and pans to be placed in the bottom rack, while still getting water spray up into the undersides of glasses and bowls in the top rack.
However, the pop-up tower or fixed tower method blocks use of the center of the bottom rack, restricting the size of objects that can fit in the bottom rack. A modification has been to reroute water flow to the mid-level spray arm using tubing directed up the back wall of the wash chamber, leaving the center of the bottom rack open and available for use.
Recently there has been the development of fixed or spinning jets on the sidewalls of the wash chamber, as "pot/pan scrubber" jets. Large objects such as cookpots often cannot lie flat if stacked with other plates and dishes, resulting in less than thorough removal of baked-on food due to indirect water spray on the surfaces. The sidewall jets allow water spray from previously unavailable directions, allowing pots and pans to be turned around facing the walls, and receiving individual attention from those specific jets.
Commercial dishwashers are able to deal with the problem of hard-to-clean deep containers by increasing the spray force and liquid volume by using a large pump motor of several horsepower, and large diameter spray arms with larger jet openings, allowing much more water spray than is possible for a residential dishwasher with limited power draw.
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, as 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.
Present-day machines feature a drop-down front panel door, allowing access to the interior, which usually contains two or sometimes three pull-out racks; racks can also be referred to as "baskets". In older U.S. models from the 1950s, the entire tub rolled out when the machine latch was opened, and loading/removing washable items was from the top, with the user reaching deep into the compartment for some items. Today, "dish drawer" models mimic this style, while the half-depth design eliminates the inconvenience of the long reach that was necessary with older full-depth models.
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 damping, and preserve heat to dry dishes faster. They also come at a premium price. 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. All European-made dishwashers feature a stainless steel interior as standard, even on low end models. The same is true for a built-in water softener.
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.
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. A bimetal switch or wax motor opens the detergent door during the wash cycle.
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. Late-model Frigidaire dishwashers shoot a jet of water from the top of the washer down into the upper wash arm, again allowing full use of the bottom rack (but requiring that a small funnel on the top rack be kept clear).
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.
Most dishwashers feature a drying sensor and as such, a dish-washing cycle is always considered complete when a drying indicator, usually in the form of an illuminated "end" light or in more modern models on a digital display, exhibits to the operator that the washing and drying cycle is now over. A dishwasher should never be emptied before a complete process has been signified to be finished by the control system, as this will often leave the contents unwashed or still in a saturated state. It is a common misconception that to empty a dishwasher before the end of a cycle will save energy, as many of the contents may need to be re-run, hence almost doubling running costs.
Dishwashers can be plumbed into either the hot or cold water supply, taking into account the maximum incoming water temperature recommended by the manufacturer.[4] Plumbing a dishwasher into the hot or warm water supply can improve cleaning performance and reduce food debris in the interior of the dishwasher.[4] A few dishwashers may spend much less time on the wash phase if the incoming water is hot, which can compromise cleaning, so results will vary.[4]
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 39 decibels or so. Undamped, low-end dishwashers generally output noise levels of anywhere from 65–70 decibels. Most manufacturers generally use their own nomenclature with trademark for sound damping.
Different kinds of dishwashing detergent contain different combinations of the items in the list below. Not all of the ingredients below are used in some detergents.
Dishwashing detergent may also contain :
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.[6]
Besides older style detergents for dishwashers, biodegradable detergents also exist for dishwashers. These detergents may be more environmentally friendly than conventional detergents.
Prior to the invention of the dishwasher in 1886, hand-washing primarily with simple detergents was common. The invention of the machine prompted the use of stronger detergents and rinse agents, thus saving time. 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 detergent and water.
Rinse aid (sometimes called rinse agent) contains surfactants that uses Marangoni stress to prevent droplet formation, so that water 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.
In some countries, especially those in Europe, dishwashers include a built-in water softener that remove calcium and magnesium ions from the water. Dishwasher salt, which is coarse-grained sodium chloride (table salt), is used to recharge the resin in the built-in ion-exchange system. The coarse grains prevent it from clogging the softener unit; unlike certain types of salt used for culinary purposes, it does not contain added insoluble anticaking agents or magnesium salts. The presence of magnesium salts will defeat the purpose of removing magnesium from the water softener. Anticaking agents may lead to clogging or may contain magnesium. Table salt may contain added iodine in the form of sodium iodide or potassium iodide, but these compounds will not affect the ion-exchange system, but adding table salt to the dishwasher's water softening unit can damage it.
If a dishwasher has a built-in water softener there will be a special compartment inside the dishwasher where the salt is to be added when needed. This salt compartment is separate from the detergent compartment, and generally located at the bottom of the wash cabinet (this is below the bottom basket). On most dishwashers, an automatic sensing system will notify the user when more dishwasher salt is required.
Pouring detergent into the salt compartment will damage the water softening system, however this can be reversed if the user acts very quickly and the dishwasher is NOT used: with a suitable wet and dry vacuum cleaner, remove the foreign substance e.g. detergent, followed by adding water again and removing the water with the wet and dry vacuum and repeating the process several times.[7]
Some newer dishwashers allow the use of "all in one" tablets/detergents (which include an amount of salt along with detergent and a rinse agent) and are marketed as an alternative to using separate salt and rinse aid, but dishwasher salt must still be added to the salt compartment in very hard water areas. The use of such "all in one" detergents does not mean that separate salt and rinse aid is not required, as omitting separate salt and rinse aid will impair the cleaning and drying results and may cause limescale damage, also, incorrect use of "all in one" tablets/detergents may not be covered under the dishwasher's warranty; it is advisable to check the dishwasher manufacturer's instruction book when using these types of tablets/detergents.[8]
Some dishwasher detergents are marketed for use in hard water areas for dishwashers which do not have a built-in water softener (and therefore do not use any dishwasher salt). These detergents use higher levels of phosphates to increase the solubility of hard water ions. In very hard water areas, the amount of phosphate may still be insufficient and the manual addition of dishwasher salt into the detergent compartment is recommended. Adding salt along with the detergent does not soften the water as does a dishwasher with an ion-exchange water softener, but the water will gain some additional ability to dissolve hard water ions. Note, however, that as water drops remaining on the dishware evaporate, deposits of the salt will likely remain. To combat this, the use of a rinsing agent which cause the water to "sheet" will help eliminate the spotting.
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.
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 less soluble in the presence of silicates in the water (added as anti-metal-corrosion agents in the dishwasher detergent). Since the cloudy appearance is due to nonuniform glass dissolution, it is (somewhat paradoxically) less marked if dissolution is higher, i.e. if a silicate-free detergent is used; also, in certain cases, the etching will primarily be seen in areas that have microscopic surface cracks as a result of the items' manufacturing.[9][10] Limitation of this undesirable reaction is possible by controlling water hardness, detergent load and temperature.[11] 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.
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[12]
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.[13] Some items can be damaged if washed in a dishwasher because of the effects of the chemicals and hot water. Aluminium items will discolour. Saucepan manufacturers often recommend handwashing due to the harsh effects of the chemicals on the pan coatings.[14] Valuable items — such as antiques or hand-painted items, 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.[15] 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 (167 F); 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.
The heat inside the dishwasher dries the contents after the final hot rinse. Plastic and non-stick items may not dry properly[16] compared to china and glass, which hold the heat better. Some dishwashers incorporate a fan to improve drying. Older dishwashers with a visible heating element (at the bottom of the wash cabinet, below the bottom basket) may use the heating element to improve drying, however this uses more energy.
Governmental agencies often recommend air-drying dishes by either disabling or stopping the drying cycle to save energy.[17]
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.[18]
Large heavy-duty dishwashers are available for use in commercial establishments (e.g. hotels, restaurants) where a large number of dishes must be cleaned.
Unlike a home dishwasher, commercial units typically are not multi-level, and only wash a single tray of dishes per cycle. This is not an inconvenience since trays are batch-processed consecutively one after the other. They can wash a rack of dishes, or a rack of 25 glasses in just approximately one minute.
Some commercial dishwashers work similar to a commercial car wash, with a pulley system that pulls the rack through a small chamber (Known widely as a "crank-back" system). Manual washers require an operator to push the rack into the washer, close the doors, start the cycle, and then open the doors to pull out the cleaned rack, possibly through a second opening into an unloading area.
In the UK, the British Standards Institution 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.[19] There are many types of commercial dishwashers including under counter, single tank, conveyor, flight type, and carousel machines.
Commercial dishwashers often have significantly different plumbing and operations than a home unit, in that there are often separate spray arms for washing and rinsing/sanitizing. The wash water is heated with an in-tank electric heat element and mixed with a cleaning solution, and is used repeatedly from one load to the next. The wash tank usually has a large strainer basket to collect food debris, and the strainer may not be emptied until the end of the day's kitchen operations.
Water used for rinsing and sanitizing is typically delivered directly from building water supply, and is not reused. The used rinse water falls into the wash tank reservoir, which dilutes some of the used wash water and causes a small amount to drain out through an overflow tube. The system may first rinse with pure water only, and then sanitize with an additive solution that is left on the dishes as they leave the washer to dry.
Additional soap is periodically added to the main wash water tank, from either large soap concentrate tanks or dissolved from a large solid soap block, to maintain wash water cleaning effectiveness.
In the European Union, the energy consumption of a dishwasher for a standard usage is shown on a European Union energy label.
Comparing the efficiency of automatic dishwashers and hand-washing of dishes is difficult because hand-washing techniques vary drastically by individual. According to a peer-reviewed study in 2003, hand washing of an amount of dishes equivalent to a fully loaded automatic dishwasher could use between 20 and 300 liters of water and between 0.1 and 8 kWh of energy, while the numbers for energy-efficient automatic dishwashers were 15 to 22 liters and 1 to 2 kWh, respectively. The study concluded that fully loaded dishwashers use less electricity, water, and detergent than the average European hand-washer.[20][21] For the automatic dishwasher results, the dishes were not rinsed before being loaded. 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; hand washers needed between 65 and 106 minutes.
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, USA, is considering a bill to limit phosphates in dish detergent to help clean up the Chesapeake Bay.[22]
In addition, rinse aids have contained nonylphenol and nonylphenol ethoxylates. These have been banned in the European Union by EU Directive 76/769/EEC.
Many recipe websites have noted that a dishwasher can be used to cook certain foods, in particular salmon. [1] [2] [3] [4] It's very important that all foods be completely sealed in a wrap such as aluminum foil to avoid contamination with soaps or rinse aids.