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Dishwasher

A dishwashing machine
A dishwashing machine

The term dishwasher can represent either a person who washes (cleans) dishes (a term commonly used in the food service industry) or a machine that performs a similar function. The latter usage is discussed in this article.


A dishwasher is a mechanical device for cleaning food utensils for preparation, keeping, serving and eating and drinking. They are found in restaurants and also in many kitchens of homes.

How dishwashers work

Unlike manual dishwashing, which relies largely on mechanical action to remove soiling, mechanical dishwashers use the circulation of quite hot (55-65 degrees Celsius or 130-150 degrees Fahrenheit) water (usually, but not always heated or brought up to temperature by an element) and very strong detergents (most far too alkaline for habitual exposure to the skin) to achieve its cleaning effect. The dishwasher therefore is mainly a device for spraying water on the dishes - first detergent-added water for cleaning purposes, then clean water (though sometimes with a rinsing aid added) to remove the detergent residue. Some dishwashers also contain a heating element to achieve fast drying of the dishes.

History

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 steam boat. 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 1920s, and 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 situations.

Brands

Higher-End

Midrange

Low-End

Characterisation

Capacity

The capacity of a dishwasher according to international standards is measured in 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 compatability before buying a dishwasher.

Size

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 mininum 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 castors 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.

Features

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.

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. Pre-rinsing is not necessary even without integrated waste disposal units - all that is required is for the larger items of food waste to be removed before placing in the dishwasher. 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.

Sound Dampening

Using blankets, panels, and sound-absorbing materials in various configurations, dishwashers can achieve sound dampening 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 dampening, i.e. QuietGuard (Kenmore), QuietPartner (Whirlpool), Whisper Package (Maytag), followed by a number. Higher numbers usually indicate higher sound dampening and thus less noise output. Thus, a QuietPartner 1 or QuietGuard 2 dishwasher, despite the "Quiet" designation, may not actually be quiet at all.

Sound dampening is the primary factor that determines the cost of a standard built-in dishwasher.

Detergent

Dishwashing detergent contains:

  • Phosphates
    - Solublises calcium and magnesium ions to prevent 'hard-water' type limescale deposits.
  • Oxygen-based bleaching agents
    - Breaks up and bleaches organic deposits.
  • Non-ionic surfactants
    - Lowers the surface tension of the water, emulsifies oil, lipid and fat food deposits, prevents droplet spotting on drying.
  • Enzymes
    - Breaks up and solublises protein-based food deposits, and possibly oil, lipid and fat deposits.
  • Anti-corrosion agents
    - Often sodium silicate, prevents corrosion of dishwasher components.

it 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
  • Perfumes
  • Anti-caking agents (in granular detergent)
  • Starches (in tablet based detergents)
  • Gelling agents (in liquid/gel based detergents)

Dishwasher detergents are strongly alkaline (basic).

 

Hazing of glassware, prohibition on dishwashing lead crystal

Glassware that is washed by dishwashing machines often develops 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:

  • Limescale deposit
    - 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 be 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.
  • Physical abrasion
    - Glassware placed such that it is physically touching can abrade and produce a milky surface.

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 is not good for the health of subsequent users.

[Comment from Retiredchemist: This description of filming/etching is incorrect. Silicate in the detergent protects glass from etching but only during the wash cycle - it is rinsed away after the wash. Etching occurs in the final (hot) rinse if the water supply has a high temporary (i.e. bicarbonate) hardness .AND. the water softener is used. The water softener replaces calcium with sodium. Sodium bicarbonate decomposes to sodium carbonate (by losing CO2)in the hot final rinse. Sodium carbonate solution is alkaline and leeches out SiO2 from the glass.]

Level of sterilisation

Domestic dishwashers do not sterilise the utensils, as proper sterilisation requires autoclaving at 121°C with pressurised wet steam for at least 15 minutes. Dishwashers (even commercial ones used in restaurants) do not do this. Commercial dishwashers can use one of two types of sanitisation methods. One is to use hot water sanitising, using final rinse water at a temperature of at least 83°C (180°F). The other method is a chemical sanitisation method, that many commercial low temperature machines use, using chlorine injected in the final rinse water.

Commercial dishwashers

Much larger heavy-duty dishwashers with high throughput are available for use in catering establishments where a large number of dishes are to be washed. Commercial machines are capable of washing a rack of dishes in just a few minutes in very hot water, which sanitises the dishes and means that they dry in air very quickly after they are removed from the machine. Many of these machines work on a conveyor-belt system, accepting a rack of dirty dishes in one side and ejecting a rack of clean dishes on the other side a minute or two later.

Sources

http://www2.whirlpool.com/html/homelife/cookin/cookdw5.htm

http://www.ccspa.org/conseducation/SDAC_autodish.html

http://www.newi.ac.uk/buckleyc/materials.htm#Glass

http://www.scienceblog.com/community/older/2003/F/20033788.html

http://www.yale.edu/ynhti/curriculum/units/1997/7/97.07.05.x.html

See also

 

External links

Brand Websites

 


 

 

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