RAI Training

Chemistry: Machine Dish Washing

Understanding the basic chemistry of a commercial dishwasher will help you diagnose what’s up if you aren’t getting good results. Everyone blames the chemical but it’s more than that. A good chemical supplier should set it all up for you and keep it running at optimum efficiency, but if you’re dealing with idiots this will help.

[important]How Dish Wash Works[/important]

It’s very simple. All automatic machines use similar processes in the same order but a professional machine is rather different to a domestic one: it needs to deliver clean, dry crockery consistently and in minutes, not hours. So, all forms of dish wash – even manual washing in a bowl or sink – rely on the same factors:

  • Mechanical removal
  • Chemicals
  • Temperature
  • Time

Commercial machines rely on the same four variables as the machine in your kitchen – it’s just that larger commercial machines need to continually process racks of crockery (each rack being similar to a domestic machine load) and deliver clean, dry plates etc in 2-3 minutes rather than 2-3 hours. A manual (hand) wash relies more on physical removal; warm water, washing up liquid and some scrubbing. But the same principles apply: try washing dishes by hand in cold water. You need an awful lot more detergent and physical scrubbing and chances are you will never get a satisfactory result without warm water.

With automatic washing there is some mechanical action of the water jets hitting soils on crockery – the reason proper stacking is critical – but the automatic process needs to rely on the right chemical environment and temperature for each stage. In a professional machine time is at a premium so reliance on chemical and temperature is far greater as cycle time needs to be short.

[important]The Process[/important]


Removal of uneaten foodstuffs to the waste followed by rinsing by a kitchen porter to remove any soiling not already scraped off. Note that often the kitchen porter will use a cloth dunked in soapy water as part of this process. If water with washing up liquid is carried over into the machine the wash tank can start to foam. Machine wash chemicals are formulated not to foam (for obvious reasons) so if you see foam in the tank this is the usual cause. You will most often see this during breakfast service as egg – largely protein – can be difficult to shift and the temperatures in the machine can set proteins.

Optional Pre-Wash

On some conveyor machines the plates are pre-sprayed to help the wash process. This occurs at 30-40°C.


The wash water is heated with an in-tank electric heat element and mixed with machine wash detergent. The machine wash is dispensed by a pump that is controlled by the machine. This can usually be set to deliver a set amount of chemical at a given point in the cycle or a conductivity probe signals when the machine needs more machine wash.

The same water is used repeatedly from one load to the next. It gradually weirs off to the waste as it is replenished by rinse water weiring over from the rinse tank. The wash tank usually has a large strainer basket to collect food debris, and the strainer is often not emptied until the end of the day’s kitchen operations. This phase usually occurs at 45-60°C.


After the wash phase the items are pulled through a curtain into the rinse on a flight machine; in a hood / under counter machine separate rinse arms kick in to wash the plates etc with clean water – that’s why the sound changes briefly during the final part of the cycle.

The rinse phase uses clean water combined with rinse aid (and sometimes machines have a pre-rinse with clean water too). This phase must occur at 80-90°C as this sort of temperature is needed to activate the rinse aid and most critically for thermal disinfection to occur. Rinse aids tend to contain a lot of alcohol; this lowers the surface tension and evaporation point of water so it sheets off items rather than beading.


There is generally a hot air fan at the end of a conveyor machine to aid drying – but the combination of temperature and rinse aid is usually sufficient for plates to evaporate to dryness very quickly. The main need for a dryer is if plastic implements are to be cleaned – water tends to bead on to these and needs blowing off.


So, irrespective of machine type the ‘journey’ for dishes is cruddy water / very alkaline chemical phase, rinse off with clean water then dry. Let’s consider the water’s journey. Essentially it goes the opposite way to the crockery. Clean (softened) water is boiled up, rinse aid added and this is used to rinse off the crockery at the end of the cleaning process. In a conveyor machine this weirs off into the wash tank, in a hood or under counter machine this water just drops into the tank of cruddy water that’s been washing the plates.