Washing machines are used to automatically or semi-automatically clean trays, racks, bins, utensils and in the case of dishwashers glasses, crockery and cutlery.
Because of the high volume of trays, baskets, tubs, containers, etc. required by many modern businesses it has become unrealistic and uneconomic to clean manually. A large bakery, for example, may require 2,500 baskets an hour to keep a continuous production flow.
Washing machines come in many shapes and sizes and are generally built for the cleaning of a specific size and type of item. Washing machines should only be used for the original purpose intended. The efficacy of a clean is dependent on correct original design of the machine (for the items being cleaned) and correct operation and maintenance. The washing machines must be managed with regular cleaning of the machine and filters, regular changing of wash solutions, inspection of wash and rinse nozzles and control / monitoring of detergent temperatures and strengths.
Washing Machines are either tunnel type machines or single tank machines.
Tunnel or Flight Machines
With tunnel machines items are placed on a conveyor which transports them through a number of stages of the washing process. Each stage occurs in a different section of the washer; pre-rinse, wash with detergent, rinse, disinfect chemically or by temperature and then dry.
Single Tank Machines
With single tank machines the item is placed in a cabinet and the wash process takes place in the cabinet by sequencing the cleaning stages (prerinse, wash, rinse, disinfect).
Cleaning Stages
The short contact time with detergent and the relatively low impact energies of the wash nozzles means that to clean effectively high chemical energies are required. In most situations a high alkaline low foam detergent at 0.25% to 1% v/v is used at 55 to 75ºC.
Ideally pre-washing should be carried out using warm water from a separate tank to avoid ‘bake on’ of protein at latter higher temperature washing stages. The water temperature must be suitable for the type of soil to be removed but in general 30-50(C. this may be heated by overflow from detergent wash or rinse tanks.
A detergent wash should be carried out from a separate tank using a low foaming detergent appropriate to the equipment being cleaned and nature of soiling. Caustic based detergents are usually the most efficient and cost effective types of detergents used although chlorinated and neutral products are sometime used. A chlorinated detergent must not be used above 55(C as it can attack stainless steel. A neutral or inhibited detergent may be employed for cleaning of sensitive items, such as aluminium trays.
Because different detergents are more suited to different water conditions, a water hardness test needs to be carried out. This will ensure that the risk of scale drop out is reduced. It may also prove beneficial in hard water areas to soften the water supply by having an on-site water softening system.
Rinsing should be carried out with water fed either directly from a heat exchanger or a separate enclosed non-recirculating tank. Where rinsing is not followed by a final disinfection stage, the rinse water should be maintained at a minimum of 82(C. Attention must be paid in hard water areas, as there could be a significant problem of scale drop out so ensure that the machine is regularly de-scaled. Where the rinse water temperature is below 82(C, this should ideally be followed by a disinfection stage; particularly in a high care environment. The disinfectant used is generally a low foam disinfectant.
Cross contamination by Washing Machines
With all modern washing machines certain cleaning solutions are reused; for instance the detergent solution. The detergent solution will become contaminated with soil from the washed items and therefore provide a transfer mechanism for allergens.
The potential for this cross contamination can be controlled by having specific dedicated washing machines for items that have allergen contamination, by washing such items separately (for instance in a three sink system) or by validating the operation of the washing machine and then putting management controls in place to ensure its correct operation.