How contamination spreads

Contamination means that something harmful or unwanted gets into food. It can be microbiological, chemical, physical or allergenic. Cross-contamination is the transfer of harmful bacteria or other hazards from a raw or contaminated source to other food, usually ready-to-eat food.

Direct cross-contamination happens when raw food touches ready-to-eat food or drips onto it. Indirect cross-contamination happens through something in between: hands, gloves, boards, knives, cloths, aprons, taps, scales, probes or packaging. Indirect routes are harder to see, so they need more management.

Plan for the main sources: raw meat and poultry, raw fish and shellfish, raw eggs, unwashed fruit and vegetables, soil, people, pests, dirty packaging and waste. Anything that touches these can become a source too.

In storage, use separate fridges for raw and ready-to-eat food if you can. If you cannot, keep raw food below ready-to-eat food, covered and in containers, so that it cannot drip.

During preparation, use separate boards, knives and utensils, often colour-coded, and separate areas. Wash hands between tasks, and always after touching raw food. Use warm running water and soap for at least 20 seconds, and dry with a disposable towel. Do not wash raw meat or poultry, because the splashes spread bacteria.

Fresh produce needs care too. Unless it is labelled ready to eat, treat it as a possible source of E. coli O157. Most of the bacteria will be in the soil. Wash produce well, rubbing it under running water or in a bowl of fresh water, and start with the least soiled items. Keep produce that will be eaten raw away from more soiled vegetables.

Gloves do not replace handwashing. Change gloves as often as hands should be washed, and wash your hands when you change or remove them. Antibacterial hand gels must not replace handwashing either. They may be used afterwards as an extra layer of protection.

Other contamination needs control too. Store cleaning chemicals away from food, in labelled containers. For physical hazards, think about glass, metal, plastic, hair, jewellery, plasters and packaging. Use blue waterproof plasters, keep jewellery to a minimum, check equipment for damage and throw away any food that may contain broken glass.

Here is an example. A supervisor in a sandwich bar finds a tray of raw bacon above an open tub of tuna mayonnaise. Moving the bacon is only the start. The supervisor must throw away the tuna if it may have been contaminated. She then cleans and disinfects the shelf, finds out why it happened, and fixes the cause, which may be that the fridge is too small for the deliveries.