Keywords or Contents or Home


Biological contamination

Chemical contamination





Cross contamination




Food handlers

Foreign bodies


Microbial contamination

Physical contamination

Unfit food


4(i) Introduction

4(ii) Microbes

4(iii) Biological Contamination

4(iv) Chemical Contamination

4(v) Physical Contamination

4(vi) Summary

4(i) Introduction

Contamination is the process by which food fit for human consumption becomes unfit by the addition of outside material. Contaminants includemicrobes, chemicals, dust, debris and physical items.

Section 4 of the 1984 Food Act (Warning UK only) makes it a criminal offence to sell food for human consumption which is unfit (eg contaminated or decomposed).

Thus it is important to look at the sources and prevention of contamination.

4(ii) Microbes

Microbes such as bacteria, yeast and moulds can contaminate food in many ways.

Raw foods especially meat contains microbes. Normally bacteria are on the outside and in the gut of animals but when slaughtered, the equipment such as knives, if not properly cleaned, are excellent at transferring the organisms into all areas of the meat. Rolling meat also moves microbes across the joint. Mincing and dicing meat also spreads microbes.

Defrosting meat, specially poultry, produces liquid (drip) rich in microbes. Drip can easily contaminate fridges, sinks and work surfaces. Poultry should be thawed in a clean area and kept separate from other foods. In catering establishments a separate fridge should be used for raw meat, poultry and fish and in home fridges poultry and raw meat should be kept on the bottom shelf and never above dairy products or other high risk foods.

Water if not properly treated or chlorinated can cause microbial contamination.

Soil is rich in microbes and as such a good way of contaminating food.

Care has to be taken when removing soil from vegetables that the soil and its microbes do not contaminate food, especially cooked and high risk vulnerable food.

Dust also contains microbes that can contaminate food if precautions are not taken. Food areas should be kept cleaned and well ventilated and the food should be covered.

Food handlers can contaminate food with bacteria if they do not follow high personal hygiene standards. Touching, sneezing, coughing and breathing over food, crockery or machinery which will be in direct contact with food, can cause contamination. Dirty hands are covered in invisible rods, spindles and bunches of poisonous berries, which can cause disease when in contact with food.

Pests and pets have very unhygienic habits so have to be kept out of food areas. Cats and dogs support large colonies of microbes on their coats or pelts and if left to wander round food areas they are very efficient at contaminating equipment. After all their pelts and coats are made of hair which is dead organic matter which is a good food material for many microbes. Other microbes just use the spaces between hair as places to hide and wait until transferred to a substance on which they will grow. Rats and mice coats are even worse, being greasy they will smear microbes on any food or equipment they rub up against. They will also contaminate by leaving a trail of faeces behind them. To protect food it should be stored in rodent proof containers and should never be stored where rodents have easy access such as on the floor or stacked next to walls.

Insects especially flies and cockroaches frequently contaminate food with microbes. Insect hair is as good as animal hair for transporting bacteria. Flies feed by sending fluids out of their mouth to digest their meal externally. This fluid often contains part of their previous meal which is just as likely to be dung or faeces as human food. They also defaecate whilst eating so can effectively contaminate food in three different ways. Hence food needs to be protected from insects by being kept covered.

Cross contamination is a very common way of contaminating food with microbes. The common way in which microbes are spread by cross contamination is HESS

a) Handler's hands.

These should be washed when entering a food area, after working with raw meat and unwashed vegetables and after dealing with waste and refuse.

b) Equipment

This includes knives, mincers, food mixers, industrial processors and fillers. All equipment should be cleaned and disinfected before use and when changing product, especially when changing from a source rich in microbes to a high risk food. eg When a slicer is used for slicing cooked meat after slicing raw meat. Also all equipment should be cleaned and disinfected immediately after use to prevent microbial growth.

c) Surfaces

Surfaces such as chopping boards, sinks and floors can be a source of cross contamination. These surfaces should be kept clean at all times and spillages should be wiped up as they arise and not left as a breeding ground for microbes.

Raw and cooked food should be dealt with on separate surfaces to reduce cross contamination. Also separate sinks should be used for hand washing and cleaning equipment.

Cloths and mops can spread microbes rather than clean, if care is not taken. The ideal is to have floors hosed down and to use disposable cloths. If this is not possible due to poor design, mops should be cleaned and disinfected after use then left to dry. Sink taps should always be cleaned along with the rest of the sink.

d) Storage areas

Raw food and cooked food should always be stored separately in hot areas, fridges, cold stores and freezers. The ideal is to use three fridges; one for raw meat and fish, one for dairy products and one for cooked foods. Unfortunately at home most people only use one fridge so raw food should be on the bottom shelf, cooked food on the middle shelf and dairy products on the top shelf.

4(iii) Biological Contamination

This is when food is contaminated by matter of a biological origin. The most common type is human hair from handlers who have failed to adequately cover all hair on the scalp or face. Hair from pets or pests can also be found if they are allowed to contaminate food. Rodent droppings can also be found in uncovered and unprotected food.

Another source of contamination is dead insects or parts of their bodies when sufficient care has not been taken over the storage and preparation of food.

A rarer type of biological contamination is when poisonous plants are mixed with similar looking food; e.g. poisonous berries mixed with redcurrants or blackberries or toadstools contaminating mushrooms.

4(iv) Chemical Contamination

This normally occurs by three methods:

a) Addition

b) Migration

c) Absorption of undesirable odours

a) Addition of chemicals

This is when chemicals, such as cleaning fluids, sterilants, or pesticides are added to food by mistake. Sometimes contamination can arise when preservatives are added in quantities far above permitted levels. In industry the chemicals used for cleaning plants (industrial equipment) or C.I.P. (Cleaning In Place) can contaminate food or be packed instead of the food product.

Chemical contamination can be avoided by always storing chemicals in labelled containers away from food. All addition of preservatives and chemicals in C.I.P. systems should be carefully monitored.

b) Migration of chemicals

This is where chemicals such as metals or chemical toxins migrate from equipment such as pipes or saucepans or storage containers such as tin cans, into food.

Only plastics which are safe for food use, should be used to store foods. Food should not be stored in open tin cans and care should be taken with aluminium saucepans when used to cook high acid foods.

c) Absorption of undesirable odours

This is when a large range of unwanted smells are 'picked up' by foods. Foods with high fat content are most prone to odour contamination. The odours 'picked up' can be anything from perfume or aftershave from handlers to paint from the walls. Chemicals used in food preparation and storage areas and even the odour of other foods can be picked up. For example eggs can pick up fish odours if stored near to each other.

4(v) Physical Contamination

Physical contamination is when physical objects are allowed to be mixed into or added to food either during preparation, processing or storage. These physical objects are called foreign bodies.

One of the most dangerous foreign bodies to contaminate food is glass. We all know how sharp broken glass is and how it can easily cut through skin. Glass in food is a lot more dangerous as it can cause internal bleeding and damage to organs which might be fatal.

Glass should not be used in food preparation, production and processing areas or even storage areas if the food is not covered or protected. In areas where food is at risk even light bulbs or filaments should have plastic coverings.

Wood especially wooden pallets or pieces of flaking wood should be kept out of food production and preparation areas to avoid food being contaminated by pieces or splinters of wood. Wooden chopping boards should be avoided and old or damaged chopping boards should be thrown away.

The decorations of walls and ceilings should be carefully maintained to avoid food being contaminated by flaking paint or plaster.

All machinery and equipment should be properly maintained to avoid parts falling off into the food. Rubbers and plastic parts such as gaskets should be regularly changed as when they age they become brittle and pieces might break off, contaminating food. Where machine oil is used care must be taken that there are no leaks, which might let the oil contaminate the food. When engineers are working on machines they must make sure that they do not leave any nuts or bolts around the production or preparation areas, to prevent them finding their way into the food. Also all nuts and bolts on machinery must be tight to prevent them falling off.

Food handlers can be a source of physical contamination if they do not follow good personal hygiene. Jewellery should not be worn as items of jewellery such as rings or gems could contaminate food if they fall in. Nails should be kept short and no varnish used as this could break off into the food. Smoking is banned in food production, preparation, and storage areas to prevent cigarette ends and ash contaminating food.

4(vi) Summary

Microbes can contaminate food via:

Raw food



Food handlers

Pests and pets

Cross contamination (HESS)

The most common types of biological contamination are human hair and dead insects.

Chemical contamination is normally by addition, migration or absorption. The most frequent risk is from unlabelled containers containing cleaning or disinfectant fluids kept near food.

Physical contamination (or foreign) bodies can arise from a wide range of articles but the most dangerous is glass.

Most types of contamination can be avoided by keeping food covered.


© R. T. L. Berg 1999