Salmonella

Salmonella bacteria are found in the gut of many animals, including cattle (which form the main source of the organism), fish and poultry, and are particularly prevalent in those farmed intensively. Modern poultry farming is overwhelmingly based on intensive methods, and chickens are the commonest source of Salmonella infection.

Chickens are reared for two purposes:

• Egg production (laying hens)

  
  The majority of laying hens are maintained in laying cages ('batteries'). Each cage is occupied by up to six birds, with an available area of 450-750 cm² per bird. The systems are highly automated, with total control of the environment. Eggs are laid on to a sloping floor so that they roll immediately on to a moving conveyor belt which takes them to the cleaning and packaging facility. Droppings fall through the cages and are removed from beneath.

  
  There are outdoor systems, known as 'free range' in which the birds must have continuous daytime access to outdoor enclosed areas with a maximum stocking density of 1000 birds per hectare. In 1994, the free range system accounted for 11% of the eggs laid in the UK.

  
  

• Meat production (broilers)

  
  Typically, these birds are raised in deep litter houses on a batch system geared to producing a group of birds of a given weight in a given time. Again, there is close control of the environment. The floor of a deep litter house is disinfected then covered with wood shavings, peat or other 'litter'. This inevitably becomes heavily contaminated with droppings which may carry Salmonella, as well as other gut bacteria and viruses. When a batch of birds has reached the required weight, the whole lot are slaughtered and the broiler house is cleaned and disinfected, ready for the next batch. After slaughter the carcasses are washed and chilled rapidly to below4.4 °C to slow down bacterial reproduction.

The close proximity of the birds in these systems (l0 000 in one deep litter unit) and the huge size of the flocks (commonly over 100 000, and up to 3 million birds) inevitably favour cross contamination and hence spread of the organism,

Salmonella typhimurium is the organism most commonly involved in causing food poisoning: Salmonella enteriditis comes a close second. (Salmonella typhi causes the much more serious disease, typhoid.)

Many broiler flocks are infected with Salmonella bacteria, as are many eggs. The organism can be transmitted to chicks by contaminated feed, and between birds in the infected droppings, with which birds are in close and prolonged contact. It is inevitable that some of the bacteria in the gut, and on the skint of infected birds remain in the carcass after slaughter. Eggs may become contaminated in the oviduct, or by faeces in the area where they are laid. The bacteria survive cold storage of the carcass.

Salmonella can enter the human body from a variety of sources, but those originating from chickens do so when the following are eaten.

• Undercooked infected chicken meat

  
  

• Undercooked infected eggs

  
  

• Food which has been cross-contaminated
  This can happen during unhygienic preparation and/or storage. People handling raw chicken, in shops for example, can spread bacteria to other foods on their hands or via knives or other utensils; pre-cooked meats which are then contaminated and subsequently eaten cold are a particular problem in this respect.

If contaminated meat has been stored at too high a temperature, the Salmonella bacteria may have had time to reproduce to numbers sufficient to cause infection. In this case, the bacteria enter and reproduce in the cells of the gut wall, causing both mechanical damage and releasing toxins which kill cells and inflame tissues. Enzyme production and absorption of both digested food and water are disturbed.

Salmonella food poisoning can also result from drinking unpasteurised milk.

AVOIDANCE OF 'FOOD POISONING' BY Salmonella
Salmonella is destroyed at temperatures above 56 °C and so normal, thorough cooking kills it.

Normal measures of personal hygiene and kitchen hygiene should be adopted. Personal hygiene has been referred to earlier in this booklet. With regard to basic kitchen hygiene, most texts which deal with food poisoning include a pictorial representation af a 'horror kitchen' in which a large number af undesirable hygiene practices are being committed; it is recommended that students prepare a critique af one af these, and/or of an actual kitchen. With regard to Salmonella, the following specific items are especially important.

• Chickens, whether fresh or ready cooked, should be refrigerated and, if fresh, cooked within two days af purchase

  
  

• Separate (preferably non-wooden) chopping boards, as well as separate knives and other utensils, for raw and cooked foods. After preparing raw food, all utensils should be thoroughly washed and dried.

  
  

• Raw chicken MUST be kept separate from other cooked meats and juices MUST NOT be allowed to drip on to other foods.

  
  

• Frozen chickens MUST be thoroughly defrosted before cooking. The instructions on the packaging should be carefully followed. Defrosting is best done in a refrigerator so that the bird remains as fresh as possible. If it is done in the open kitchen, the carcass should be covered to exclude flies, dust and airborne bacteria.

  
  

• Before starting to cook the bird it is essential to make sure that no ice remains in the body cavity and that the thighs have softened. If the chicken is not totally defrosted, it may not become thoroughly cooked right the way through to the centre. For this reason, it is also better to place stuffing under the skin at the neck end, rather than in the body cavity.

  
  

• Do not keep a cooked chicken carcass warm for a long period of time. Do not leave to cool for more than one hour before refrigerating.

  
  

• Do not reheat chicken, or any other food, more than once and then it must be made piping hot all the way through. (There are particular risks here in relation to barbecue meals).

  
  

• Do not eat raw eggs, or home-made uncooked egg dishes such as mayonnaise, mousses or ice cream

  
  

• Do not drink unpasteurised milk.

  
  

• 'Sell by' and 'Use by' dates should be strictly observed.