Bacteria from the gut and hide of animals contaminate the meat when the animals are slaughtered and the meat is processed.

Raw meat is then chilled and spoilage is caused by cold-loving (psychrophilic) bacteria. Once they have used up the sugars in the meat, they break down the proteins, forming slime and a putrid odour.

Cured and fermented meats are dry and often contain salts. Cured meats spoil by:

• yeasts which produce aromatic smells and slime
• salt or acid tolerant bacteria which cause souring.

Raw fish and seafood rapidly go off, even though they are stored in ice. There are no carbohydrates in the flesh. So bacteria from the gut and gills start work on the proteins straightaway. They produce the mix of chemicals that gives bad fish its unique smell.

Shellfish should be kept alive as long as possible before cooking and eating to avoid spoilage.

Raw milk can contain many bacteria and it will soon go sour, even if it is chilled. It can also harbour pathogenic microbes and in the past presented a danger to human health from diseases such as TB.

To make it safe and prolong the storage life, most milk is pasteurised. Even so, the milk eventually spoils, due to the growth of cold-tolerant bacteria. However, pasteurised milk tends to go rancid and clotted rather than sour.

Cream is sometimes ‘bitty’ due to the growth of Bacillus cereus bacteria.

Dairy products are made with the help of microbes. But sometimes other, undesirable ones will cause spoilage.

Butter is high in fat with not much water. This means it is slow to go off. Some bacteria, yeasts and moulds may grow and make it rancid and tainted.

Yoghurt is made when bacteria ferment milk. It is acid and can be spoiled by yeasts and moulds.

Cheese is a solidified mixture of milk protein and fats. Its typical flavours are dependent on microbial action during manufacture. The combination of salt, low water content and acidity in mature cheese mean that it keeps well and any spoilage is by yeasts and moulds.