Microbes and food 1. Menu - you are what you eat
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Cheeses photo
Picture 1.17a there is a huge variety of cheeses available.
1.17 Cheese
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What is it?
Cheese-making started around 8,000 years ago and today there are probably 1,000 different types of cheese. Cheese is made from milk, by fermenting it with special types of bacteria. The characteristic flavours of cheeses come from the type of milk used and from the action of microbes during manufacture and storage. Turning milk into cheese considerably extends its storage life.
How is it produced?
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Milk contains the sugar lactose and the protein casein as well as fat, water, minerals and vitamins. During the cheese-making process the sugar is converted to acid and the protein is coagulated. The water is removed, leaving a solid or semi-solid product.

The flow chart below shows how it is made. Roll your cursor over the headings (starting with 'Milk') and the arrows to find out more.

It is important to use high quality milk. This is usually pasteurised for safety reasons. A mixed starter culture of lactic acid bacteria such as special types of Lactobacillus, Lactococcus, and Streptococcus is added to start the fermentation.

The bacteria convert the milk's lactose to lactic acid.

Curdled milk. Rennet is added to the ripe milk. It contains the enzyme chymosin which causes the casein to coagulate, trapping moisture and fat globules. This solid yet soft gel is called the cheese curd. Rennet used to be extracted from calves’ stomachs but now chymosin is often used which is produced by fungi commercially – another example of a microbial food ingredient. The timing and amount of enzyme added differs according to the type of cheese.
Picture 3. Flow chart of cheese production.
Ripe milk. The fermentation runs for about an hour and the pH falls due the production of lactic acid.
Curds are mostly protein and fat. They are washed, salted, chopped and pressed into moulds. The salting helps to control enzymic activity as well as the growth of microbes.
Whey is over 90% water and is left behind when the curd is formed. It also contains about 5% lactose.

Whey is dried for use in animal feeds and is also used in a variety of food products.
Soft cheeses like cottage cheese contain more water than hard cheeses.

Cheeses like Brie and Camembert are sprayed with Penicillium camembertii which forms a soft white mycelium on the surface, giving the cheese its characteristic flavour and texture.

Blue cheeses are inoculated with cultures of different moulds and yeasts. Sometimes the microbes are added to milk or curds in the vat, others are inoculated into the formed cheeses using wires. Penicillium roquefortii is used to produce Gorgonzola, Stilton and Dolcelatte. The spores quickly germinate into fungal threads which run along cracks in the cheese.
Hard cheeses like Cheddar contain less than 40% water. During manufacture, the cut curds are heated to 38-42°C which makes them shrink and become firmer. The starter culture can survive the heat and continues to grow and produce acid. When the acid level has reached around 0.25% the whey is run off and the curds are formed into blocks in a manual process called ‘cheddaring’. More whey comes out. Then the blocks are chopped into small pieces to ensure even distribution of salt before being reformed into blocks which are pressed to get rid of any remaining whey and trapped air. The cheese is then left to mature.
How does it spoil?
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Cheese in the fridge photo
Picture 1.17 Cheese keeps well in the fridge, but will eventually go mouldy.
Cheese keeps well due to its low pH (about 5.0 in Cheddar), the low water content resulting from whey removal and the high salt content. Any spoilage is caused by yeasts and moulds which can grow in these conditions. They are controlled by excluding air from the cheese by either giving it a wax coating (the traditional method) or by some form of vacuum packaging. Refrigeration also prolongs storage life. Hard cheeses keep much better than soft ones as they contain less water.

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Can it be harmful?
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Cheese is considered to be a safe food due to its acidity, salt content and low water activity. Enormous quantities are made and eaten each year without problems. The greatest risk of food poisoning comes from three types of bacteria, although others have been implicated in cases throughout the world.

Salmonellae can grow in cheese during manufacture and storage, but they are unlikely to be present if the cheese has been made from pasteurised milk. Most cases of illness have been due to the cheese being contaminated due to poor hygienic practices.

Listeria monocytogenes is more likely to be found in soft, mould ripened cheeses which are still made with raw milk in some countries. This bacterium does not generally affect healthy people, but the young, elderly, immuno-compromised and pregnant women are at risk. The bacteria can cause flu-like symptoms, blood poisoning or meningitis and can harm unborn babies.

E. coli O157 is acid tolerant, unlike most coliforms, and may be found in cheese that has been badly handled or has been made from raw milk. It can cause severe illness in the young, elderly or immunocompromised.

Control of unwanted microbes in cheese is achieved by good manufacturing practice and attention to hygiene. Sometimes preservatives are added to cheese. Storage and packaging are also important.