Nutrition
Milk
Cows’ milk is the type of milk most commonly consumed. Other types that are also used are ewes’ and goats’ milk. Most milk undergoes some form of heat processing such as pasteurisation, sterilisation or ultra high temperature (UHT) treatment. This is to ensure that any harmful micro-organisms are destroyed before the milk is consumed and to improve keeping qualities.
Milk has been derived from many types of mammals and put to different uses over the centuries. This has resulted in the development of a number of milk products. These include butter, cheese, cream and yogurt.
Types
Milk is an emulsion, ie a suspension of tiny fat droplets in water. The fat droplets are coated with a thin layer of protein which helps maintain the suspension. The water also contains protein, sugar (lactose), mineral salts and water-soluble vitamins.
Cream is made by separating the fat and solids from milk. It is a fat-in-water emulsion. When whipping or double cream is whipped it changes from a liquid to a foam. This is due to the partial denaturation of the proteins, which stabilise the mixture by trapping air.
Cheese can be classified in a number of ways, for example by the place where it is produced or the method of production. Generally it is composed of milk solids, including some water and other ingredients. An enzyme called rennin is used to clot the milk and produce the milk solids (casein curd) and liquid (whey) which is drained off. The different kinds of cheese result from different methods of production and raw ingredients.
Yogurt is milk which has been coagulated and soured by lactic acid. The lactic acid is produced by the addition of harmless bacteria.
Butter is a water-in-oil emulsion made from cream. Its composition is controlled by law:
* Milk fat, at least 80% but less than 90%
* Dry non-fat milk material, no more than 2%
* Water, no more than 16%
CHARACTERISTICS
Milk : When milk is heated a skin may develop on its surface. This is due to the coagulation of proteins. To avoid this, the milk should be agitated (ie whisked).
Cheese : Cheese may become tough and rubbery due to over-cooking. The protein shrinks and squeezes out fat and water. This is known as syneresis. If cheese is added to sauces it must be grated or cut into small pieces and added at the end of the cooking process. This is so that it can melt quickly, without leaving lumps.
Butter : Butter can be used for a variety of functions, including:
Aeration - Products such as cakes need air incorporated into the mixture in order to give a well risen texture. This is achieved by creaming butter with caster sugar. Small bubbles of air are incorporated and form a stable foam.
Shortening - Short crust pastry, biscuits and shortbread rely on fat to give their characteristic crumbly texture. The fat coats the flour particles and prevents them from absorbing water. This prevents gluten formation.
Flavouring - Butter is often used in cakes, biscuits and sauces because of its rich distinctive flavour.
Retention of moisture - Butter can help retain a bakery product’s moisture and therefore increase its shelf-life. It can be used to baste food being cooked by dry heat.
NUTRITION
Milk and milk products (cheese and yogurt) are important sources of protein, B vitamins such as riboflavin and B12, and minerals such as calcium, zinc and magnesium. These days, they are available with a range of fat contents.
STORAGE
Most milk and milk products should be stored in a refrigerator to slow down the growth of micro-organisms, eg mould on cheese. Milk, cheese and yogurt, once opened, should be stored and used by the date specified on the packaging.
Cereals
Wheat is the cereal most commonly used for the manufacture of food products, although many other types of cereals are used. Each cereal has unique properties which make it suitable for a variety of food products. Cereals require different conditions to grow. For example, rice is grown in damp tropical climates, eg India and China, and oats in cold temperate climates, eg Scotland. Cereals are also known as ‘staple foods’ as they often make up the bulk of the diet since they are relatively cheap to produce.
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