• 2 Dairy products • a) Characteristic, composition and factors affecting:• proteins in milk• caseins• whey proteins• lactalbumin• lactoglobulin• lactose content• b) Processes involved and reasons for:• milk processing• pasteurisation• homogenisation• sterilisation• c) The processes used to produce:• dried milk• evaporated and condensed milk• skimmed milk• cheese• butter• yogurt• ice cream• d) The characteristic changes in:• the souring of milk• mould attacks on cheese

Milk composition

• Milk contains hundreds of types of protein, most of them in very small amounts.

• Classified in various ways according to their chemical or physical properties and their biological functions.

Casein

• Casein protein makes up 80% of the protein.

• Only found in milk.

• Coagulates when milk sours because of the acid causing milk to curdle.

Whey protein

• Whey is the serum or watery part of milk that is separated from the curd in making cheese, the watery part of milk produced when raw milk sours and coagulates.

• Curds are the solid lumps you find in milk when milk sours.

Proteins of milk

• Lactalbumin and lactoglobulin are not coagulated by acid or rennin and remain in the whey.

• These proteins are coagulated by heat and can be seen as a skin on the surface of boiled milk.

Lactose

• Lactose is the sugar found in milk. • The bodies ability to digest lactose depends on

the presence of the enzyme lactase in the intestine.

• If lactase is low, lactose cannot be broken down to its component sugars, glucose and galactose.

• Lactose therefore passes undigested into the large intestine and can give rise to symptoms such as abdominal pain, bloating, flatulence and diarrhoea.

Types of milk

• Most common in the UK is still cow's milk, but others include sheep and goat's milk, as well as a number of plant-based substitutes - including Soya, rice, oat and almond milk - for those with lactose intolerance.

Fat content

• Whole or full-fat milk contains about 3.5% fat

• Semi-skimmed contains about 1.7% fat

• Skimmed milk contains 0.1 to 0.3% fat

• Some supermarkets sell milk with a 1% fat content which has almost half the fat of semi-skimmed milk but retains a more creamy flavour.

Milk nutrition

• Cow's milk is packed with a range of nutrients, including protein, calcium, zinc, vitamins A and B, and iodine.

•

Homogenisation

• Forcing milk through a small passage at high velocity. This breaks down the fat globules in milk into much smaller ones and creates a stabile fat emulsion.

• A uniform distribution of fat, no cream layer on the top

• Full-bodied flavour • Whiter, more appetising colour • Longer shelf life

• Jimmy D video

Heat treatment

• 95% of UK milk is heat treated.

• Main methods;– Pasteurisation– Sterilisation– Ultra-heat treatment

Pasteurisation

• Most popular method of heat treatment. Kills harmful bacteria without significantly affecting the nutritional value or taste of the milk.

• Heating milk to a temp of no less than 71ºC for a minimum of 15 seconds. This process is known as High Temperature Short Time (HTST).

• Following heating, the milk is cooled rapidly to below 6ºC using chilled water on the opposite side of the plate. This process also extends the keeping quality of the milk.

A typical pasteuriser is completely

automatic. Pour milk in one end and it flows

between a set of heating pipes for a set

period of time (long enough to kill off most of

the harmful bacteria), then between a set of

cooling pipes, before emerging from an

outlet pipe into the bottles:

Sterilised

• Available in whole, semi skimmed and skimmed varieties. It goes through a more severe form of heat treatment, which destroys nearly all the bacteria in it.

• Milk is pre-heated to around 50oC, then homogenised, after which it is poured into glass bottles which are closed with an airtight seal.

• Filled bottles are carried on a conveyor belt through a steam chamber where they are heated to a temperature of between 110-130ºC for approximately 10-30mins. Then they are cooled using a cold water tank.

• This results in a change of taste and colour and also slightly reduces the nutritional value of the milk, particularly the B group vits and vit C.

• Unopened bottles or cartons of sterilised milk keep for approximately 6 months without the need for refrigeration.

UHT

• Ultra heat treated milk is a form of milk that has been heated to a temp of at least 135ºC in order to kill off any harmful micro-organisms. The milk is then packaged into sterile containers.

• UHT milks have a longer shelf life as a result of the higher temperatures to which they are heated and the packaging used to store them.

• UHT milk is available in whole, semi skimmed and skimmed varieties.

Evaporated

• Doesn’t have added sugar

• Sterilised in the can

• Loses 60% of Vit C and 40% of thiamin.

• Pronounced flavour and tan colour due to the reaction between lactose and proteins.

• Long storage life.

Condensed

• Concentrated in the same way as evaporated milk, but with the addition of sugar.

• Homogenised and heated.• Passed through an evaporator and boiled

under vacuum. Then cooled and sealed in tins.

• Storage life is indefinate. Sugar content has preservation effect.

Dried Milk Powder• Produced by evaporating the water from the milk using

heat. The milk is homogenised and heat treated before drying.

• 2 ways to produce milk powder, spray drying and roller drying.

• Skimmed milk powder can be mixed easily with water; however whole milk isn’t easily reconstituted due to its higher fat content.

• Whole milk powder contains all the nutrients of whole milk with the exception of Vit C, thiamin and Vit B12. Skimmed milk powder contains hardly any fat and therefore no fat soluble vitamins. However, the protein, calcium and riboflavin content remain unaffected.

• If stored correctly, skimmed milk powders can be kept for up to one year.

Souring of milk

• Bacteria and a sugar, lactose, are present in fresh milk. • The bacteria feed on the sugar and cause the milk to

sour. • The souring of milk is a fermentation process.• The pH of the milk decreases as the milk gets older. • The souring of milk is brought about by fermentation

carried out by bacteria. The lactose sugar is converted to lactic acid which causes the pH to fall.

• Souring of milk

Curdling can be caused by heat, acids, enzymes and gelatine.

• Write about the effects in heat, acids, enzymes and gelatine on milk

• Page 97 in Food Facts.