Yoghurt & Ice cream

Pasteurisation  Milk is heated to 72 0 C for 15 seconds. It is then rapidly cooled to below 10 0 C. This is continuous process is known as HTST. Special plate heat exchangers has milk on one side and steam on the other. A special holding tube is used through which the milk passes to the cooling section and then to bottling  What does HTST mean? Over 99% of bacteria is killed and although it is not sterile all harmful organisms are destroyed. Any left over organisms including spores are inactivated in the cooling stage. What are the pros of pasteurisation? Which Vitamins are damaged in the pasteurisation process?

Homogenisation   Recap what is homogenisation and why do we do it?

Sterilisation  Sterilized milk is milk which has been homogenised, filtered and subjected to heat treatment so it will remain in good condition in an unopened bottle for at least a week but usually several weeks.  After homogenisation the milk is filtered, sealed into narrow necked bottles and then heat treated. In practice higher temperatures and shorter time is used eg C for 15 minutes this was done in bottles and then cooled with water spray to about 75 0 C and then allowed to cool to room temperature.  Most sterilised milk in the UK is produced by UHT sterilisation. This is carried out before the milk is bottled. The milk is homogenised and then heated to no less than C for 1 second by flowing over a heated surface. A completely sterile product is obtained and once cooled is packed into sterile containers.  What changes occur in sterilisation?  Lactalbumin and Lactoglobulin are coagulated some calcium phosphate is precipitated and about 35% Thiamin and 50% ascorbic acid is destroyed.  What are the pros of sterilised milk?

Dried milk  Drying removes most of the water from the milk and is an excellent means of preservation. The process can produce a great product which when reconstituted differs very little from fresh milk. There is little loss of nutrition if drying is carried out correctly.  Over heating will cause Vitamin loss, protein damage and will encourage browning by non enzymic Maillard reaction.  When dry it must be kept away from moisture and ideally air. If there is more than 5% moisture the Maillard reaction will slowly take place turning the powder from white to light brown.  Most milk powders are very fine which can float on the surface, have poor dispersability and poor wettability so to combat this instantisation can be used in which the powder is re wetted so that it clumps together. These clumps act like sponges and absorb the water and disperse rapidly.  Traditionally this was done on roller driers. The roller dryer consists of a hollow drum internally heated by steam to which a film of milk sticks and as the drum rotates the milk dries to give a flaky powder. This almost sterile as the heat treatment is severe but can also damage the protein,  Spray drying is the most common as it produces a product which is more soluble and of a better flavour and colour. Milk is concentrated in an evaporator and then sprayed when still hot (80 0 C) into a chamber where the spray meets a blast of hot air (180 0 C) and dries instantly.

Roller drier and spray drier

Evaporated milk  Evaporated milk is made by evaporating water from milk to reduce the water content to about 70%. The product sometimes has poor granular texture so they improve this by adding either sodium citrate, disodium phosphate or calcium chloride. It has to be homogenised as it is perishable. It is then sealed into cans and heat processed in retorts at C for about 15 minutes depending on the can size.

Condensed Milk  This is similar to evaporated milk but it is sweetened which means it is well preserved due to its sugar content. The milk is pasteurised as it is not heat processed. It is not cooled as it passes to the evaporator. A sugar syrup is added to the evaporator and the whole milk is concentrated under vacuum at 50 – 55 0 C. The product is then cooled and agitated at the same time. Often small crystals of lactose are added to ensure rapid crystallisation of the lactose. The addition of the sucrose forces lactose (less soluble) out of the solution). If the lactose is allowed to crystallise slowly it produces large crystals sometimes as big as marbles. Agitation and seeding with crystals ensures small lactose crystals which are unnoticed in the product.  The product is poured into sterilised cans and sealed without further heat treatment.  It is mostly used in chocolate and confectionary.

Skimmed milk  Skimmed milk is milk in which nearly all the fat has been removed. However a small fraction of the fat 0.1% is almost impossible to remove by the standard method using a centrifugal separator. Removing the fat from the milk will also remove the fat soluble vitamins and so skimmed milk must not be used to feed babies.  Why is skimmed milk so popular?

Butter  Butter is made by churning cream. The churning process causes an inversion of the colloid. Cream is an emulsion (which one?) where as butter is the opposite (which is?). This involves the expulsion of most of the water from the original cream. Cream of 35 – 38% fat and 60 – 68% water is converted to butter of 80% fat and 16% water. (this is the legal maximum was water content)  Fresh cream separated from milk before churning is used for butter making but it is also possible to ripen or sour cream to give added flavour.  Special cultures of organisms are added to the cream and are allowed to produce about 0.25% lactic acid. Lactic acid producing bacteria are used. Butter can be salted with up to 2% added. Unsalted butter is more popular WHY?  Before churning, the cream is pasteurised and cooled to about 10 0 C at which the churning is most efficient. Churning is done by violent agitation of the cream which takes about 30 minutes. Small pieces of butter at first appear and then the butter milk is removed. The butter is washed, salted and then worked to obtain the desired consistency with a final water content of up to 16%.  The nutritive value of butter is somewhat variable as it depends on its fat content and fat soluble vitamins, mainly retinol (A) and Vitamin D. Carotenoids are the reason for the yellow colour of butter and in the summer the butter fat is richer because of the grass which the cows eat.

Yoghurt   Yoghurt is the best known fermented milk. Yoghurt is milk concentrated or with added milk powder which has developed acidity and flavour due to two microorganisms.  The two organisms must be in equal amounts and they must no outgrow the each other or a bitter/ too acidic product will result.  Yoghurt contains more protein, thiamin and riboflavin than milk. Low fat yoghurt contain less fat soluble Vitamins.  Yoghurt falls into different categories  Set yoghurt  Stirred yoghurt

Set Yoghurt and Stirred yoghurt  Set Yoghurt - The fermentation is allowed to take place in the container in which the yoghurt is sold.  Stirred Yoghurt – This is fermented in bulk and then packed later.  The ingredients for yoghurt include:  Whole milk  Skimmed milk  Evaporated milk  Dried milk  Stabilisers and thickeners  Fruit flavours  Colours  Sugar Generally low fat yoghurts are produced so that the fat is separated from the milk using a centrifugal separator. The solids in the milk is increased by evaporation of some water, the addition of evaporated milk or the addition of skimmed milk powder. Sugar, stabilisers, flavours and colours are blended into the milk base. The mix is homogenised, then pasteurised at 90 0 C for 30 minutes or by the HTST method to kill all miccroorganisms. The mix is cooled to 44 0 C and then the starter culture is added.

Ice cream  Ice cream is an emulsion of fat in a complicated solution which is made up of both colloidal and true solutions. Ice cream contains very small crystals ice, air sacs, fat globules, colloidal suspensions of casein, stabilising agents, flavours, colour and sugar solutions.  There are two main types:  Soft ice cream  Hard ice cream Soft Ice Cream – This tends to be made in small batches. The secret to good ice cream is getting the ingredients in the correct balance. What do you think would happen if you have;  Too much milk powder?  Too little milk powder?

Set Yoghurt  To make a set yoghurt the mix is incubated at 44 0 C for about 1 ½ hours and then poured into containers which are kept warm until the yoghurt has fully coagulated. The yoghurt is then cooled to C and held at this temp until consumed, ideally within 14 days.  Natural yoghurts are usually of the set type.  Stirred Yoghurt  To make stirred yoghurt which offers advantages in continuous manufacture incubation is at a lower temperature so that the yoghurt becomes thicker but not coagulated and continuous stirring ensures that no curd is formed.  Fruit and syrup are added into the containers followed by the yoghurt mix which is then cooled and stored the same as a set yoghurt.  New developments include frozen yogurt and long life yoghurt where aseptic is used following heat treatment of the yoghurt.  Bio yoghurts claim health advantages as they have living cultures which can help with stomach problems.

Soft Ice cream  Stabilisers and emulsifiers are used extensively in ice cream and these include gelatin, alginates, modified celluloses, carrageen, pectin's and various gums. These prevent the formation of large ice crystals during freezing and allow only small ice crystals.  Stabilisers give body to the product and improve the melting resistance of the ice cream. Stabilisers absorb large quantities of water without them the ice cream would have a poor texture and melt rapidly.  The ingredients are mixed together, homogenised then pasteurised. Air is whipped into the ice cream as its frozen usually at about 5 0 C. It usually increases by about 50%., this is called “Overrun”. It is usually served straight from the freezer.

Hard Ice cream  At least 8% fat should be used if 9%or more is used the ice cream can become very heavy as not enough air would be mixed into it.  In both types of ice cream no more than 15% sugar should be used because it may crystallise. Small amounts tend to encourage the growth of large ice crystals. The water must be warm which aids dispersion of the ingredients. The mix is homogenised and pasteurised and then frozen. During the freezing the mix is stirred to incorporate more air than in soft ice cream otherwise it would freeze into a hard mass. An overrun of 100% is essential in hard ice cream. Once frozen the ice cream may be cut into blocks and wrapped and then hardened at C.  There have been very few food poisoning cases and it’s a great energy supplying food. It doesn’t contain a lot of protein.