- 1 Concept Map
- 2 Objectives
- 3 Introduction
- 4 Effect of Heat on Milk
- 5 Activity
- 6 Activity
- 7 Why does milk boil over?
- 8 Coagulation of Milk
- 9 Activity
- 10 Acid Coagulation of Milk
- 11 Coagulation of Milk due to Enzymes
- 12 Summary
- 13 Activity
- 14 Assignments
- 15 Discussion Forum
This OER enables you to:
- explain the various changes which take place in milk on heating
- apply this information while making preparations from milk
- identify the different methods by which curdling of milk takes place
- apply this information while handling milk
Several changes take place in milk on cooking. Let us try and understand them.Some of these changes take place
- • because of the action of heat on the milk proteins.
- • because of the action of acids or enzymes on the milk
Effect of Heat on Milk
Scenario: It is your birthday, and your mother has prepared vermicelli kheer/payasam which is your favourite sweet. You are looking forward to eating it, but the moment you taste it you find it has a burnt flavor and you waste it. You may have come across several such instances. This has occurred due to what is known as scorching.
The white deposit that you see at the bottom & sides of the vessel is due to the coagulation of the milk proteins lactalbumin & lactoglobulin due to the action of heat on them. If this deposit gets burnt due to high temperatures this is known as scorching.Scorching of milk gives it a burnt unpleasant flavor which should be avoided because this flavor cannot be masked and remains in any of the products prepared from milk.
You can prevent scorching by:
- Continuously stirring the milk to reduce the quantity of deposit
- Using a vessel which has a thick base or a copper or sandwich bottomed vessel
- Not heating milk on a very high flame
Scenario: You are heating the milk on the gas range for preparing a cup or coffee. You hear the doorbell ring and you go to answer it. When you get back to the kitchen you find the milk has boiled over and your gas range is a mess
Why does milk boil over?
- When you heat the milk,the milk proteins lactalbumin and lactoglobulin coagulate and instead of being suspended in the milk come up to the surface and form a thin skin.
- As you go on heating the milk this skin gradually thickens. At the same time some of the cream also starts separating from milk and comes up on top. Thus a thick layer or scum is formed on the surface.
- As you know milk contains a large percentage of water. On heating, the water evaporates and the water vapour formed rises and get trapped under the skin and cannot escape.
- It pushes the skin upwards and the milk boils over. You can prevent this from happening by stirring the milk with a spoon, thus breaking the film , and preventing the milk from boiling over.
Coagulation of Milk
Keep a cup of milk overnight in the kitchen. Heat it up next morning. What happens?
- It turns sour &curdles
- It remains unchanged
Acid Coagulation of Milk
- When you add acidic ingredients such as lime juice, vinegar, raw mango, orange juice etc to milk, the pH is lowered and the protein casein coagulates.
- Milk which is usually a homogenous liquid separates out into solid curds and liquid whey. This milk is also said to be curdled.
- The solid portion of milk consists mainly of casein and fat. The liquid portion or whey contains the lactose, water soluble vitamins & minerals and the proteins lactalbumin and lactoglobulin.
- This can also occur if milk is left at room temperature for a considerable length of time permitting bacteria naturally present in milk to multiply. These bacteria have the ability of fermenting milk sugar lactose to lactic acid thereby lowering the pH of milk. When this milk is boiled it curdles.
- Acid coagulation of milk can be desirable and undesirable in milk cookery
- How acid coagulation of milk can be useful
- Acid coagulation of milk is useful in preparing two commonly prepared items from milk
- Yogurt or Curds
- Paneer or Channa
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Paneer is used for many preparations such as paneer mutter, palak paneer, sweets such as rassogolla, sandesh etc. Click on the following link to see a video on making paneer The media player is loading...
How acid coagulation of milk can be a problem
- Acid coagulation of milk which has been not stored under proper conditions can be a problem especially if the milk curdles while preparing items like tea, coffee or sweets such as kheer, basundi etc.
- To overcome this problem milk, after boiling, should be cooled and then refrigerated at temperatures between 1 and 4 deg Centigrade
- Milk is often heated in the presence of other foods and coagulation may then occur after a very short period of heating. For example milk may curdle when certain vegetables are cooked in it due to the acids & salts found in the plant tissues which are released into the milk during heating.
- When tomatoes are combined with milk to make cream of tomato soup coagulation may occur. Therefore care should be taken while combining the tomatoes and milk and the time and temperature of heating will determine whether curdling occurs. The puree should be added to the béchamel sauce and not the other way round to prevent it from curdling.
Coagulation of Milk due to Enzymes
- Another method of curdling milk which is used commercially is with the help of proteolytic enzymes i.e. enzymes that break down proteins.
- One such enzyme which is used very commonly in the cheese making process is rennin which is extracted from the stomach of young calves.
Cheese is prepared from milk coagulated with the help of the enzyme rennin
Changes take place while cooking or handling milk
- • Changes taking place due to action of proteins on milk include
- Boiling over of milk
- • Coagulation of milk takes place because of
- Acid coagulation of milk
- Action of enzymes of milk
Take a quiz given below
- Refer and find out the names of different products that can be prepared from or yogurt and paneer or channa or where yogurt and paneer are used in the following categories
- i. Sweets
- ii. Raitas
- iii. Main Course/Curries/Gravies
There are many traditional ways by which boiling over of milk can be prevented. One such method is by using a milk saver. Have a discussion on the different ways by which boiling over of milk can be prevented .