Lesson 6. CONDENSED MILK
Condensed milk is a milk product obtained by evaporating part of water of whole milk, or fully or partly skimmed milk, with or without the addition of sugar. They are intended for use as such or for pre-condensing the fluid milk or fluid milk by–product preparatory to the manufacture of dried milk products.
The term ‘condensed milk’ is commonly used when referring to full cream sweetened condensed milk whereas the term evaporated milk is generally used while referring to full cream unsweetened condensed skim milk. Skimmed milk products are known as sweetened condensed skim and unsweetened condensed skim milk respectively.
6.2 Sweetened Condensed Milks
Sweetened condensed milks are milk products which can be obtained by the partial removal of water from milk with the addition of sugar, or by any other process which leads to a product of the same composition and characteristics. The fat and/or protein content of the milk may have been adjusted, only to comply with the compositional requirements by the addition and/or withdrawal of milk constituents in such a way as not to alter the whey protein to casein ratio of the milk being adjusted.
It may sometimes contain added refined lactose, calcium chloride, citric acid and sodium citrate, sodium salts of orthophosphoric acid and polyphosphoric acid not exceeding 0.3 per cent by weight of the finished product.
Such kind of addition need not be declared on the label. Sweetened condensed milk should contain not less than 9.0 percent milk fat, and not less than 31 per cent milk solids and 40.0 per cent cane sugar.
They may be
2. Sweetened condensed skimmed milk
3. Sweetened condensed partly skimmed milk
4. Sweetened condensed high-fat milk
6.3 Evaporated Milks
Evaporated milks are milk products which can be obtained by the partial removal of water from milk by heat, or by any other process which leads to a product of the same composition and characteristics. The fat and/or protein content of the milk may have been adjusted, only to comply with the compositional requirements by the addition and/or withdrawal of milk constituents in such a way as not to alter the whey protein to casein ratio of the milk being adjusted.
It may contain added calcium chloride, citric acid and sodium citrate, sodium salts of orthophosphoric acid and polyphosphoric acid not exceeding 0.3 per cent by weight of the finished product. Such addition need not be declared on the label. Unsweetened condensed milk should contain not less than 8.0 percent milk fat, and not less than 26 per cent milk solids.
They may be
1. Evaporated milk
2. Evaporated skim milk
3. Evaporated partly skimmed milk
4. Evaporated high-fat milk
5. Evaporated Filled Milk : Evaporated filled milk is a prepared blend of skim milk, vegetable oil, stabilizers and vitamins.
Because of its concentrated form, evaporated milk is a multipurpose, convenient dairy product ready for every milk use. Pouring directly from the can, evaporated milk can be used in countless applications: creaming coffee or tea, poured on cereals and fruits, providing consistency in meat patties and loaves, coatings for baked or fried meats, or in place of milk in the manufacture of candies, frostings and pies.
6.4 Advantage of Condensing Milk in Vacuum
The chief advantages of condensing milk in vacuum over evaporation under atmospheric pressure are three fold. They are
1. Economy of operation
The total heat units expended when condensing in vacuum pan at the usual temperature of about 55 to 60°C are slightly greater than the total heat units required for evaporation under atmospheric pressure. There is no saving in total heat units due to evaporation under reduced pressure. However evaporation in vacuum has the distinct advantage of making possible the economical utilization of steam at low temperature and of exhaust steam. Hence where exhaust steam is available there is opportunity for a considerable saving in fuel by its use.
The fact that under reduced pressure milk boils at relatively low temp (55 to 60°C) makes possible the use of low pressure steam such as exhaust steam, without sacrificing the rapidity of evaporation. This assists in saving fuel and reduces operating cost. In short, it helps reutilization of steam used in condensing and adoption of double and triple system of condensing. Other advantages are:
- About 90% of the water contained in the milk is removed in the evaporator and only 10% in the spray dryer. However, the energy required per Kg water evaporated in the spray dryer is 16-20 times the energy required per Kg water removed in the evaporator.
- In absolute terms, the energy consumption of the dryer is approximately twice that of the evaporator if a six effect evaporator is used. If a 4 effect evaporator is used the energy consumption of the evaporator and the dryer will be approximately the same. The above illustrates that although only 10 % of the water is removed in the dryer, this should not lead to the assumption that the efficiency of the dryer is of minor importance.
- In an effort to improve the overall efficiency of the process, the tendency has been to concentrate the milk as much as possible prior to drying. The importance of such tendency is illustrated by the fact that if for a given milk throughput, the evaporator concentration falls from 50 to 48%, the dryer evaporative load is increased by 8.7%, while if the evaporator concentrations increases to 52% the dryer load is reduced by 7.5%. Since, the energy supplied to the dryer is about 60% of the total required for drying process; these variations have a marked effect on the overall economy.
- In modern plants, skim and whole milks are concentrated to 48-50 % solids and whey to 55-60 %.
- In view of the above, it has been suggested that evaporators and dryers should be developed that can handle milk of higher total solids. However, the use of higher concentrations is not an engineering problem related to the design of evaporators and dryers, it is a technological problem.
2. Rapidity of evaporation
The fundamental factor that determines the rapidity of evaporation is the rate of heat transmission or the amount of heat transmitted by the steam to milk per unit area of heating surface per hour. The rapidity of heat transmission in turn depends largely upon the temperature difference between steam and milk. The greater the difference, the more heat is transmitted. The rapidity of evaporation also depends upon surface area available and viscosity of the product. The rapidity of evaporation is necessary:
(1) To check the growth of microorganisms that may have survived higher heat treatment.
(2) To have economy of evaporation.
(3) To protect the milk against heat damage.
3. Protection of milk against heat damage during operation
Condensing in vacuum pan makes possible evaporation at a relatively low temperature (55 to 60°C). With present evaporating equipment, milk temperature of 52 to 57°C is usually maintained. Exposure to high evaporating temperatures hampers the quality. The low boiling point saves the milk from heat damage.