Milk and Milk Products Technology-vat: Dried milk

Dried milk

DRIED MILK

Drying is the separation of a liquid from a compound or blend of compounds by evaporation. The product to be dried exists either in a solid or solid–liquid phase. The separation is into a solid and a gas phase. Dried milk products are manufactured to reduce bulk so as to reduce packaging and the transportation costs, to improve the storage life of the product, etc.,

Definition

“Dried milk or milk powder is the product obtained by the removal of water by heat or the suitable means, to produce a solid containing 5 per cent or less moisture. The dried product obtained from whole milk is called Dried whole milk or whole milk powder (WMP); and that from skim milk is known as Dried Skim Milk or Skim Milk Powder (SMP), or Non-fat Dry Milk (NFDM).

PFA Legal Standards

“Accordingto the PFA Rules, (1976), whole milk powder is the product obtained from cow or buffalo milk, or a combination thereof, or from standardized milk, by the removal of water. It may contain calcium chloride, citric acid and sodium citrate, sodium salts of ortho phosphoric acid and poly phosphoric acids, not exceeding 0.3 per cent by the weight of the finished product, and 0.01 per cent of butylated hydroxy anisole (by weight) of the finished product. Such additions need not be declared on the label. Milk powder should contain not more than 5.0 per cent of moisture and not less than26.0 per cent of fat. The total acidity expressed as lactic acid should not be more than 1.2. per cent. The standard plate count may not exceed 50,000/g and the Coliform count may not exceed 90/g. The maximum solubility index should be 15.0 for roller dried and 2.0 for a spray dried product.

“Accordingto the PFA Rules, (1976), skim milk powder is the product obtained from cow orbuffalo milk, or a combination thereof, by the removal of water. It may contain calcium chloride, citric acid and sodium citrate, sodium salts of ortho phosphoric acid and poly phosphoricacids, not exceeding 0.3 per cent by the weight of the finished product. Such additions need not be declared on the label. Skim milk powder should contain not more than 1.5 per cent of fat and the moisture may not exceed 5.0 percent. The total acidity expressed as lactic acid should not be more than 1.5 per cent. The standard plate count may not exceed50.000/ g. and the Coliform count may not exceed 90/g. The maximum solubility index should be 15.0 for roller dried and 2.0 for a spray dried product.

Composition

The average percentage composition of whole milkand skim milk powder is given in the following table: (Percentage)

Food and nutritive value

Under modern methods, the nutritive value of milk is preserved to a great extent. While there appears to be only a slight destruction of lysine in spray drying, the severe heat treatment of roller drying destroys more lysine. Dry whole milk is a good source of vitamin A,calcium and phosphorus. It is also a valuable source of riboflavin.

Milk drying systems

The manufacturing process consists of unit operations of milk selection and pre-treatment, concentrate manufacture,homogenization, drying and filling/packaging. The basic difference is in the type of drying, and can be achieved in several ways and means.

Drying by Cold Treatment

Drying milk by freezing out the water and centrifuging. This system was proposed and patented as early as 1884 and is nowobsolete.
Drying milk by freezing and sublimation: This freeze-drying method, which seems to have been developed in 1945, consists of freezing the product and supplying heat, so that moisture is removed by sublimation by maintaining a vacuum in the vaporizing chamber.

Drying by application of heat

Pre-requisites

Milk selection and pretreatment

Milk is selected according to the requirementsand clarified and standardized in the usual ways.

Heat Treatment

Milk or milk products must be heat treated by an officially approved pasteurization process in order to inactivate possible pathogenic or other harmful germs and to inactivate most of the enzymes. Another intensive preheating, such as is normal for evaporated milk, is not required here, because the recontaminating germs cannot grow at the moisture levels available in the powders, and protein stabilization is not necessary. On the contrary, very gentle pasteurization is required in order to have the least whey protein denaturation.
Among the other factors, the quality of skim milk powder is measured by the non-denatured whey-protein-nitrogen index (WPN index), which characterizes the heat treatment involved. From this heat classification the application or utilization of the powder can be derived. For certain applications, e.g.,baby food, the classification of the gently heated proteins by the WPN is not sufficient, and other criteria such as the protein number (ratio of casein nitrogen and whey protein nitrogen Vs. total protein nitrogen), organoleptic evaluation of the reconstituted milk or the coagulation and acidification capability (e.g., for cheese manufacture) must be taken into account.

For the manufacture of “low-heat” skim powder, the short-time heat treatment must be used; for “high-heat” skim powder, the process temperature is90ºC for 30minutes.
For the production of whole milk powder base,milk is heated to 90-95º C and held for 15-30 should, which is considered optimal.

To improve the fat distribution, homogenization is advantageous during the manufacture of whole milk powder. If the base is fat-standardized by adding cream, partial-flow homogenization is possible before concentration. Other wise the concentrate is homogenized.

Concentration

The objective of concentration is to remove a maximum amount of water from the milk and obtain a maximum dry matter content, as the drying process requires three times more energy than the removal of water in the evaporator.
The milk is concentrated to its flow limits,i.e., dry matter will be 40-50 %.
The concentration ratio selected is controlled by the finished product and the drying process; the ratios are as follows:-
Drum-dried milk powder 1 : 3.5-4
Drum-dried skim milk powder 1 : 4-6
Spray-dried whole milk powder 1 : 3.5
Spray-dried skim milk powder 1 : 4-5
A further increase of the concentration ratio in the evaporator is characterized by a significant increase in viscosity and is very difficult due to the flow limits of the concentrate.

Last modified: Monday, 16 April 2012, 5:57 AM