In this article we will discuss about:- 1. Introduction to Dried Milk 2. Objects of Production of Dried Milk 3. History and Development 4. Definition 5. Standards 6. Composition, Food and Nutritive Value 7. Role of Milk Constituents.
Introduction to Dried Milk:
Today, the production of dried milks and milk products has become an increasingly important segment of the dairy industry. The world’s recorded annual output of dried milks and some milk by-products in 1969 was 4.2 million tonnes, the contribution of India being 31.1 thousand tonnes, i.e., 0.7% of the total.
There are definite indications of greater future production of dried milks and milk products due to their better keeping quality, their requiring less storage space and lower shipping costs- all of which are economically attractive. More non-fat dry milk production takes place than whole milk powder production, due to the high keeping quality of the former.
The ultimate aim of the industry is to obtain dry products which, when recombined with water, give little or no evidence of detrimental change compared to the original liquid product. While the reconstituted non-fat dry milk is nearly comparable in flavour to the original skim milk, much improvement needs to be made with dry whole milk. The other aims are to ensure good keeping quality and a low manufacturing cost.
Dry milk provides a means of handling the excess milk supply in a dairy factory during the flush season, while in the lean season, that meant for the production of dry milk can be diverted to market milk. Thus dry milk may be called the balance wheel of the dairy industry today.
In India, the first-ever commercial production of spray dried milk was started by AMUL Dairy, Anand, in 1955. The production of some dried milk products in this country during the past few years, is shown in Table 9.1.
Objects of Production of Dried Milk:
The objects or purposes of drying milk (and milk products) are:
(i) To remove the moisture so as to reduce bulk, thereby effecting a saving in storage space and packaging costs;
(ii) To reduce the cost of transportation (due to reduced bulk);
(iii) To improve the storage-life of the product (due to low moisture content);
(iv) To provide a product which can be utilized for many food manufacturing operations;
(v) To conserve, as far as possible, the natural properties of the original raw material.
History and Development of Dried Milk:
Marco Polo, the celebrated Venetian traveller of the thirteenth century, reported that the soldiers of Kublai Khan, a Mongol emperor, carried dried milk when on an excursion. Before it was used, water was added to a portion of the dried material. Mixing was accomplished by the horses’ movements while travelling.
Next we hear of dried milk in tablet form made in 1810 by the French scientist, Nicholas Appert, by the air-drying of milk solids concentrated to a ‘doughy’ consistency.
In 1855, Grimwade of Britain developed a modified dry product from highly concentrated milk to which was added sodium (or potassium) carbonate and sugar. This semi-solid material was extruded into thin streams and dried in trays.
In 1902, Just of the USA was among the first inventors of a drum drier with two rolls. Numerous other types of drum driers were invented, but most of these were used to a limited extent. Vacuum drum driers were designed between 1889 and 1909.
Among the early inventors of spray drying equipment were Percy of the USA (1872) and Stauf of Germany (1901). Peebles of the USA is known for his pioneering research to improve the reconstitutability of non-fat dry milk in water.
Definition of Dried Milk:
Dried milk or milk powder is the product obtained by the removal of water from milk by heat or other suitable means, to produce a solid containing 5 per cent or less moisture. Whole milk, defatted milk or skim milk may be used for drying.
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). The various dried milk products are given specific names.
Standards of Dried Milk:
A. PFA (Legal):
(i) Whole Milk Powder:
According to 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 orthophosphoric acid and polyphosphoric acids, not exceeding 0.3 per cent by weight of the finished product, and 0.01 per cent of butylated hydroxy anisole (by weight) of the finished product.
Such addition need not be declared on the label. Milk powder should contain not more than 5.0 per cent moisture and not less than 26.0 per cent fat. The total acidity expressed as lactic should not be more than 1.2 per cent. The standard plate count may not exceed 50,000/g. and the Coli count may not exceed 90/g. The maximum solubility index should be 15.0 for a roller-dried and 2.0 for a spray-dried product.
Note:
The process of drying must be mentioned on the label. The spray dried product must be packed in hermetically sealed containers when the net quantity exceeds 510/g.
(ii) Skim Milk Powder:
According to the PFA Rules (1976), skim milk powder is the product obtained from (the skim milk of) cow or buffalo milk, or a combination thereof, by the removal of water. It may contain added calcium chloride, citric acid and sodium citrate, sodium salts of orthophosphoric acid and poly- phosphoric acid, not exceeding 0.3 per cent by weight of the finished product. Such addition need not be declared on the label.
Skim milk powder may not contain more than 1.5 per cent milk fat, and moisture may not exceed 5.0 per cent. The total acidity expressed as lactic acid should not exceed 1.5 per cent. The standard plate count should not exceed 50,000/g. and the Coli count must not exceed 90/g. The maximum solubility index should be 15.0 for a roller-dried and 2.0 for a spray-dried product.
Note:
Same as for whole milk powder.
B. ISI:
According to the Indian Standards Institution, the specifications for dried milks are as given in Table 9.2.
C. ADMI:
The American Dry Milk Institute standards are given in Table 9.3.
Composition, Food and Nutritive Value of Dried Milk:
The average percentage composition of whole milk powder and skim milk powder is given in Table 9.4.
Food and Nutritive Value:
Under modern drying 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.
Role of Milk Constituents in Dried Milk:
(a) Milk Fat:
This may be present in dried milks or milk products either in the original globule, i.e. in emulsion form, or as free fat, i.e., in de-emulsified form. Free fat makes the dry product greasy and leaves an oily film on the reconstituted milk.
The unsaturated fatty acids of milk fat, e.g., oleic, play a profound role in oxidative deterioration. Free fat promotes oxidation. The phospholipids of milk appear to be responsible for the oxidized flavour. The rancid flavour of milk fat is primarily due to the liberation of butyric acid as a result of hydrolysis of glycerides through lipase action.
(b) Milk Proteins:
Many of the problems of dairy technology revolve around the behaviour of the caseinate system and particularly on the aggregation of casein particles by heat, salts, acid, etc. Milk proteins are readily subject to changes during heat-processing and to some extent during storage: the changes adversely affect their solubility, i.e. dispersibility.
The heat-treatment destabilizes the proteins by inducing first stage denaturation and then irreversible denaturation (coagulation) as it becomes more severe. Some destabilization also occurs slowly during prolonged storage. Some proteins, probably β-lactoglobulin, appear to be the source of sulfhydryl compounds associated with a cooked and caramelized flavour.
(c) Milk Sugar:
This has an important role to play in the manufacture and storage of dried milk products. In freshly dried milk, the lactose is still present in the amorphous or glass (non-crystalline) state. In this form it is very hygroscopic and readily absorbs moisture upon exposure to the atmosphere, which induces crystallization. This absorbed humidity then causes the powder to become sticky, and then to cake. (This tendency to stickiness and caking increases in the presence of lactic acidity.)
Browning (colour defect) of dried milk products may be caused during manufacture, chiefly due to excessive temperature and long exposure to heat. Browning-discolouration may also occur during storage; especially under conditions of high moisture, high storage temperature and packaging in air-packed containers—all of which hasten and intensify browning.
(d) Milk Salts:
These play an important part in influencing the physical state and physical stability of the milk proteins, particularly the caseinates. The well-known ‘salt balance’ theory of Sommer and Hart holds that optimum stability depends on a certain ratio of calcium and magnesium ions to those of citrates and phosphates.
Minor changes in salt balance and pH easily upset this equilibrium and tend to destabilize and precipitate the casein particles. The resulting dried milk will have a low solubility (dispersability). Copper and iron exert a catalytic action in the oxidation of milk lipids, which produces undesirable flavours in the dried milk products.