In this article we will discuss about:- 1. Instantization of Dried Milk 2. Packaging of Dried Milk 3. Storage 4. Yield 5. Keeping Quality 6. Judging and Grading 7. Uses.
Instantization of Dried Milk:
Instantization refers to the process by which dried milk and milk products are made instant-soluble.
Purpose:
The chief purpose of instantizing is to improve the rate and completeness of the reconstitutability of dry milk and milk products. The instantizing process affects wettability, sinkability, dispersability and solubility of the particles; however, total solubility is not improved.
Products:
Mainly applied to skim milk powder/non-fat dried milk. However, whole milk powder and other milk fat containing dry dairy products are also being instantized.
Characteristics: D
(i) Lowers bulk density and hence increases packaging costs;
(ii) Increases moisture content of the finished product, which lowers the keeping quality.
Factors Influencing Instantization:
Dry milk that is manufactured especially for instantization usually yields the best results. Moisture content and particle size should be as uniform as possible. A minimum of fine particles, less than 20 µM in diameter, is desired, while particles in the range of 25 to 50 µM are preferred.
The skim milk powder for agglomeration should be low in fat content. Low heat or medium heat skim milk powder is normally used, since high heat skim milk powder shatters much more easily in handling after instantization.
Success Factors:
The success of the instantizing operation depends on:
(i) Uniform and constant rate of powder distribution into the wetting zone.
(ii) Avoiding over- or under-wetting of particles by providing uniform moisture conditions. (Over-wetted particles dissolve slowly and inadequate wetting permits excessive shattering during handling.)
(iii) Stabilizing air movement to ensure optimum particle collision. (Excess movement causes the product to adhere to the equipment lining.)
(iv) Controlling redrying air temperature and flow rate (which is necessary for adequate moisture removal without heat-damage to the instantized product).
Major Systems/Processes:
These are:
(i) The Peebles process;
(ii) The Cherry-Burrell process, and
(iii) The Blow-Knox process (all American).
In addition, there are NIRO and ANHYDRO Agglomerators, both Danish. The equipment of each system varies in detail.
Common Features:
These are:
1. Wetting of the surface of the particles with steam, atomized water, or a mixture of both (in a tube or chamber);
2. Agglomeration, which occurs because the particles collide with each other due to turbulence, and adhere to each other to form clusters (in a tube or chamber or cyclone);
3. Re-drying with hot filtered air (in a chamber or cyclone or deck);
4. Cooling;
5. Sizing to eliminate the very large agglomerates and the very small particles, and
6. Sifting and packaging.
Particulars of Major Processes:
These are outlined in Table 9.5.
Packaging of Dried Milk:
Before packaging, the sheets of roller-dried milk must be broken down to powder form. This is often done with a simple brush-type sifter totally enclosed in design, to reduce costs. If a very fine powder is desired, a ball mill can be used. With spray powder, it is only necessary to remove any coarse aggregates by passing the powder through a screen sifter.
Packaging by hand is undesirable as it is difficult to avoid dust, wastage, manual contact and general discomfort. Powder-filling machines are normally used, the type depending upon the nature of the container. Semi-automatic single-head machines may be used for large containers, while fully automatic multi-head rotary machines are available for the rapid filling of small containers.
In vacuum-operated fillers, the package (which must be rigid) is raised to the filling head and makes an air-tight joint against a rubber-faced filling nozzle. A vacuum pump withdraws air from the package, through the gauze and the powder is sucked into the package from a supply hopper until the gauze is completely filled.
The vacuum is then broken and the container is removed from the filler-head. Vacuum filling involves an enclosed circuit, which permits faster, dustless filling without wastage. No moving parts are required to make contact with the powder and removal of air enables a denser pack to be obtained.
The powder packaging department should be completely isolated to reduce the spread of powder dust to the drier building. A pneumatic conveying system is preferable for transferring powder from the drier to the packaging rooms; apart from the advantage of an enclosed system, the powder is cooled by the air current and can be moved over considerable distances.
A suitable container for dry milk should be impervious to moisture, light, gases, and insects; should be durable for handling; resistant to corrosion; of low cost; and be relatively easy to fill, seal, handle and empty.
The retail package should have a reclosable opening. Because of the hygroscopic nature of dry milk, the packaging materials must provide a good vapour barrier. Of the several types in use, one is the fibreboard carton with an overwrap of foil laminated to paper. Another consists of a fibreboard carton with an inner lining of foil laminated to paper.
A polyethylene bag inside the fibreboard carton is also used. Other combinations of layers of polyethylene, foil and paper either as lining or overwrap, are available for packaging. In India, the tin-can, though expensive, is still the most widely used retail container. Plastic-coated paper bags are the latest development in packaging technology.
Powder packaging should be carried out:
(i) In a dry atmosphere between the drier and sealed package, and
(ii) Promptly in a hermetical, moisture-proof package.
This is because lactose is present in a glass/non-crystalline/amorphous state in freshly prepared dried milks and milk products. In this state, the powder is very hygroscopic. If exposed to a humid atmosphere it will readily absorb moisture, become sticky and cake in storage.
Packaging of Whole Milk Powder:
Fat decomposition resulting in a tallowy flavour is a major storage defect in whole milk powder, and less often in skim milk powder. The rapid flavour deterioration in whole milk powder (and other fat-containing products) due to oxidation necessitates inhibitory measures. One of these consists on packaging the product with a low oxygen content.
The general procedure is to immediately remove oxygen by subjecting the product to a vacuum of 71 cm. (28 in.) of Hg within 24 hours of drying, with final packaging within a few days. Less than 2 per cent of oxygen in the headspace gas of the package is considered satisfactory for most storage conditions. Good quality whole milk powder with a low oxygen content can be expected to withstand room temperature for six months or more without an oxidized off-flavour.
To obtain a low level of headspace-oxygen in whole milk powder, a double gassing technique is applied. The customary procedure is to collect filled, air-tight metal cans (with a small brogue hole) on trays to be conveyed into the vacuum chamber/cabinet. The air is removed rapidly (in 60 seconds) under a very high vacuum of 74 cm. (29 in.) of Hg. After a 2-5 minute hold, the pressure is restored with nitrogen to 0.5 to 1.0 psi above atmospheric pressure.
Nitrogen may be replaced with a mixture of nitrogen and carbon dioxide, the latter being restricted to 5-20 per cent. Once removed from the chamber, the containers are sealed by soldering the 1-2 mm. holes on the lid. The containers are then held for oxygen desorption.
When an oxygen equilibrium has been attained in the headspace, usually within a week, but at the most within ten days, the cans are punctured and the vacuum treatment, pressure restoration with nitrogen and sealing steps are repeated.
Gas packaging of whole milk powder should not be delayed after drying. Delay causes a deterioration in quality during the holding period. Warm powder directly from the drier tends to have a more rapid rate of oxygen desorption under vacuum. (If the production is not large, whole milk powder may be placed in metal drums and the air exhausted.
By holding the product under partial vacuum for oxygen desorption the first gassing step in the package may be eliminated, and yet the final maximum of 2 per cent oxygen can be attained.) Gas packing is the most effective method of extending keeping quality in tropical climates.
Note:
Keeping quality of dried milk and milk products can be increased by the addition of anti-oxidants such as hydroquinone, ascorbic acid, ethyl or propyl gallate, nor dihydro guiaretic acid, etc., to the liquid milk product before dehydration. Many countries prohibit the use of anti-oxidants. In India, butylated hydroxy anisole at 0.01 per cent by weight of the finished product is permitted for use in dried milks.
Packaging of Skim Milk Powder:
For industrial use and storage, skim milk powder may be packed in barrels, drums and bags; for retail purposes in metal cans, glass jars or cartons. A 2-mil polythene bag inside a 4-ply kraft paper bag is a common package for domestic commercial trade. Cartons of fibreboard, foil and plastics have largely supplanted glass and metal as retail containers.
Packaging/Marking:
The following particulars are expected to be marked or labelled on each container of dried milks and milk products:
(i) Name and type of the material (Trade/Brand name, if any);
(ii) Name of manufacturer and address;
(iii) Batch or code number;
(iv) Net weight;
(v) Process of manufacture (spray or roller-dried);
(vi) Date of manufacture and/or expiry date;
(vii) Directions for use;
(viii) Composition, etc.
Storage of Dried Milk:
Since high storage temperatures are detrimental to the keeping quality of dried milks and milk products, temperatures lower than 24°C (75°F) are desirable.
To ensure maximum keeping quality, the dried product should be stored in a vapour-proof, moisture-proof, sealed package in a dark, cool, dry place. Refrigerated storage should be used for long storage in warm climates. Retail distribution should be done in the original container.
Yield of Dried Milk:
The yield of milk powders (whole or skim) will depend on the per cent total solids in the raw material, moisture retained in the finished product and losses in manufacturing.
Problem:
Given 10,000 kg. milk testing 7.0% fat and 9.5% SNF. Calculate the yield of whole milk powder.
Solution:
According to the PFA standard, whole milk powder shall contain not more than 5.0% moisture and not less than 26.0% milk fat.
Keeping Quality of Dried Milk:
Keeping quality or shelf-life of dried milk or milk product refers to the time (in days or months) it retains its edible qualities after production.
Factors Influencing Keeping Quality:
Generally the deterioration occurring during storage involves flavour, colour and solubility index.
The rate of spoilage is influenced by the following factors:
(a) Composition;
(b) Quality of milk used in production;
(c) Care during production;
(d) Care during handling and storage;
(e) Manufacturing conditions, especially heat-treatment;
(f) Moisture content;
(g) Metallic contamination;
(h) Packaging conditions (materials and methods), and
(i) Storage conditions.
Note:
In commercial practice, the three most important factors influencing keeping quality are: fat content, moisture percentage and storage temperature. Since milk fat deteriorates rapidly, its content in skim milk powder should be restricted to 1.25 per cent or less. Likewise, moisture adversely affects storage life as the content increases above 4 per cent.
To prevent absorption of moisture during storage, the package should be relatively impervious to moisture vapour. High storage temperatures are detrimental to the keeping quality. Hence temperatures lower than 24°C (75°F) are desirable. If possible, refrigerated storage should be used for long term storage in tropical countries.
Major Changes during Storage:
Micro-organisms do not generally proliferate in dried milks and storage defects are purely chemical in nature. There are two major types of storage deterioration, viz., one affecting fat and the other affecting lactose-protein.
I. Fat Decomposition:
This may be divided into oxidation and rancidity (hydrolytic).
(a) Oxidation:
Whole milk powder and other dry high-fat milk products undergo oxidative deterioration (resulting in production of tallowy flavour) in storage. Chemical changes occur due to the addition of oxygen to the double bonds of unsaturated glycerides, producing first peroxides and later aldehydes, ketones and various carbonyl compounds.
Assuming average conditions of milk quality and processing, an oxidation off-flavour (oxidized flavour) becomes noticeable by sensory tests in roughly three months when the dry whole milk/milk product is held at 21-24°C (70-75°F) under air- packed conditions.
Various factors influence the rate of milk-fat oxidation.
These are:
(i) Storage Temperature:
The higher the storage temperature, the higher the rate of oxidation, and vice versa. (Hence low storage temperature is recommended.)
(ii) Presence of Copper and Iron:
The higher the copper and iron content, the higher the rate of oxidation, and vice versa. Both act as catalysts. To prevent a rapid oxidized flavour development, not more than 1.5 ppm of copper and 10 ppm of iron are recommended in dried milk and milk products. (Hence milk/milk product should come in contact only with stainless steel/aluminium surfaces during production, handling and processing.)
(iii) Acidity:
The higher the acidity, the higher the rate of oxidation, and vice versa. (Hence, use of fresh, sweet milk is desirable.)
(iv) Sunlight:
Accelerates fat oxidation. (Hence transparent packaging materials should be avoided.)
(v) Pre-Healing:
Proper pre-heating/forewarming of milk will delay oxidative changes during storage of dry whole milk/fat-rich products. This beneficial effect of pre-heating has been attributed to the release of sulfhydryl compounds, which act as anti-oxidants. The improvement may also be due to the more complete inactivation of lipase with the higher temperature-time required for pre-heating.
(vi) Addition of Anti-Oxidants:
When added in small amounts to the liquid product before dehydration, anti-oxidants increase the keeping quality of the dried product. Under the PFA rules (1976) butylated hydroxy anisole may be added to whole milk powder at the rate of 0.01 per cent by weight of the finished product.
(vii) Gas-Packing:
This is the most practical commercial procedure for increasing the keeping quality of dried products. By reducing the oxygen content to 2 per cent or less (by volume) in the headspace gas, the keeping quality is roughly doubled. The product may be vacuum-packaged with the low oxygen level. However, replacing the partial vacuum with nitrogen is more common in the dry milk industry.
(b) Rancidity (Hydrolytic):
True rancidity may be due to the hydrolysis of fat through lipase enzymes, leading to the production of free fatty acids, such as butyric. However, this defect is of minor importance in fat-rich dried products.
II. Lactose-Protein Changes:
During storage, the dry milk/milk products may develop an off-flavour, off-colour and insolubility, all due to lactose-protein changes.
(a) Staleness:
This storage defect is increased by a high moisture content and/or a high storage temperature. (Hence a low moisture content and a low storage temperature are recommended.)
(b) Off-Colour:
The principal change in colour is called browning-discolouration, which is attributed to the Maillard Reaction (lactose-protein reaction). The higher the moisture content and/or storage temperature, the faster the browning, and vice versa. Delayed cooling of the dry milk/milk product after drying is a contributing factor. (Hence, a low moisture content, prompt removal of the dried product from the drying chamber and a low storage temperature are recommended.)
(c) Insolubility:
This is measured by determining the solubility index, which measures the extent of protein-denaturation. The higher the storage temperature and moisture content of the dry milk and the longer the storage period, the higher the rate of increase of the solubility index, and vice versa. (Hence a low moisture content and low storage temperature are recommended.)
Recent Developments:
The manufacture of dried milks/milk products has increased more than that of any other milk product in the world in recent years. Not only have new uses for dried milks been found, but it has also been suggested in leading dairying countries that instant milk powder will, to a great extent, be a substitute for fresh milk in the future.
The dry milk industry today has become far more sophisticated than it was before. In the larger plants, automation has set in. Control of the air-inlet and air-outlet temperature of a spray drier is usually provided. The feed rate to the atomizer is adjusted automatically to give a constant air-outlet temperature (which controls the moisture content of the powder).
A new important development is to place the drying chamber externally, i.e., in the open air. This saves building costs. The external insulation of the chamber is increased up to 20 cm. thickness and the chamber looks like a silo.
Judging and Grading of Whole Milk Powder and Skim Milk Powder:
Score Card:
This is given jointly for both whole milk powder and skim milk powder in Table 9.8.
Procedure of Examination (Canned Product):
(a) Sampling:
Select a can of the product at random for examination.
(b) Sequence of Observations:
Cut more than three-fourths of the top of the can and turn it back.
Then examine it in the following order:
(i) Appearance of the Can:
Look out for signs of rust, etc., both outside and inside (when emptied).
(ii) Flavour:
Note the smell (odour) immediately on opening the can. Then reconstitute with distilled water to the original concentration (according to directions given on the can-label), and after about an hour examine flavour (smell and taste) by tasting 5-10 ml. on the tongue.
(iii) Appearance of the Product:
Examine uniformity of colour and absence of lumps, foreign specks, scorched/burnt particles, etc.
(iv) Fineness and Homogeneity (Body and Texture):
Pour the product from the can into another container and note the flow characteristics.
Requirements of High-Grade Whole Milk Powder and Skim Milk Powder:
The physical (sensory) qualities of high-grade milk powder will chiefly depend on the method of drying, viz., roller or spray, and also to some extent on whether they have been prepared from cow or buffalo milk and whole or skim milk.
Spray-dried whole milk should be clean, rich, sweet and very pleasant in flavour; fine, smooth and homogeneous in body and texture; uniform in colour—which should be normally light yellow for cow and creamy white for buffalo—and devoid of foreign specks, and scorched or burnt particles.
Skim milk powder, when reconstituted, should be similar in flavour to that of fresh skim milk for a spray product, although it may possess a slightly cooked or heated flavour for a roller-dried product; it should be fine, smooth and homogeneous in body and texture if produced by the spray process, but may be coarse and less homogeneous if produced by the roller process. If not finely pulverized, it should be uniformly yellowish white for cow and chalky white for buffalo milk; foreign specks, and scorched or burnt particles should be absent.
Uses of Dried Milk:
(a) Skim Milk Powder:
(i) In the preparation of toned, double toned and recombined milks;
(ii) In tea and coffee (during milk shortage);
(iii) In ice cream manufacture;
(iv) In candy and confectionary;
(v) In prepared foods;
(vi) In breads and rolls, biscuits, etc.:
(vii) In cultured products;
(viii) In the preparation of a few Indian sweets.
Note:
A spray-dried product is preferred where complete reconstitution is required; if not, a roller-dried product, being cheaper, is used.
(b) Whole Milk Powder:
(i) In the preparation of reconstituted milk;
(ii) In tea and coffee;
(iii) In baby-food preparation;
(iv) In candy and confectionary, bakery products, etc.
Note:
The production and consumption of whole milk powder is normally very low compared to that of skim milk powder throughout the world. This is mainly due to the much shorter shelf-life of the former as compared with the latter.