The properties of dry milks may be divided into two categories, viz., physico-chemical and sensory. The relevant physico-chemical properties include: particle size, surface and shape, structure, density, flowability, dustiness and reconstitutability. The sensory characteristics of chief importance are flavour, colour and appearance, and tactual sensations in dry or reconstituted forms.
1. Structure:
Dry milks have a dual structure, viz., primary and secondary. The primary structure comprises the milk solids in which are dispersed the moisture and air cells. The physical mass of the particle, both in skim milk powder and whole milk powder, is dominated by the lactose in which presumably the protein, fat and minerals are more or less dispersed.
The bulk of the particles surrounded by air (whole particles are not in contact) constitute the secondary structure. The size and shape of the particles and the degree of uniformity of these characteristics affect the secondary structure.
(a) State of Lactose: D
Lactose in dry milk and milk products generally exists in an amorphous or glass (non-crystalline) state. It is very hygroscopic and readily absorbs moisture when the relative humidity of the surrounding air is 50 per cent or above. As sorption occurs, the lactose becomes sticky and this initiates the adherence of the milk particles to each other. Then solidification, commonly known as caking, occurs.
Note:
No crystallization of lactose-glass occurs in dry milks with a moisture of 5 per cent or less. The critical moisture level for crystallization is 7 per cent or more.
(b) State of Protein:
The heat-treatment given to milk during the drying process destabilizes the milk proteins (casein and serum proteins) by first inducing reversible denaturation and then irreversible coagulation.
(c) State of Fat:
The fat may exist in dry milks as globules surrounded by a membrane covering of proteinous nature or as ‘free’ fat. It is considered de-emulsified in the Free State. Free fat makes the dry product greasy and produces an oily film on reconstituted milk. In drum-dried whole milk powder, most of the fat (85 to 95 per cent to total) exists in the Free State and much of it is on the surface of the particles.
This high percentage may be caused by the rupturing of the globule-membrane by the hot drum and the scraping action of the knife when removing the film. In spray-dried whole milk powder, deviations from the standard drying operation or improper storage increase the free fat. Free fat promotes oxidation, especially free fat on the particle surface.
(d) Moisture Content:
The normal satisfactory range of moisture content is 3-4 per cent for skim milk powder and 2-3 per cent for whole milk powder. The rate of several deteriorative changes is influenced by the moisture content of dry milks.
2. Density:
Importance:
The density of dry milks and milk products has several practical implications. The chief one is its pronounced influence on packaging costs. Recovery of milk particles in cyclone separators is also affected, and density directly contributes to the proportion of the milk-dust nuisance.
Classification:
Densities are classified into:
(i) Bulk/apparent density,
(ii) Particle density, and
(iii) True density.
(i) Bulk/Apparent Density:
This is regarded as weight per unit volume and generally expressed as g./ml. It is divided into packed and loose density. The packed bulk density of skim milk powder is generally 0.50-0.60 g./ml. for a spray-dried product. Whole milk powder has a slightly lower bulk density than skim milk powder.
Foam spray-dried has the least, drum-dried the next and spray-dried the highest. The degree of pulverizing drum-dried milk in the hammer will affect bulk density. Agglomeration reduces it roughly by 40 to 60 per cent. Dry milk from a centrifugal spray has a slightly lower density than a pressure-spray product.
The manufacturing process and conditions greatly influence bulk density. Steps to reduce occluded air will increase bulk density. One method is to minimize the air content of the concentrate before drying; increasing the total solids of the concentrate is another.
(ii) Particle Density:
This is chiefly influenced by the amount of entrapped air. Centrifugal spray-dried milks contain more entrapped air than pressure-spray products and hence have a lower particle density.
(iii) True Density:
This refers to the density of air-free solids and may be calculated by the formula:
The true density of skim milk powder is 1.44-1.48 and whole milk powder 1.26-1.32 g./ml. The moisture content and the ratio of solids-not-fat- fat are the two chief factors affecting it. In a dry high-fat product, an increase in moisture reduces density and the opposite is true for skim milk powder. A decrease in fat will increase true density.
(iv) Increasing Density:
With air within and around the dried particles, there is scope for compression. The application of 7-10.5 kg./sq. cm. (100-150 psi) pressure to whole milk powder can reduce its volume by half. The densities of dried milks are given in Table 9.7.
3. Flowability:
This is affected by size, shape, density and the electrostatic charge of the dried milk particles.
The following properties improve flowability:
(i) Agglomeration;
(ii) Spherical shape;
(iii) High particle density;
(iv) Smooth surface;
(v) Dryness.
Note:
Addition of free-flowing agents, such as calcium silicate @ 1.5 per cent, sodium-aluminium silicate @ 0.5-1.0 per cent, etc., improves flowability.
4. Milk Dustiness:
Small, light dry milk particles, otherwise called milk dust, easily become air-borne. They cause higher entrainment losses, are a health hazard to people allergic to protein and create sanitation problems in the factory. Milk dustiness is increased by low solids in precondensed milk, low moisture content, etc.
5. Reconstitutability:
This refers to the sum total effect of solubility, dispersability sinkability and wettability. Lactose, undenatured serum protein and a portion of the salts are the most soluble. The casein is dispersible. Sinkability refers to the ability of the dried particles to penetrate the surface tension of water.
Wettability is the penetration of water into the particles. Reconstitutabiiity includes all those functions which take place in the process of recombining dried milk with water. The ideal dry milk is one which rapidly recombines with water without agitation to give the characteristics of regular milk.
The factors affecting reconstitutability include:
(i) Physical properties, viz., size, shape, density, uniformity, air content, composition (especially ratio of solids-not-fat: fat of the particles) and presence of additives;
(ii) Chemical properties, viz., extent of protein denaturation (by heat or storage), and
(iii) Conditions of recombination, viz., the temperature of water and dried milk, the nature and extent of hardness in water, and the time and nature of agitation.
Note:
The physico-chemical properties of milk powder, which influence reconstitutability, are affected by various factors. These may be grouped into two, viz., equipment and processing conditions- equipment includes the drier and system of atomization; and processing conditions include heat-stability of the milk, pre-heat treatment of the milk and the concentrate, total solids percentage of the concentrate, outlet-air temperature and contact time, recycling of fines, selectivity of screening, agglomeration, and storage temperature and time.
(a) Dispersability:
This is affected by the total heat-treatment on the casein during processing. Higher heat-treatment with a higher percentage of total solids causes a greater degree of irreversible denaturation; and the denatured casein under normal reconstitution procedures does not form a stable dispersion.
Drum drying causes a higher degree of denaturation than spray drying; however, heating the milk concentrate too much or for too long, or subjecting milk particles to above-normal temperatures in the drier in spray drying (when attempting to obtain production above the normal capacity of the drier) will increase percentage denaturation. Large particles of dry milks are generally recognized as being good for dispersability.
(b) Wettability:
The tendency of dry milks to form lumps upon addition of water indicates lack of wettability. The factors which favour wettability are large-sized and irregularly shaped particles (which provide more space in the interstices for water penetration for wetting); agglomeration favours wetting by providing both the above conditions.
The amount and dispersion of fat influences wettability. The milk dried with low melting point fats has better wettability. The addition of up to 25 per cent sugar (sucrose) to milk before it is dried, or dry blending the sugar in granulated form (powdered sugar is less effective), improves wettability and dispersion.
(c) Sinkability:
This is closely related to wettability. The amount of occluded air within the particle has a pronounced influence on sinkability. Foam spray-dried milk has very poor sinkability. Agglomeration of regular spray-dried milk improves sinkability by increasing the aggregate weight. The effect of agglomeration on sinkability is greater in skim milk powder than whole milk powder.
5. Flavour:
When made from high-grade milk, the flavour of dried whole milk should be clean, rich, sweet and very pleasant; and that of dried skim milk similar, when reconstituted, to that of fresh skim milk. However, in practice, there is some degree of burnt flavour, which is greater in drum-dried milks than in spray-dried ones. Normally, drum-dried milks always possess a distinct cooked flavour.
The intensity with which milk is heated (mainly the pre-heating temperature-time combination) considerably influences the development of a cooked and caramelized flavour in spray-dried milks. ‘Low-heat’ and ‘medium-heat’ powders give the reconstituted milk a flavour close to that of pasteurized milk; however, ‘high-heat’ powder, when reconstituted, gives a definite cooked flavour.
Note:
In practice, during the storage of milk powders, various flavour defects occur which are of concern to the user.
6. Colour and Appearance:
The colour of whole milk powder should be uniformly light-yellow for cow and creamy-white for buffalo milk; while that of skim milk powder should be uniformly yellowish-white for cow and chalky-white for buffalo milk. The yellow colour varies in intensity with the season of the year. Small errors in operation during drum drying can readily produce a darker shade of colour.
Discolouration, often associated with a cooked or caramelized flavour, results from high heat-treatment. A more common defect in appearance is the presence of burnt particles, commonly in drum drying and seldom in spray drying. A general brown discolouration may develop as a storage defect of both roller and spray powders (mainly when a powder with a high moisture content is exposed to high temperatures).