Quality attributes of dried milks

Milk powders possess various organoleptic, physico-chemical and reconstitutional properties, which are important to both industrial and consumer use. These properties are the basic elements of quality specifications for milk powders. These properties of the powder are affected by the milk quality, the design of the evaporator and dryer, and by the process conditions. During drying process, care is taken to conserve as much as possible the natural properties of the original raw milk. Quality of dried products should be such that when reconstituted with water, give little or no evidence of detrimental change compared to the original liquid products.

i. Flavour

Milk powders are expected to demonstrate a slightly sweet, clean and pleasant flavour, though other dried milk products may be expected to confirm to certain other specific requirements. Non-fat dried milk does not have rich flavour because of absence of fat.

ii. Colour

Milk powder should be uniform in colour, free from foreign specks and burnt particles.It should exhibit greenish white or creamish white colour, respectively in buffalo and cow milk powders.

iii. Physical Properties

Particles shape: Spray dried skim milk powder particles have been found to be regular spherical or slightly oval with different surface structures, as revealed by electron microscopy. Some particles may be smooth, but most of them are severely wrinkled with deep surface folds and having “apple-like structure”. The internal structure of almost every particle of spray dried milk powder shows a large vacuole, in which smaller globular particles are trapped.

Particle size distribution: The particle size distribution of a powder is a measure of the average particle diameter and of the spread of sizes on either side of this average. However, the general rule is that the mean particle size (estimated by sieving test should not be smaller than a set size. For example, for instant whole milk powder,the mean particle size suggested is 180 ìm with not greater than 20 % fraction above 500 ì and not greater than 10%. fraction of below 125 ì. The particle size of powders affect their appearance, flowability, bulk density, Dust characteristics and reconstitution .

Bulk density: Bulk density, also known as apparent or packing density, is a measure of the mass of milk powder that occupies a fixed volume. It is normally expressed in g/ml. The bulk density of normal spray dried powder may vary between 0.5 - 0.8 g/ ml.

The bulk density of milk powder is economically, commercially and functionally an important property. When shipping powders over long distances, the producers are interested in high bulk density to reduce the shipping volume, since in most cases the transportation costs are charged by volume. Also, high bulk density saves in packaging material. In some instances producers may be interested in low bulk density to supply apparently larger amounts of powder on the retail market than that of their competitors.The way, in which particles pack together also influence the bulk density. It depends on the particle size range. The wider the range of sizes, the more likely it is that small particles will pack in the voids left between large particles and the higher will be the bulk density. As smaller particles contain proportionately less occluded air than large particles, the removal of fine material, for instance in the manufacture of instant whole milk powder, affects the bulk density both by decreasing the average particle density and by decreasing the number of small particles available to fill the voids between the larger particles. If a powder is subjected to severe mechanical action,there may be a breakdown of individual particles, forming more fine material and decreasing the number of internal voids, thus increasing the bulk density.

Flowability: The flowability of a powder refers to the ease with which the powder particles move with respect to one another. Flowability is one of the major functional properties of dried milks, with considerable implications in packaging, metering (or, quantitative transfer) into a process vessel, etc. Based on flowability, the product may be termed “free flowing”, “moderately cohesive” (less flowable) or “very cohesive” (least flowable). The main factors controlling flowability are:

•  Particle size distribution - cohesion or resistance to flow, of spray dried milk powder increases with decreasing particle size
•  Surface texture 
• Milk fat is an important factor in cohesion. Increasing fat content up to 20 % increases the resistance to flow.
•  Pressure nozzle powders are superior in this respect to rotary atomizer powders and two-stage drying gives better result than single-stage drying.

 Other factors improving flowability are:

•  Agglomeration
•  Low percentage of fines
•  Addition of free-flowing agents (e.g. sodium aluminium silicate)
•  Addition of components by dry mixing (e.g. sugar, whey powder)
•  Low free fat content
•  Low relative humidity of atmospheric air

Skim milk powder is more free-flowing than whole milk powder. With increase in water content (e.g. > 5%), free-flowingness considerably decreases. At lower temperature, the free-flowingness is slightly better. Anti-caking agents (‘free flowing agents’ or ‘flow conditioners’) are believed to reduce interparticle forces, and as such they are expected to increase the bulk density of powders and, therefore, their flowability. Such compounds often act by preferentially absorbing moisture owing to their high moisture-holding capacity. Thus they prevent caking and minimize flowability loss during storage.

iv. Reconstitution and Instant Properties

Reconstitutability refers to the ease with which a powder can be dissolved in water and it consists wettability, sinkability, dispersibility and rate of solution. Ideally, high rates of wettability and rapid solubilization with a minimum of scum remaining are required. The milk powder is said to have good reconstitution properties if the reconstituted liquid milk differs little or not at all from homogenized whole milk prepared from sweet fresh milk. The solubility of milk powders produced in modern driers approaches 100%. For skim milk powder and whole milk powder to be graded as instant the values of dispersibility in percent should be above 90 and 85,respectively.