Manufacture of Spray Dried Milk Powder

The predominant method of drying milk and milk products is spray drying. The condensed milk from the evaporator is pumped to a balance tank and from there passed through a filter and introduced into drying chamber with or without preheating.

The solubility of skim milk powder is not affected up to a preheating temperature of 80°C. Feeding cold concentrated milk to the drier may result in sticking to the chamber causing more burnt or scorched particles in the final powder. Either a pressure nozzle or spinning-disc is used for atomising the concentrate into fine droplets and exposing to a current of hot air in the spray-drying chamber. Due to increased surface area, and to the high latent heat of water evaporation (2.26 MJ/kg), sprayed particles release their moisture quickly, thereby causing an immediate temperature drop of the incoming air. While the inlet temperature reaches up to 215oC, the temperature in the chamber drops down almost instantly to the temperature of outlet air (about 95oC in one stage drying). Residual moisture is one of the most important properties of milk powder both from a quality and an economic point of view. The outlet temperature is usually used as the parameter by which the final moisture of the product is controlled. The outlet temperature depends on a large number of factors.

The chamber design, the residence time of milk particles in the chamber, the final desired moisture content in the powder and the design of powder collecting systems controlling the air inlet and outlet temperatures. Dry product is taken away immediately after drying. The powder coming out of the drier is preferably cooled to 30°C by a secondary air stream before packaging and storage. Cooling is done to prevent clumping, sticking and heat-damage to the product. Prolonged heating causes staleness in non-fat dry milk and jeopardizes the flavour and keeping quality of dried milks. On prolonged exposure of the powder to heat, the fat has a tendency to melt and oil off. The fat thus becomes the continuous phase. Some of it covers the surface of the dried milk particles and is exposed to air and light and becomes subjected to oxidation.

In two-stage drying, it is also important to control the intermediate moisture, i.e. the moisture of the powder at exit from the drying chamber because it influences many other properties including solubility index, particle density, bulk density, agglomeration, etc. Because there is some gain in moisture content during pneumatic conveying and blending and to a lesser extent during storage, it is normal to produce powder from the dryer of lower moisture content than that called for by the specification so that the final powder remains within specification.

Spray drying has numerous important advantages compared to other drying techniques: The whole process proceeds very rapidly; air residence time in the chamber is up to 30 seconds. Because of this and because drying is accomplished at lower drying temperatures, the product has excellent properties.

Certain disadvantages of spray drying include very large size of the drying chamber,expensive equipment, high electricity and steam consumption and low bulk density of the dried product. However, the bulk density of milk powder obtained by spray drying is still higher than that of roller-dried powder. Because of high investment cost, Spray drying installation is economically justified only for large capacities, over 100,000 kg of raw milk per day. Today there are specialized drying plants with modern facilities, producing 18 tons/hour of powder.

FOAM SPRAY DRYING

Foam spray drying can be accomplished by forcing the gas into liquid through the pump before atomising nozzle supplying the drier. Air is commonly used as the added gas for making foam spray non-fat dry milk. Nitrogen is commonly used for making foam dried whole milk. Foam spray drying provides a means of using most conventional spray drying equipment for drying liquids up to a maximum of 60% totals solids as compared to 50% on a particular drier, for obtaining an instant type powder and provides a procedure for increasing the capacity of conventional equipment and profits as well. Foam spray drying gives more uniform particle size in the dried product. Foam spray dried powder has improved dipersibility, but poor sinkability due to occluded air when reconstituted in water. However, the foam spray milk powder has lower bulk density and hence higher packaging, storage and transportation costs.