Roller Drying process for milk

Roller Drying

• The roller drying process is becoming comparatively obsolete because of the inherentdis advantages like low solubility of powder, cooked flavour and smaller capacity. However,the roller-dried powder finds special application in the manufacture of confectionery, baby foods, ice-cream, milk sweets, etc.

• The product to be dried is spread as a thin film on to the smooth surface of a continuously rotating hot drum roller or cylinder, and the film of the dried product is continuously scrapped off by a stationary knife, located opposite the point of application of milk. The film has to be ground to obtain powder.

Roller Drying Operation

• The product may be placed in its natural form or pre-condensed in a vacuum pant evaporator before it is fed to the drum drier. The pre-heated product is pumped intoh The reservoir between the upper portion of the rotating drums (14-19 rpm) to provide a thin layer over the turning drums (Fig. 4.7). The drums are heated internally by steam Usuallyat 4.2 to 4.9 kg/sq.cm (60-70 psi) with suitable arrangements for steam intakea and condensate outlet. The cylinders are mounted parallel to each other about 0.5 to 0.75 mm apart. One drum can be moved to provide the desired clearance between the drums The drums should be properly aligned and the surface must be kept free of rust and pits. The doctor blade, which is made of spring steel is mounted on strong blade holders and pressed against the drum at an angle of 15 to 30°.The knife should be sharp and reground frequently. The film of dry milk forms a continuous sheet form the knife tothe auger trough, which is about level with the bottom of the drum. The auger for each drum discharges the product into the elevators, then to a grinder which pulverizes the product, after which it is sifted, packed and stored. Water vapour above the drier has a lower density than the air surrounding the unit and hence rises upwards. A hood is placed over the drums for the vapours to escape.

Salient Features of Roller Drying of Milk

• The skim milk is pasteurized or pre-heated to preferably 80°C with or without holding before condensation to 20-22 per cent total solids. Beyond a total solids concentration of 25 per cent difficulties arise in drying. The capacity of the drier and final moisture contenta arecontrolled by the pressure of steam inside the drums, the speed of the drum, the temperature of the inlet milk, the level of milk between the drums, the total solids andi Viscosityof concentrated milk and the gap between the drums. The product is in contact withthe drum for three seconds or less at a temperature of about 150°C depending one The steam pressure. Scorched and burnt particles are to be expected in roller drying.