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Industrial drying technologies were developed during the 20th century. Nowadays, spray drying is the most commonly used method in dairy industry, whereas roller drying is only used to meet very specific requirements.
Roller drying (drying on heating cylinders) consists in obtaining a dried product by boiling. Evaporation of water is proportional to the latent heat of vaporization input. Such input is created by conduction, from the contact between the product and a heated surface.
The material used for this technique is made of rotary cylinders with horizontal axes, which are set up close to each other. These cylinders are steam heated from the inside.
The product in liquid form (more or less pasty) is poured between the cylinders. Afterwards, a thin layer of dried product appears on the drums and is later removed from them with a scrapping tool. A hood, set up above the drums, is used to vacuum out the excess of steam.
The thermal shock thus created leads to physicochemical reactions that are most valued by the chocolate and biscuits industries. Indeed, when undergoing roller drying, lactose caramelizes and the product browns (Maillard reaction).
Spray drying (drying by atomisation) is the most used technology in dairy industry. The product to dry (in liquid state or in suspension) is sprayed into a hot gas stream which leads to a quick dehydration. The insertion of a moist body into a hot and dry gas stream (air for example) creates between them a temperature and partial water pressure gradient..
Three types of spray dryers may be used : centrifugal, pressure nozzle and two-fluid pressure nozzle.
Atmospheric air is vacuumed up through filters which are adapted to the product to dry and local conditions. The air may be heated in two ways: by direct heating (electricity or gas) or by indirect heating (steam, oil, gas).
Two spray drying principles exist:
Technological advances concerning spray drying have enabled many improvements regarding: general performances, product quality, thermal performance, hardward, atmospheric releases.
Spray drying meets the current quality requirements of the dairy industry (solubility > 99%, dispersibility > 95%, WPNI maximum, low free fat content, etc.).