Manufacture of dairy powders includes several unit operations,
two of which are to remove water and convert a liquid product into
a powder, i.e. concentration by evaporation and spray-drying.
Evaporation is based on heat transfer between condensing steam
as the heating medium and the liquid to be concentrated.
Falling-film evaporators are widely used in the dairy industry. In
such equipment, the liquid enters at the top of the vessel, and it
is distributed so that it flows down tubes as a film and then the
vapor and the concentrate leave the bottom of the vessel
(Morison et al., 2006). The advantages of such evaporators are minimal
fluid residence times and high transfer rates, with relatively
low temperature differences (Salvagnini and Taqueda, 2004).
They are quite suitable for heat-sensitive products as evaporation
is performed under vacuum to reduce the boiling temperature.
Spray-drying is based on the transfer of both heat and water
between a liquid product and hot drying air: the product, in liquid
or in suspension form, is sprayed into a hot air stream in which the
transfer of water takes place. This drying technique is appropriate
for heat-sensitive products as dehydration is very rapid, taking a
few seconds (Bimbenet et al., 2002).