The feed rate
(from 100 to 230 L h1) and steam pressure are the operating variables
used to increase total outlet solids. The duration of a run at
stable operating conditions was about 2 h. The spray atomiser was
equipped with a pressure nozzle (0.63 mm diameter orifice; N72)
and a 4-slot core (0.51 mm nominal width; N21) providing a 55
spray angle. In the case of the highly viscous skim milk concentrate
exiting the CT evaporator, the core was replaced by another with
narrower slots (0.41 mm nominal width; 66 spray angle; N17) in
order to control the droplet diameter, which would otherwise be
increased with higher viscosity. The temperature of the concentrate
before drying was about 40–45 C. The operating parameters were
set at a constant feed flow rate of 100 L h1, a final target moisture
content of the powder of 4.0% (w/w) and a final target water activity
of 0.2. The parameters used for drying the whey protein and skim
milk concentrates are detailed in Table 1.