Two types of samples were used in this study. They were whole milk (nonsticky, NS) manufactured by Dutch Lady Malaysia and orange juice concentrate (sticky, S) manufactured by Barkath Co-Ro Manufacturing Sdn. Bhd., Malaysia. Whole milk was heated up to 42±2°C before feeding to the spray dryer. For spray drying of orange juice powder, maltodextrin DE 12-15 with 6% moisture content (Epic Chemicals Sdn. Bhd, Malaysia) was used as the drying agent material. An aqueous solution with 30 °Brix was prepared by dispensing maltodextrin in warm water (about 50 °C) with constant stirring. Then, this solution was added to orange juice concentrate.
A nozzle atomizer spray dryer, model Lab-Plant SD 05 Laboratory Scale Spray Dryer (L.P. Technology LTD, Huddersfield, U.K) was employed. The air flow rate was kept at 62 m3/h and the atomizer pressure was 2.1 bar. A preliminary study on the effect of several inputs on the outputs was studied. For spray drying of whole milk powder, inlet air temperature (165-185 °C) and feed flow rate (400-800 ml/h) were investigated. For spray drying of orange juice powder, inlet air temperature (130- 150 °C) and maltodextrin concentration (7-21%) were investigated. These experiments were carried out in triplicate. Outlet air temperature and ambient air temperature with relative humidity were recorded each 10 seconds. Relative humidity and humidity for inlet and outlet air were determined using CYTSoft Psychrometric Chart Version 2.2. The powder moisture content was determined using Ohaus MB-45 Moisture Analyzer (Ohaus Corporation, Switzerland) at 105°C. Upon investigation, the most influential input on the outlet air temperature and moisture content will be selected as the manipulated variable. Drying air parameters and the characteristics of liquid feed were observed in both processes.
For empirical modeling, the sample was fed into spray dryer at constant manipulated variable for 10 minutes, with 2.1 bar of atomizer pressure and 62 m3/h of air flow rate. Then, a step change of the manipulated variable was applied. Outlet air temperature was recorded continuously until the process reached steady state. The experiments were carried out in three replications. The data obtained was plotted and the model obtained resembles first order process with time delay (FOPTD). The obtained model needs to undergo diagnostic evaluation and was verified with additional data before being used for process control.