Investigating the Drying Kinetics of Goat Whey and Casein Protein Using the Thin-film Drying Method

Abstract

This research was initiated to investigate the drying characteristics and drying kinetics of goat whey proteins and caseins. A thin-film drying apparatus was designed and successfully built with the aid of 3D-printing technology. The structure of the drying channel was designed and set up to achieve a relative good thermal stability. The standard water sample test was conducted to ensure the effectiveness of the equipment and to determine the basic parameters of the system. Goat whey isolate (WPI) and milk micellar casein (MC) were reconstituted to a concentration of 10% w/w for both proteins with several MC: WPI ratios (100:0, 80:20, 50:50, 20:80, 0:100). The determined drying curve illustrated that while the overall drying rate of MC and WPI is quite similar, their drying characteristic was slightly different. While both the drying rates of MC and WPI increased with an increase in the drying temperature and drying air velocity, higher air velocity tends to have more effects on MC, especially at higher temperatures. It was observed that a layer of crust was formed during the drying of MC and WPI. This crust formation was more obvious when drying MC, and there was apparently more moisture retained by the solidified layer. The observations of surface layer morphology at different drying stages have also supported this phenomenon. Accordingly, while the apparent equilibrium moisture content of MC and WPI were numerically similar at 1 m/s of drying air velocity, the apparent equilibrium moisture content of MC remarkably increased when the drying air velocity became higher. The critical moisture content was read from the approximated drying curve using the piecewise theoretical fitting method. The characteristic drying curves of MC and WPI were obtained, which proved the drying rate of both proteins followed a convex falling relationship. With the data obtained from the thin-film drying experiments and water tests, the Characteristic Drying Curve (CDC) model for MC and WPI was developed and validified. These CDC models were improved by introducing a characteristic parameter, ‘n’, indicating the deviation from the possible linear drying reduction characteristics as observed in the bovine milk. The improved CDC models showed good agreement with the experimental data. Based on the apparent equilibrium moisture content data obtained from the thin-film drying experiments, specific functions could be obtained to predict the apparent equilibrium moisture content which also agreed well with the experimental data. The analysis of the drying characteristics of micellar casein may lead to meaningful correlation with the morphology and structural properties of micellar casein, which was reported in literature. The CDC models as established for the MC and WPI were found to be desirable, with a good potential for further application in the CFD simulation.