Modelling diffusion of moisture during stoving of starch-molded confections

Starch molding is a widely used shape forming technique in the confectionery industry. Candy is deposited into cavities imprinted in dry granular starch contained in a shallow tray. For products that cannot be deposited at their final moisture content, a drying or stoving stage follows deposition, and largely determines the productivity of the process. In the present work a finite difference model was developed that quantitatively predicted the moisture diffusion from a single candy piece into the starch bed or air at a given temperature (50°C). The model suggested that a 'skin' or region of low moisture content (3-4% wet basis) and diffusivity (10^-13 m² s^-1) was formed near the surface of the candy at normal operating conditions of the drying room that then slowed the drying process, and that the spacing between the candies does not affect the drying rate at typical print densities. The model adequately predicted experimental results from model systems and in production. Controlling the relative humidity of the drying air, the initial moisture content in the starch bed, and the temperature of the drying chamber may avoid skin formation, improve drying rate and minimize the drying energy required.