Thin Layer Modelling of Recirculatory Convective Air Drying of Curd (Indian Yoghurt)

Dahi is one of the most popular fermented milk products consumed in India with excellent therapeutic properties. The initial moisture content of dahi is approximately 5.7 kg water kg dry solid−1. The shelf life of dahi is limited and hence the long term storage is possible in the form of dahi powder which can further be used as a base for formulation of health drink mix. Dahi was dried in a laboratory scale recirculatory convective air dryer to a final moisture content of 0.04 kg water kg dry solid−1. Drying characteristics of dahi were investigated under varying conditions of dahi thickness (0.003 m, 0.004 m and 0.005 m) and drying air temperatures (45°C, 50°C and 55°C) and velocities (1.5 m s−1, 2.0 m s−1 and 2.5 m s−1). Different drying models were used to simulate the observed drying data. The mathematical models were compared based on R-square and reduced chi-square values. The drying characteristics were satisfactorily described by Page, modified page, logarithmic and Midilli et al. models. The Midilli et al. model followed by modified page model provided the best representation of data. Effective moisture diffusivity computed on the basis of Fick's law varied between 2.52 × 10−10 m2 s−1 and 1.3 × 10−9 m2 s−1 under experimental drying air temperatures and sample thicknesses at air velocity 2.5 m s−1. The temperature and thickness dependence of effective moisture diffusivity was expressed by an Arrhenius type of equation. The equation showed best fit for diffusivity data at 2.5 m s−1 and varying temperatures and sample thickness.