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A diffusion‐based drying model has been used to model the single‐layer drying of whole hazelnuts (hull, air‐gap and kernel). The drying of whole hazelnut was simulated by means of the numerical solution of Fick's second law for a composite solid consisting of an air gap which is between two solid bodies. The Crank–Nicholson implicit numerical method has been applied to solve the diffusion equation with variable mass diffusivity for whole hazelnut. The numerical data so obtained has been presented in the form of a series of curves which represent the drying characteristics of a whole hazelnut. Experimental results from whole hazelnuts were correlated with the theoretical results. A correlation describing the diffusivity of the whole hazelnut with moisture content and temperature was established, when the expression of diffusivity for hazelnut was used. Predicting the drying behaviour of the hazelnut for a known value of initial moisture content, equilibrium moisture and equivalent radius, a good agreement was obtaind between the experimental data and prediction. Whole hazelnut diffusivity was determined at 25–45°C, air velocity 0·2–0·3 ms−1, 60% relative humidity of air, using the theoretical curves that best fit the data. The diffusivity of whole hazelnut was determined as a function of drying air temperature and velocity. © 1998 SCI.

In the theoretical analysis of convective drying process, two boundary conditions are common for concentration: constant concentration and convection. In this study, these two boundary conditions were comparatively examined by comparing theoretical results obtained with regard to experimental ones. Pumpkin (Cucurbita pepo) was considered as the product to be dried while air was the drying medium. The drying characteristics of pumpkin were determined for various values of drying air parameters, including temperature, velocity and relative humidity. Sorption isotherms of the dried pumpkin were also determined for different temperatures and water activities. The values of the effective moisture diffusivity, Deff, and the convective mass transfer coefficient, hm, were predicted, and these values were found to agree fairly well with those available in the existing literature. PRACTICAL APPLICATIONS Convective drying as well as other drying techniques are used in order to preserve and store agricultural products for longer periods by removing some of their moisture content. Drying is a complicated process involving simultaneous heat and mass transfer under transient conditions. Understanding the heat and mass transfer in the product will help to improve drying process parameters and hence the quality.