Models for food particle and dryer design should be as simple as possible. A dryer can be visualized as formed by a number of thin layer beds, ideally one particle deep. Therefore, thin‐layer dehydration studies are an essential step to developing methods for industrial equipment design. In this work, drying curves were experimentally determined for sweet cherry fruits, using air temperatures of 50, 60, 70 and 80C; air relative humidities of 5 and 50%; and air velocities of 1, 2, 3 and 5 m/s. A short time analytic predictive model for diffusion inside solids, fast to run on the computer, was selected to interpret the data with satisfactory accuracy. By this fitting procedure, diffusion coefficients of water in sweet cherries were determined to vary between 6.814 × 10−11(50C) and 3.474 × 10−10(80C) m2/s. The relationship of diffusivities with temperature was accounted for by using an Arrhenius‐type equation. PRACTICAL APPLICATIONS The determination of intrinsic drying properties such as thin layers, which are kinetic parameters, becomes an important issue as far as industrial dryer design is concerned. In thin‐layer‐dryer analyses, the food particle whose transport (drying) properties need to be evaluated is exposed to constant drying air conditions for a given length of time, so all variations occur within the product, and so the drying parameters thus measured can be related to those constant air conditions. The conclusions obtained at the thin‐layer level, which are interesting in themselves as drying information, must be consistent enough to allow their application at the deep‐bed level, for equipment simulation and design. Therefore, thin‐layer dehydration studies are an essential step to developing methods for industrial equipment design.

he aim of this work was to experimentally determine drying curves for thin layer and bed drying of rosehip fruits, with and without pretreatments, to reduce processing times as a function of drying air operating variables, to propose dehydration kinetics of fruits and to determine its kinetic parameters for further use within drying simulation software. Fruits were pre-treated both chemically and mechanically, which included dipping the fruits in NaOH and ethyl oleate solutions; and cutting or perforating the fruit cuticle, respectively. Simulation models were then adopted to fit the kinetics drying data considering fruit volume shrinkage. These simple models minimized the calculation time during the simulation of deep-bed driers. Results show that pre-treatments reduced processing times up to 57%, and evaluated models satisfactorily predicted the drying of rosehip fruit. Effective mass diffusion coefficients were up to 4-fold greater when fruit was submitted to mechanical pretreatments.

Macro and micro-structural changes take place during food dehydration. Macro-structural changes encompass modifications in shape, area and volume. Studies of such changes are important because dehydration kinetics (essential for calculating industrial dryers) may be highly influenced by changes in food shape and dimensions. The overall changes in volume, surface area ("shrinkage") and shape (Heywood factor, with provides a close description of food shape) were determined experimentally, and the results were correlated with simple expressions. Hence, although dehydration kinetics can be modeled with simplified overall shrinkage expressions, the possibility of selecting a suitable geometry and predicting the characteristics dimensions will provide higher accuracy. An additional unresolved problem is the lack of a general model that predicts macro-structural changes for various foods and diverse geometries. In this work, based on experimental data of sweet and sour cherries, and rose hip fruits, a simplified general model to predict changes in volume and surface area are proposed. To estimate how the changes in characteristic dimensions affect the kinetic studies, experimental drying curves for the three fruits by means of a diffusional model considered the following variants for the characteristic dimensions: (i) The radius of the fresh food, assumed constant; (ii) The radius of the partially dehydrated product; (iii) The radius predicted by the correlation for structural changes, especially volume, obtained in this work and generalized for the three fruits, and (iv) to demonstrate the need to study the macro-structural changes for all dehydrated foods, also be present the case of a restructured food.

Macro and micro-structural changes take place during food dehydration. Macro-structural changes encompass modifications in shape, area and volume. Studies of such changes are important because dehydration kinetics (essential for calculating industrial dryers) may be highly influenced by changes in food shape and dimensions. The overall changes in volume, surface area (“shrinkage”) and shape (Heywood factor, with provides a close description of food shape) were determined experimentally, and the results were correlated with simple expressions. Hence, although dehydration kinetics can be modeled with simplified overall shrinkage expressions, the possibility of selecting a suitable geometry and predicting the characteristics dimensions will provide higher accuracy. An additional unresolved problem is the lack of a general model that predicts macro-structural changes for various foods and diverse geometries. In this work, based on experimental data of sweet and sour cherries, and rose hip fruits, a simplified general model to predict changes in volume and surface area are proposed. To estimate how the changes in characteristic dimensions affect the kinetic studies, experimental drying curves for the three fruits by means of a diffusional model considered the following variants for the characteristic dimensions: (i) The radius of the fresh food, assumed constant; (ii) The radius of the partially dehydrated product; (iii) The radius predicted by the correlation for structural changes, especially volume, obtained in this work and generalized for the three fruits, and (iv) to demonstrate the need to study the macro-structural changes for all dehydrated foods, also be present the case of a restructured food.