Reassessment of Thin-Layer Drying Models for Foods: A Critical Short Communication

Drying kinetic, thermodynamic and quality analyses of infrared drying of truffle slices

Kinetic and thermodynamic parameters are the most important part for making a suitable tool for drying agricultural products. Moreover, calculation of the energy required for the drying of product, the properties of the rehydration ratio, the food appearance changes, and the evaluation of the microstructure of food are crucial. Since the thermodynamic properties of truffle slices have not yet been reported, this study aims to establish a mathematical model to describe drying process of agriculture product, evaluate the effective moisture diffusion coefficient (Deff), determining the activation energy (Ea) to elucidate the thermodynamic characteristics, measure color characteristics, and rehydration ratio (RR) during the drying process of truffle slices. Truffle slices were dried in an infrared (IR) dryer at four temperatures of 50-80°C and two thicknesses of 0.5 and 1 cm. The best model to describe the drying process of truffle slices was Midilli et al.'s model. The value of Deff, SEC, and RR were in the range of 3.06 × 10-8 to 2.48 × 10-7 m2/s, 79.68-191.271 kWh/kg, and 5.99-7.49, respectively. The Deff of truffle slices increased with the above-mentioned parameters of the samples. The Ea obtained was 26.62-27.43 kJ/mol. The results indicated that enthalpy and entropy decreased with increasing drying temperature, while Gibbs free energy improved. The enthalpy, entropy, and Gibbs free energy values changed between 24.48-25.28 kJ/mol, -130.47 to -122.63 J/mol °K, and 63.97-70.17 kJ/mol, respectively. In addition, the results of color attributes decreased with increasing temperature, while chroma oppositely increased.

Intensification of moisture separation in the pulp convective drying process with ultrasound-assisted method

Traditional pulp convective drying (CD) is time-consuming and energy-intensive. This study aimed to assess the drying performance of pulp using ultrasound-assisted drying (UAD) and compared it with CD to intensify moisture separation. UAD was found to be fast and efficient with high effective moisture diffusivity of 2.77 × 10-10 ∼ 3.20 × 10-10 m2/s, low activation energy of 20.2 kJ/mol, and short drying time of 21.0 ∼ 16.5 min. It demonstrated that applying ultrasound could promote moisture separation with 26 %∼42 % reductions in drying time and 42 %∼22 % savings in energy consumption. The constant rate period was not presented and no significant differences in drying rates were observed when the moisture ratio was below 0.43 under the investigated conditions. The kinetics modeling results indicated that the Page model was the best to predict the pulp drying kinetics for both methods. It may lead to an alternative efficient approach for decarbonizing the drying process in pulp and paper production.

Impact of Fermentation Pretreatment on Drying Behaviour and Antioxidant Attributes of Broccoli Waste Powdered Ingredients

Valorisation of fruit and vegetable wastes by transforming residues and discards into functional powdered ingredients has gained interest in recent years. Moreover, fermentation has been recalled as an ancient technology available to increase the nutritional value of foods. In the present work, the impact of pretreatments (disruption and fermentation) on drying kinetics and functional properties of powdered broccoli stems was studied. Broccoli stems fermented with Lactiplantibacillus plantarum and non-fermented broccoli stems were freeze-dried and air-dried at different temperatures. Drying kinetics were obtained and fitted to several thin layer mathematical models. Powders were characterized in terms of physicochemical and antioxidant properties, as well as of probiotic potential. Fermentation promoted faster drying rates and increased phenols and flavonoids retention. Increasing drying temperature shortened the process and increased powders' antioxidant activity. Among the models applied, Page resulted in the best fit for all samples. Microbial survival was favoured by lower drying temperatures (air-drying at 50 °C and freeze-drying). Fermentation and drying conditions were proved to determine both drying behaviour and powders' properties.

Critical Analysis of the Use of Semiempirical Models on the Dehydration of Thin-Layer Foods Based on Two Study Cases

Moisture transport during food drying can be phenomenologically described by Fick’s second law and by the so-called anomalous diffusion model. However, in the literature, many studies have shown the extensive use of empirical/semiempirical models (EMs/SEMs) to adjust experimental data for the drying of thin-layer foods. This research aims to perform a critical analysis of the most commonly used EMs/SEMs and compare them with Fick’s second law and an anomalous diffusion model using two different sets of hot-air drying data. Two waste byproducts from the food industry, spent coffee grounds and passion fruit peels, were selected for analysis. The selected EMs/SEMs were found to be mathematically interrelated (i.e., some are a subset of others), and their appropriateness was incorrectly justified mainly by their statistical goodness-of-fit. As shown, it is highly recommended that researchers start analyzing drying data with phenomenological models. The extensive use of EMs and SEMs can be replaced by the anomalous diffusion model, which has a high capacity to adjust empirical data and a sound phenomenological description of the process.