Study of the drying kinetics and desorption isotherms of eggplant in convective air drying

• Autor(a) principal: Barroca, Maria João
• Data de Publicação: 2021
• Outros Autores: Guiné, Raquel P. F.
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.19/6818
• topic: eggplant; hot air drying; thin layer model; drying kinetics; sorption isotherms

Eggplant is a high-yielding vegetable crop well adapted to hot and wet environments that can provide significant nutritive benefits to human health like vitamins, minerals, phenolics and antioxidants. Therefore, the crop has been used in traditional medicine but due to their perishability it cannot be preserved long even when refrigerated and thus it is necessary to freeze or dry it to extend its shelf-life. The present study aimed to evaluate the effect of air temperature on the drying kinetics and sorption isotherms of eggplant for hot air temperatures ranging from 50 to 80 0C. Eggplants were obtained from a local market and, after removal of the peel, they were cut into slices with 6 mm thickness. Convective drying with hot air was performed in a drying chamber with an air flow rate of 0.5 m/s and at constant temperatures of 50, 60, 70, and 80 0C, for the different essays performed. The drying operation was undertaken until the final product reached a moisture content of about 10% (wet basis). The experimental data were fit to seven mathematical models commonly used to describe the drying kinetics of food products in terms of the dimensionless moisture ratio. The desorption isotherms were obtained measuring both the moisture content (halogen moisture analyser) and the water activity (hygrometer, coupled to an isothermal bath) along drying and the experimental values were adjusted using models presented in the literature. The results showed that the drying process at 80 0C provides a very important energy saving, since it allowed a reduction of about 67% in the drying time, as compared with the temperature of 50 0C. The statistical indicators obtained by fitting the experimental data to the tested kinetic models allowed concluding that the Page, logarithmic and Wang & Singh models satisfactorily described the drying behaviour of eggplant slices, but the best was the Vega & Lemus model. Furthermore, a nonlinear relation was obtained between the Vega & Lemus k constant and temperature. According to the Brunauer–Emmett–Teller (BET) classification, the moisture sorption isotherms of eggplant fall into category III. For increasing temperature, the amount of adsorbed water decreased for the same water activity, making eggplant less hygroscopic at higher temperatures. Furthermore, the Chen model was confirmed to be accurate in predicting the equilibrium moisture content of the eggplant slices in the temperature range studied. The shelf-stable dried eggplant should contribute to better exploitation of this crop and foster its use as an ingredient in the development of flours to be used as a functional ingredient in different kinds of meals, like soups, sauces and baked goods.