Influence of power density and geometry of young cactus cladodes (Opuntia ficus-indica (L.) Mill.) on intermittent microwave drying kinetics

Due to its multiple uses, the production and consumption of nopal have increased worldwide in recent years. The influence of power density (60.3 to 538.9 W g_db^-1) on the intermittent microwave drying of young cladodes, of different sizes, was studied. In general, all drying treatments showed a sigmoid shape and three drying periods: heating (I), constant rate (II) and falling rate (III). Empirical models were used to model drying kinetics. However, although they had a good fit (R² from 0.965-0.998) they do not exactly represent the changes between drying periods. According to microstructural analysis of dried samples, the water was transferred from the inside to the surface of the cladode by the sides, where there is no cuticle o epidermis (which was removed by the thorn quitting process). It was determined that the drying rate in period II depends exclusively on the power density applied and not on the cladode’s geometry. However, in period III the data of effective diffusivity (D_eff, 2.20 x 10^-6to 5.59 x 10^-5 m²s^-1) showed that the drying rate is affected by the size and thickness of cladodes.

Keywords: Microwave intermittent drying, Cactus cladodes drying, Effective diffusion coefficient, Drying periods, Activation energy.