Diffusivity, shrinkage and simulated drying of litchi fruit (Litchi Chinensis Sonn.)

Dielectric properties of litchi fruit (Litchi chinensis Sonn) at microwave frequencies

The dielectric properties of litchi fruit were determined using the open-ended coaxial probe method. The measurements were performed in the frequency range from 0.5 to 20 GHz during 3 days of storage at room temperature (~  24 °C). The dielectric properties increased with storage time. Additionally, measurements at different temperatures (24, 30, 40 and 50 °C) were determined. The dielectric constant (ε') decreased with increasing temperature in a frequency range of 0.5-5 GHz; at higher frequencies, ε' increased with increasing temperature. The loss factor (ε″) value increased at frequencies higher than 2 GHz and decreased with increasing temperature. The results will be useful for further applications using microwaves, such as microwave-assisted drying, sensing of quality parameters, modeling, and heating to protect against molds or insects, among other applications.

Effect of Intermittent Microwave Volumetric Heating on Dehydration, Energy Consumption, Antioxidant Substances, and Sensory Qualities of Litchi Fruit during Vacuum Drying

To examine the processing characteristics and high quality of an improved microwave vacuum drying system, litchi fruits were dried using intermittent microwave volumetric heating while microwave vacuum drying at 2 W/g was carried out for comparison; the intermittent microwave heating profiles were set as (1) 5 min drying-on, 5 min drying-off; (2) 5 min drying-on, 10 min drying-off; and (3) 5 min drying-on, 15 min drying-off. Energy consumption during drying was determined, and physicochemical properties such as moisture content, vitamin C, total phenolics, color, and sensory evaluation of dried products were assessed. In microwave vacuum drying, intermittent microwave volumetric heating was found to be energy-efficient (about 32 KJ/g to 45 KJ/g) and saved at least 31% of energy consumption compared with microwave vacuum drying as well as decreasing product browning. In addition, microwave volumetric heating had no substantial effects on sugar and protein contents, while antioxidants were affected significantly (p ≤ 0.05). Moreover, sensory evaluation showed that intermittent microwave-assisted vacuum drying (IMVD) increased the acceptance of the dried product compared with microwave vacuum drying (MVD).

Moisture Diffusivity of Seedless Grape undergoing convective drying

Seedless grape (Sultana grape) is a very important commercial fruit grown in large quantities in Tunisia. This product is characterized by a high initial moisture content (the initial wet basis moisture content of the fruit is more than 80 %), and thus a high shrinkage during drying. The mature seedless grape is a spherically shaped fruit. Thermo-physical properties and drying kinetics of seedless grape is essential for the optimization of its drying processes. This paper is composed of two parts, the first one is reserved to the experimental study of seedless grapes, such as the establishment of the desorption isotherms which were determined at 40, 50, 60 and 70 °C by using static gravimetric method and these desorption data were fitted by GAB model. Then we were interested in measurement of the axial hydrous shrinkage of a grape berry and it was expressed as a function of moisture content. Indeed, the drying kinetics under different controlled conditions of air temperature and relative humidity were realized. In the second part, the moisture diffusivity of the seedless grape was determined by minimizing the sum of square of deviations between the predicted and experimental values of moisture content of convective drying kinetics. The adopted approach was based on a numerical solving of the conservation equation of the solid phase and the equation of diffusion/convection of liquid phase for spherical geometry, coupled by the solid phase velocity due to shrinkage. The moisture diffusivity of seedless grape increased with temperature and was correlated by an Arrhenius-type equation. Indeed, the effect of moisture diffusivity was expressed by an exponential function. The moisture diffusivity of seedless grape ranged between 3.5610−10 (m2/s) and 12.610−10 (m2/s). Activation energy was found equal to 57.76 kJ/mol.