Effects of time related operating conditions on the effective drying time and drying rate during interval IR drying of squash (Cucurbita moschata)

Abstract

Infrared drying is increasingly used in the food industry. Infrared energy can improve drying operations and reduce energy consumption compared to convective drying. Often energy is lost during drying in the form of sensible energy, which increases the temperature of the sample and thus causes irreversible damage. Interval drying was applied to infrared by well-defined intervals. The new drying process IIRAD (Interval Infrared Airflow Drying) was implemented using infrared energy coupled with ambient temperature (18 ± 1 ◦C) airflow (1.41 m s − 1) drying and 0.7 W cm− 2 as IR power. In order to estimate the operating time intervals, the calculation of the energy necessary to evaporate a thin layer of water was estimated according to the initial moisture content. The intervals were then defined with 5s of action time (tON) and a tempering time (tOFF) of 2 min. A second set of time related operating conditions was achieved by modulating tON during the drying process: constant tOFF = 2 min while tON varied from (i) 5 s during the first 240 min (till water content reaches W = 4 g H2O/g db), (ii) tON = 4 s during the next 273 min (till W = 2.6 g H2O/ g db), and finally (iii) tON = 2 s till the end of the process. The three modes, continuous, IIRAD type I and II were compared through the drying kinetics, the effective drying time and the energy consumption. The moisture content rapidly decreased during IIRAD type I and II compared to continuous IR drying. The effective drying time was significantly reduced, which allowed a significant energy gain. The moisture migration from the deep layers to the surface of the samples  mainly occurred during the tempering time, which accelerated the water evaporation during the following active times. Calculation and experimental tests showed that the energy required decreased throughout the drying process with water content decrease. Indeed, reducing tON from 5 to 2 s had a positive effect on drying since a higher evaporation rate was noted.  Interval Infrared drying appears to be a promising method to intensify drying and save energy. The intervals must be defined according to the sample nature, the operating conditions and the water content of the sample which varies throughout the process.