Physical and chemical characterization of Corchorus olitorius leaves dried by different drying techniques

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Molokhia, Corchorus olitorius, is a popular leafy vegetable, known in many world regions as a good source of nutritional and medicinal properties. Due to its short shelf life and the limited harvesting time, processing such as drying techniques permit to preserve and provide it throughout the year. In the present study, it was attempted to reveal the main physical and chemical characteristics of molokhia leaves. Also, three drying techniques, shade drying (SHD), convective drying (COD), and microwave drying (MID), have been applied to study the kinetics and their main physical and chemical effects. The analysis demonstrated that molokhia leaves are a good source of phenolic compounds, flavonoids, and chlorophylls pigments. Those bioactive compounds have provided the leaves with considerable antiradical scavenging and reducing capacities. Drying time decreased from days, in the case of SHD, to some hours when using COD, and less than 20 min when using MID. Increasing drying temperature and power input have increased the drying rate. Modelling of drying kinetics of MID three power inputs (350, 500 and 750 W) and COD at 60 °C exhibited a high fitting for most empirical models (R2 > 0.980). SHD was less deleterious on leaves colour. Also, it preserved the content of phenolics, flavonoids, and thus the antioxidant activity of leaves. On the contrary, COD at 80 °C had a detrimental effect on previous components and their activity. Vega-Gálvez model can be presented as the best-fitted model to describe the rehydration kinetics of dried leaves. Rheological analysis of the aqueous extracts of the leaves demonstrated the effect of time and grinding on the increase of mucilage diffusion. The obtained results could help industrials to choose the convenient drying method and more analysis on the subject are recommended.