Comparison of effect of convection drying and osmo-convection drying on drying behaviour and colour indexes of peppermint leaves

The effect of convective drying and osmo-convective were investigated on drying behaviour, drying characteristics and colour indexes of peppermint leaves. Drying experiments werecarried out at 50, 60, 70 and 80°C and the osmotic dehydration pretreatmentwith a 5% (w/w) salt solution at 40°C, 80 rpm agitation and 60 min immersion time.The moisture diffusion coefficient and activation energy were calculated by the second law of Fick and the Arrhenius equation, respectively. The results show thatdrying behaviour, drying characteristics and peppermint colour were influenced by drying method. Drying times of peppermint leaves by convective drying was shorter than those dried by osmo-convective drying. The thin structure and large area of the leaf surface was caused a rapid decrease in moisture content.Therefore, the drying wasoccurred at a falling rate. Osmotic dehydration due to thepresence of infused solids increased cell wall thickness, decreased drying rate and effective moisture diffusion.The effective diffusion coefficient increased while increasing temperature, it is indicating intensity of interactions at higher temperatures. Thetotal colourchangesof dried peppermint leaves by convective drying was less than osmo-convective drying method, which indicates that the osmotic dehydration was not efficient to prevent colourchanges of peppermint leaves.