Review of leaf drying: Mechanism and influencing parameters, drying methods, nutrient preservation, and mathematical models

Fresh leaves used for culinary and medicinal purposes are highly perishable by nature. Quite often, post-harvest losses occur and there is noticeable deterioration in quality due to moisture enhanced enzymatic and microbial activity, climate changes, improper handling, delayed transportation, improper storage and delay in sales. To preserve leaves for a longer duration and to ensure their easy availability for off-seasonal use without considerable deterioration in nutrient levels, an appropriate drying method is essential for the removal of moisture to a safe activity value. Low moisture content of products made from dried leaves helps improve their shelf life, reduce shipping weight and minimize the transportation cost. A controlled heat treatment process is employed for the removal of the required water content from the leaves. An optimized drying process is necessary not only for the preservation of leaves to achieve concentrated nutrients, but also to minimize the energy consumption to make it eco-friendly. The optimized process of drying leaves is to ensure desired final moisture content retaining the original high level of nutrients as that of fresh leaves. The selection parameters for the drying technique of individual leaves is based on local climatic conditions, drying air temperature, relative humidity of air, drying time, size, shape and age of leaves, etc. The present review work explores the influencing parameters on water loss in leaves, drying kinetics, various available drying methods, range of operating conditions, and the effect of different drying methods on nutritional properties. This research paper highlights the best fit thin-layer models employed for drying of different leaves. The major challenges faced by the drying industry such as energy conservation while drying, emission reduction and hot spots for possible future research are also reviewed in this paper.