Investigations on effect of elevated temperatures and reduced relative humidity on drying of grapes

Abstract

Drying of grapes is a complex process involving physical or chemical pre-treatment to create micro-cracks in waxy cuticle on berry skin followed by drying to 15-16% moisture level by different methods. Weather factors prevailing during grape drying particularly temperature and relative humidity play a very important role in determining the quality of raisins. Since last few years, major grape drying areas in India are experiencing untimely rains during grape drying season resulting in the production of inferior quality raisin. To address such recurring problems, modified grape drying chamber containing heavy duty heaters and fans was fabricated at ICAR National Research Centre for Grapes, Pune. Berries of Thompson Seedless and Manik Chaman were dried in single layer in this chamber in March 2017 and April 2018. Average temperature and average relative humidity during the drying duration and drying periods required to reduce moisture level in the berries up to 15-16% moisture level were compared with drying in chamber having ambient conditions. Average temperatures and relative humidity in heated chamber were high and low, respectively than in ambient chamber in both the years. Thompson Seedless required 25% and 28.6% less drying period in 2017 and 2018, respectively. Whereas, Manik Chaman required 37.5% and 42.9% less drying period in 2017 and 2018, respectively. Under ambient drying conditions, both varieties exhibited similar drying periods (8 days in 2017 and 7 days in 2018) in both the years, however, faster drying of Manik Chaman in heated chamber in both the experimental years may perhaps be due to different wax structure on berry skin and its faster degradation under high temperatures in this variety. Drying under heated chamber conditions would be beneficial to grape growers as the grapes are dried at faster rate and are not exposed to unfavorable drying conditions created by the untimely rains for a longer time. The quality of raisins is also expected to be far better due to absence of dust and other debris which are otherwise present in open drying conditions.