Drying Characteristics of Onion-A Review

Onion drying an important post harvest process to extend shelf life of harvested onion, and thereby improve bargaining position of the farmer to maintain relatively constant price of his product. Drying reduces post harvest losses and transportation costs since most of the water are taken out from the product during the drying process. In India, sun drying is the most commonly used method to dry the agricultural materials like grains, fruits and vegetables. In sun drying, the onion is spread in a thin layer on the ground and exposed directly to solar radiation and other ambient conditions. The rate of drying depends on various parameters such as solar radiation, ambient temperature, wind velocity, relative humidity, initial moisture content, absorptivity and mass of product per unit exposed area. Direct sun drying has been a popular method of crop drying in the farms and villages. Sun drying allows for the production of a product rich in colour and translucent in appearance, but it has some disadvantages (e.g. it is a time-consuming process, weather depended, labour demanded and it is greatly exposed to possible environmental contamination. Solar driers using natural convection or forced circulation have been investigated to overcome these problems. For commercial applications, the ability of the drier to process continuously throughout the day is very important to dry the onion to its safe storage level and to maintain the quality.