Which of the following is usually the longest part of drying operation

Phases of drying 1. Initial warm up period 2. Constant drying rate period 3. Falling drying rate period In hygroscopic food material more than one falling rate period occurs. In the first period plane of evaporation moves inside the food and water diffuses through the dry solids to the drying air. It ends when plane of evaporation reaches to the centre of food and the partial pressure of water falls below the saturated water vapour pressure. Second falling rate period occurs when the partial pressure of water is below the saturated vapour pressure and the drying is by desorption. Falling rate period is the longest period during drying of food product. Equilibrium moisture content (EMC) occurs when dry spots develop at the surface so less area exposed to dry air and evaporation decreases.