Control of citrus surface drying by image analysis of infrared thermography

Citrus surface drying is one of the most important unit operations in a fresh fruit processing plant. A common risk in citrus surface driers (CSD) is using an excessive air temperature or keeping the fruit in the drier too long, since an important loss in sensorial quality may occur as well as a decrease in the fresh fruit shelf life. Nowadays, most of the new CSD use systems to control air temperature, but do not include elements to monitor the process by defining the required drying time. A new system to control the surface drying time by image analysis of the fruit surface temperature distribution, using infrared techniques was tested. The control of fruit surface temperature during drying allowed us to determine the moment when the surface drying finishes and the peel drying begins. Oranges (var. Valencia Late) washed with water or covered with a commercial wax were dried at 20, 25 and 35 °C with 1, 1.5 and 2 m/s air velocities. During the first drying step the lowest surface temperature of the fruit was measured from infrared images captured with an AGEMA 470 camera and was assumed to be the wet bulb temperature. Drying time could be well established when temperature at any point on the fruit surface exceeded this value. An empirical model was developed to correlate drying times with air conditions. Parameters of the model may be used in control systems for industrial CSD equipment.