Two-stage drying of tomato based on physical parameter kinetics: operative and qualitative optimization

The industrial drying of tomatoes using hot air is a widespread practice. However, air drying has several issues in terms of time, energy consumption and qualitative characteristics of the final product. The study aims to improve drying through the monitoring of the tomato’s physical characteristics. The study was divided into two parts. In the first part, the kinetics of moisture, color, volume, and rehydration ratio during the drying of tomatoes were modeled according to the temperature used (50, 60, 70, and 80 °C). Moisture loss kinetics has an initial zero-order, followed by first-order kinetics once the critical moisture is exceeded. For color, a first constant trend is observed followed by a steadily increasing or decreasing variation according to the color considered (i.e., red, or brown). The use of low temperatures guarantees the least changes of physical characteristics but significantly increases the time (+ 189%). Conversely, high temperatures guarantee faster drying but cause greater tomato browning (+ 200%). During the drying, the volume decreased linearly according to the considered temperature. In the second part of the work, a tentative drying optimization was performed, based on color kinetics. A two-stage process using two temperatures was developed to reduce drying times without affecting the product quality. Also, two control tests were conducted at 50 °C and 70 °C constant temperatures. No significant quality differences were observed between tomatoes dried at 50 °C and those using the two-stage process (p < 0.05). This type of approach did not produce a qualitative deterioration of the final product and at the same time allowed a significant reduction (− 27.6%) in time.