Study of three-stage intermittent drying of pears considering shrinkage and variable diffusion coefficient

An intermittent drying process of whole pears of Rocha variety was performed in order to reproduce the traditional sun drying of the Portuguese São Bartolomeu regional variety. This traditional drying originates a much appreciated product. The results show that the "Rocha" pear presents a similar kinetic behaviour with diffusion coefficients ranging from 2.5 × 10-9 m2.s-1 to 6.0 × 10-11 m2.s-1, at 40 °C, and from 2.4 × 10-9 m2.s-1 to 1.9 × 10-10 at 50°C, considering shrinkage. A total time reduction of 28.7% was achieved when drying at 50°C instead of 40°C, thus concluding that the air temperature has a much greater influence on the drying kinetics than the airflow velocity. The intermittency of the process contributed to decrease the total time of convective drying, achieving a reduction of 26.2% when using three 14 h pauses in the set of tests performed at 50°C with an airflow velocity of 2.7 m.s-1. It was also verified that shorter pause periods are disadvantageous and lead to longer drying time, because the homogenization of the radial moisture distribution is incomplete before starting the next convective drying phase. The methodology used in this study can be an alternative to traditional drying and be suitable to an industrial context, applying a correct strategy for the duration and number of pauses, with gains in productivity and energy savings.