Evolution of physicochemical properties of pear during drying by conventional techniques, portable-NMR, and modelling

The knowledge of changes in the properties of foods that occur with processing is needed for designing better drying methods that preserve desirable characteristics and minimize/eliminate undesirable ones. To this aim, this study analysed the effect of convective drying at 45–55 °C on physicochemical properties of pear. The drying kinetics, shrinkage and SEM images were evaluated. Portable-NMR was used to determine the drying moisture profile of pears and thickness reduction. Portable-NMR also allowed to investigate water mobility in fresh and dried pears by measuring the longitudinal and transverse relaxation times, and the self-diffusion coefficient. Results showed that at 55 °C the faster drying rate and corresponding energy saving were combined with a more uniform water distribution in pear tissues. Finally, a diffusion model considering the shrinkage during drying was developed. It assumes that all water in pear is free-water, according to NMR result that most of the water lost was from vacuole.