Influence of Osmodehydration Pretreatment and Combined Drying Method on the Bioactive Potential of Sour Cherry Fruits

The aim of this study was to determine the effect of osmotic dehydration (OD) time in apple concentrate on dehydration kinetics and quality parameters (polyphenol content and antioxidant activity) of sour cherries before further processing. Additionally, the study was focused on the effects of combined drying (convective drying + vacuum-microwave finish drying (CD-VMFD)) applied to fresh and osmodehydrated sour cherries in respect to drying kinetics, including the temperature evolution of dried material with time, as well as on quality factors of the finished product. Analyses were conducted for four kinds of sour cherries: frozen with stone (FS), frozen without stone (FW), thawed with stone (TS), and thawed without stone (TW), which were osmodehydrated for 180 min in the same operating conditions. The study revealed that the decrease of water loss to solid gain (WL/SG) ratio during OD of frozen sour cherries in apple concentrate associated with increased values of WL/SG was considerably enhanced by stone removal and the process of thawing. A logarithmic model was proposed for overall description of drying kinetics of combined drying. While OD time was increasing, the drying constant was decreasing for CD and obtained the maximum value for VMFD at the optimal time of OD, which was associated with achieving the peak temperature of VMFD material. It was found that the best variant of pretreatment of sour cherries, to be used for OD in apple concentrate, was freezing the fruits without stones. These fruits were characterized by a high content of polyphenols, high antioxidant activity, and the largest WL/SG ratio during OD process. It was also stated that the optimal method for sour cherry drying was the combined method consisting of OD (120 min) followed by CD (90 min) and VMFD, while maintaining the initial parameters.