Comparative Evaluation of the Properties of Deep-Frozen Blueberries Dried by Vacuum Infrared Freeze Drying with the Use of CO2 Laser Perforation, Ultrasound, and Freezing-Thawing as Pretreatments

Existence of cuticle wax layer not only reduces the drying rate of blueberries but also causes the fruit to burst during drying. Such a phenomenon results in undesirable appearance as well as in losses of bioactive compounds responsible for health benefits and in sugars adhering to the surface and hence increased moisture adsorption capability of the dried fruit. In this study, uses of CO2 laser perforation, ultrasound and freezing-thawing as skin pretreatments prior to infrared freeze drying and their effects on drying characteristics as well as selected properties, i.e., shrinkage, color, rehydration capacity, as well as total anthocyanins and phenolics contents, of blueberries were investigated. Fourier transform infrared (FTIR) spectra of the dried fruit were also determined. Pretreatments increased drying rate and rehydration capacity of blueberries; shrinkage reduced from around 57 to 25%. Laser perforation and freezing-thawing but not ultrasonic pretreatments exhibited no significant effects on color of blueberries. Laser perforation and ultrasonic pretreatments exerted positive effects on the total phenolic and anthocyanin contents of the dried samples, respectively. FTIR spectra illustrated that all pretreatments did not alter the chemical fingerprints of the dried fruit powder.