Development of several hybrid drying methods used to obtain red beetroot powder.

Reduction of bioactive compounds sensitive to heat, light and oxygen could be attributed to drying. The effectiveness of microwave pretreatment at 525 or 420 W and the ohmic heating at 17.5 V/cm over the properties of beetroot powder were compared. This study aimed to develop novel hybrid drying methods. The effects of microwave and ohmic heating pre-treatment and subsequent microwave-convection drying were studied. Several investigations were performed, such as phenolic content, antioxidant activity, betalains content and color. The samples pretreated by microwave have registered higher polyphenols content and antioxidant capacity than those ohmically heated. The ohmic heating pretreated samples registered highest values for ΔL = 7.33-7.65 and ΔH = 8.18-10.02. Confocal microscopy reveals that the MW heating favors cellular lysis and cell content agglutinates. In conclusion, even if the MW preheating has certain disadvantages, in some cases it provides increased bioavailability generated by the vegetal cell disruption and the release of the bioactive compounds.