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.

Influence of different drying methods on the physicochemical properties of red beetroot (Beta vulgaris L. var. Cylindra).

There is an increased interest in preserving fruits and vegetables by drying. The novelty of this study consists in the combination of the following three drying methods: free convection (at 50, 60, and 70°C), forced convection at 40°C and 315W microwave power. The aim of this study was to investigate the effects of the drying conditions on red beetroot (Beta vulgaris L.) in terms of betalain variance, and polyphenol, microstructure changes (SEM). A strong thermal shock, provided by convection at 60° followed by microwave wattage 315W/9min, leads to a better preservation of bioactive compounds content (0.631±0.0042mg/g of betacyanin and 0.795±0.0019mg/g betaxanthin) when compared to convection at 50, 60 and 70°C. The results showed that combined drying methods led to a significant preservation of the phytochemical content as compared to the traditional methods.

Effect of different drying methods on moisture ratio and rehydration of pumpkin slices.

This study was carried to determine the influence of hot air drying process and combined methods on physicochemical properties of pumpkin (Cucurbita moschata) samples. The experiments in hot air chamber were lead at 50, 60 and 70 °C. The combined method consists of a triple combination of the main drying techniques. Thus, in first stage the samples were dried in hot air convection at 60 °C followed by hot air ventilation at 40 °C simultaneous with microwave. The time required to reduce the moisture content to any given level was highly dependent on the drying conditions. So, the highest value of drying time in hot air has been 540 min at 50 °C, while the lowest time has been 189 min in hot air combined by microwave at 40 °C and a power of 315 W. The samples dried by hot air shows a higher rehydration capacity than samples dried by combined method.