The purpose of this study was to produce red beetroot pulp powder using foam-mat drying technique and to investigate the effect of foam thickness on drying kinetics and physicochemical and microstructural properties of the resulting powder. To prepare foam, ovalbumin as foaming agent at a concentration of 2% (w / v) was added to the red beetroot pulp and was stirred for a specific time and speed. Prepared foam was spread on aluminum plates at thicknesses of 5, 6 and 7 mm and then dried in a hot air cabinet dryer at 50 ºC and constant air velocity of 1 m/s. Results showed that increasing foam thickness from 5 to 7 mm had no significant effect on moisture content and moisture ratio but significantly (p <0.05) reduced the drying rate by about 29.63%. The effective moisture diffusion coefficient was significantly affected by the foam thickness and ranged between 5.550 × 10−9 and 7.388 × 10−9 m2/s. Also, with increasing foam thickness, due to increasing drying time and more denaturation of proteins, an increase in Carr index and Hausner ratio was observed, resulting in a decreasing trend in the flowability of the powders. Bulk and tapped densities of powders were significantly decreased due to the change in moisture content, caused by the increase in foam thickness. Thickness had no significant effect on water solubility index and water absorption index. The microstructure analysis of the produced powders by field emission scanning electron microscope showed that with increasing foam thickness, cracking and surface roughness of powder particles increased. The results of this study and further studies can lead to optimization of red beetroot powder production as a natural food coloring and better preservation of its nutritional and functional properties.
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