The objective of this paper was to determine the stability of foamed concentrated apple juice and to identify the effect of air temperature and foam composition on the kinetics of drying process. A clarified apple juice (21Brix) was foamed with 1 % methylcellulose or with methylcellulose (1 %) plus maltodextrin (5 %) added. The foamed juice was dried using convection, and forming a 4 mm layer at 60, 70, and 80°C. Furthermore, a non-foamed apple juice was dried at a temperature of 60 °C. The density of foams and their stability were determined based on the kinetics of drainage. As opposed to the non-foamed concentrated apple juice, the application of foam drying makes it possible to produce a dry, porous material during a drying period lasting up to 70 minutes. The drainage of foams with methylcellulose and maltodextrin was lower compared to foams produced only with the addition of methylcellulose, and this fact proved a higher stability of foams with methylcellulose and maltodextrin. The drying rate of foamed juices significantly depended on the temperature of drying air. The increased temperature from 60 to 80 °C resulted in a doubled increase in the drying rate with 1.0 kg/kg dm water content. The material darkened while being dried at 80 °C, thus, the temperature of 70 °C was applied and the juice was foamed with methylcellulose and maltodextrin; with this measure used, it was possible to obtain a better quality powder during a shorter drying period.
foam drying, drying kinetics, stability of foams