Characteristics of selected functional properties of apple powders obtained by the foam-mat drying method

Foam-mat drying is an alternative method which enables the removal of water from high-sugar content juices and purees that are difficult to dry. The aim of this study was to investigate the effect of different foam-mat drying methods on some physical properties and aroma of apple puree powder. Apple puree was foamed with the addition of foaming agents. Foamed puree was dried as a layer using different techniques: convection drying, microwave drying, and freeze-drying. Also foamed puree with and without maltodextrin (6% or 15% w/w) was dried. Selected properties such as hygroscopicity, wettability, solubility, colour, water content, water activity, bulk density and the changes in retention of aroma compounds of apple powder were determined. The drying method affected the physical properties of apple powders. Foaming prior to drying allowed us to obtain less hygroscopic powders with low moisture content and water activity. The freeze-dried samples were characterized by lower wettability and solubility than powders obtained by air (convection) drying and microwave drying. The increase of maltodextrin addition from 6 to 15% resulted in a decrease of hygroscopicity, bulk and tapped density, and flowability of apple juice powders were also improved. The addition of maltodextrin to apple puree reduced the wetting time of freeze-dried powders but only high concentration of the carrier significantly improved wettability of powders The convection drying of foamed apple puree enabled us to obtain instant powder with higher retention of apple aroma.