Solar thermal drum drying performance of prune and tomato pomaces

Fruit and vegetable pomaces are co-products of the food processing industry; they are underutilized in part because their high water activity (aw) renders them unstable. Drum drying is one method that can dry/stabilize pomaces, but current drum drying methods utilize conventional, high-environmental-impact heating mechanisms. In this work, a small-scale double drum dryer (20 cm length × 15 cm diameter) was interfaced with a 98.3 m2 External Compound Parabolic Concentrator (XCPC) [solar thermal collector] array designed to produce up to approximately 40 kW of heating power. The conditions for drying prune and tomato pomaces were optimized on this system via a split-plot design. The design had 4 variables: added water, added maltodextrin carrier, dwell time, and drum surface temperature. Moisture content, aw, and color of the dried pomaces were assessed to determine the effectiveness of the drying. Both pomaces were rendered shelf-stable (aw < 0.6) for all tested conditions. However, prune pomace exhibited a narrower range of aw values than did tomato pomace. Conditions for adequate drying with minimal color change (and thus expected minimal nutrition loss) were established. This work demonstrates the potential for solar thermal energy to provide the heat for drum drying fruit and vegetable pomaces.