Solar thermal drum drying performance of prune and tomato pomaces

Authors: Milczarek, Rebecca; FERRY, JONATHAN - University Of California; Alleyne, Fatima; Olsen, Carl; Olson, Donald; WINSTON, ROLAND - University Of California

Submitted to: Food and Bioproducts Processing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2017
Publication Date: 9/4/2017
Citation: Milczarek, R.R., Ferry, J., Alleyne, F.S., Olsen, C.W., Olson, D.A., Winston, R. 2017. Solar thermal drum drying performance of prune and tomato pomaces. Food and Bioproducts Processing. 106:53-64.

Interpretive Summary: When fruits and vegetables are peeled or processed into juice, processors are left with a damp co-product known as pomace. The pomace consists of the peels, seeds, and other insoluble parts of the fruit or vegetable. Pomace contains many nutrients and fiber, but it is often used for low-value applications such as fertilizer or animal feed because it is unstable (will get moldy quickly at ambient conditions). So, fruit and vegetable processors would benefit from an environmentally-friendly way to quickly dry and stabilize pomace so it could be put to more valuable uses and transported more easily. Drum drying is a well-established drying technology, but current drum drying equipment utilizes conventional, high-environmental-impact heating mechanisms (such steam created from boilers running on fossil fuels). In this project, we demonstrated that a drum dryer heated by solar thermal energy could effectively dry tomato and prune pomaces. We tried different combinations of pomace blend ingredients (water and a starch-like drying aid) and processing conditions (drum surface temperature and drying time) to find the best combination of these variables that would dry the pomace completely and minimize the change in color. Thus, this study demonstrated that prune pomace and tomato pomace can be dried on a drum dryer run by solar heating and established the best formulation and processing conditions drying each of these materials.

Technical Abstract: 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 x 15 cm diameter) was interfaced with a 106.6 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.