Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.
The purpose of this work is to develop and implement a simple and robust preservation technology that can enhance food and economic security and thereby improve the lives of people in developing countries. The technique under development is termed “solar assisted pervaporation (SAP)”. It is a sustainable technology whereby fruit juices or purées are concentrated using reusable pouches made of “breathable textiles”. The active layer of the textile is permeable to water vapour but not liquid water, and the process is related to the techniques pervaporation and membrane distillation. The pouches are filled with the juice/purée, hung from a tree or placed on a roof or in a solar dryer, and then exposed to sun and air for a specified period of time. A storage stable concentrate can be produced within 10 to 45 hours (depending on the conditions) and the entire concentration process is done using only solar energy. The fruit juice concentrate can be stored in the household or sold, thereby increasing both food and economic security in local communities.
To demonstrate the feasibility of this technique, three different fruits/juices were concentrated by placing prototype pouches in a convective dryer to simulate a ventilated solar collector, or by exposing the pouches to a solar lamp equipped with convective air circulation to simulate ambient drying with direct sunlight exposure. Drying curves were obtained experimentally and the concentrates were analysed in terms of moisture content, water activity and degrees Brix. Depending on the drying time and type of fruit, the resulting product can be syrup-like, pasty, or have the texture of fruit leather. Under certain drying conditions, the water removal rate is actually higher for the pouch versus an open dish of the same dimensions because the pouch has two sides for mass transfer. Horizontal drying was found to be more effective than vertical drying and dissolved solids content was found to affect drying rate. Initial tests with a solar lamp have shown that drying rate increases with increasing radiant energy flux. It was also found that composition (fiber vs. sugar content) and form (purée vs. juice) of the fruit determines its compatibility with the pouch and drying technique. The results indicate that the technique is feasible and have given insight into how the bag should be designed.