Solar Assisted Pervaporation (SAP) : A process using membrane pouches and solar energy for the dehydration and preservation of fruit juices in rural and remote areas

Abstract: Drying has been used for thousands of years to preserve foods. One of the first methods used was open air sun drying which exposes foods directly to solar radiation and ambient air. This method is still used today around the world but it remains underdeveloped on the small-scale for two main reasons. The first is that it can be unhygienic since the food is easily contaminated by pests, dust or microorganisms in the air or surroundings. The second is that it is difficult to control due to changing weather patterns and unpredictable cloud cover. Solar dryers are one alternative as they provide some protection from larger pests in the surroundings and can be used to gain more control over the process. A contamination risk still remains, however, unless an air filter is used to remove microorganisms from the air before it contacts the products.The overall aim of this research is to develop a safe and practical fruit preservation technology that is suitable for rural and remote areas of developing countries. The technology under development is termed Solar Assisted Pervaporation (SAP) and involves solar drying of membrane pouches filled with fruit juices to create shelf-stable fruit concentrates. The membrane pouches are permeable to water vapour but not liquid water or other nutrients. The hygienic membrane layer of the pouch also prevents the product from contamination during drying because it is impermeable to microorganisms. The technology is meant for small-scale use and could be especially suitable for rural and remote areas of developing countries where infrastructure is limited and fruit spoilage is high.This licentiate thesis has focused on assessing the feasibility of the process under realistic drying conditions, evaluating the possible effects of internal mass transport on food safety, and investigating the transport phenomena that describe the system. SAP involves complex heat and mass transfer where the principles of drying are combined with membrane transport theory. The rate limiting step for the evaporation process can be one of three mechanisms: the internal diffusion mass transport of water from the center to the inner surface of the membrane, the permeation of water through the membrane or the external transport of water from the outer surface of the membrane to the air. Due to the complexity of the system, certain mechanisms were studied in isolation. Internal mass transport was investigated from a food safety perspective. External mass transport and the permeation through the membrane were studied together using water as a model substance in order to identify the rate limiting step that would be expected during the constant rate drying period of a fruit juice/purée drying process. In terms of process feasibility, the findings indicate that reasonable drying times can be achieved (i.e. 2 to 3 days assuming only 8 hours of active solar drying per day) with realistic drying conditions. When testing was performed with a solar simulating lamp, there was a negative effect on the drying flux when ambient air under forced convection was applied. This was due to the cooler air reducing the amount of energy available for the latent heat of evaporation. Drying fluxes in an indirect solar dryer simulator at 40.9°C, 13.9% relative humidity and 51.6°C, 7.5% relative humidity were comparable to the fluxes obtained at 620 W/m2 without forced air convection. This motivated the choice to further investigate the SAP pouch transport phenomena in an indirect solar dryer since drying fluxes are comparable and photo-oxidation of the product no longer plays a role. With regards to food safety, it was found that viscous, fibrous and starchy fruit purées are more likely to dry inhomogeneously compared to fruit juices. The inhomogeneous drying may result in local wet spots and/or crust formation which could pose as a food safety risk. The internal mass transport was studied with apple purée and found to be diffusive. This supports the local wet spot observation since a lack of internal mixing would prevent re-distribution of the wet spots. The investigation into the limiting transport phenomena showed that for pouches dried in an indirect natural convection solar dryer simulator (within the following operational limits: 20.2°C to 53.9°C, 17% to 38% relative humidity, 0.15 to 0.39 m/s air velocity), the evaporation flux in the constant rate drying period was limited by the membrane resistance rather than the convective heat transfer from the air to the surface. Air velocity had a slight but statistically significant effect on the apparent convective mass transfer coefficient, which suggests that a boundary layer exists at these low air velocities and that the thickness of this boundary layer can be reduced by increasing the velocity of the air passing over the pouches. Temperature did not have any noticeable or statistically significant effect on the apparent convective mass transfer coefficient for the membrane-bulk transport. These findings have implications on the design of an indirect solar dryer as they show that the two inputs that should be controllable are the air velocity and the water vapour partial pressure difference between the inner surface of the membrane and the bulk air (Δp).The findings above suggest that when drying a food matrix, such as a fruit juice/purée, in a SAP pouch, it may be possible to control the onset of crust formation and reduce uneven drying by controlling Δp and air velocity in an indirect solar dryer. This hypothesis will be tested in the second half of the doctoral studies.Solar Assisted Pervaporation challenges the traditional view on open sun drying, and has the potential to help people in rural and remote areas add value to fruits that would otherwise spoil, and does so in an environmentally sustainable way.