Solar Assisted Pervaporation : A process for the concentration of fruit juices in membrane pouches with solar energy
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 a 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. However, there is still a risk of contamination since microorganisms in the air can be transferred to the product during the drying process. This means there is a need for improved solar drying practices that can prevent contamination from the air during drying, especially if the weather conditions are poor or drying is to continue over many days and nights.The overall aim of this research was to develop and evaluate a safe and practical fruit juice concentration process that is suitable for rural and remote areas of tropical countries, where solar energy is abundant and fruit spoilage rates are high. The process has been termed Solar Assisted Pervaporation (SAP) and involves the concentration of fruit juices in membrane pouches with solar energy to create shelf-stable fruit juice concentrates or marmalades. 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 process is meant for small-scale use and is especially suitable for rural and remote areas of low-income countries where infrastructure is limited.This doctoral thesis has focused on evaluating the SAP process under controlled laboratory conditions and under realistic, outdoor conditions in Mozambique. In terms of process feasibility, the findings indicate that it takes 3 to 6 days to produce a marmalade with a soluble solids content of at least 65 degrees Brix when membrane pouches are solar dried in an outdoor, tropical environment. With regards to food safety, it was found with neutron imaging that viscous, fibrous and starchy fruit purées dry inhomogeneously compared to fruit juices. Local regions of high moisture were also observed in the purées, which means there is an increased probability of microbial growth and spoilage in these regions. The mass transport in the purées was also found to be by diffusion. The results of the field study show that by using a solar dryer, the total drying time can be reduced by 50%. The field study results are in-line with the results obtained under controlled laboratory conditions using a climate chamber, where it was shown that increasing the air velocity had a positive effect on drying flux. A resistance-in-series model was used to model the overall mass transfer resistance and assess how changing internal conditions (i.e. viscosity changes) affect the SAP process. In terms of the microbiological quality, active and passive solar drying were found to significantly reduce the probability of fermentation by yeast during drying. Pasteurisation as a juice pre-treatment was found to significantly reduce the total aerobic count, whereas solar drying setup had a negligible effect on the aerobic count. For all samples tested in Mozambique, mold, lactic acid bacteria and Enterobacteriaceae were not a concern. Mold was visible on the exteriors of some pouches yet not detected in the final samples, which indicates that the pouches provided a hygienic barrier during drying. The recommended SAP process is to add sucrose and pasteurise the tangerine juice before adding it to the pouches and then use an active solar dryer to reduce the drying time and the probability of spoilage and fermentation during drying. Pasteurisation and hot filling into sterilised jars is recommended after drying, if the marmalades are to be sold commercially.
CLICK HERE TO DOWNLOAD THE WHOLE DISSERTATION. (in PDF format)