Method to assess the drying performance of water vapour-permeable membrane pouches for fruit juice preservation

The aim of this work was to develop a method for assessing the drying performance of a hygienic, small-scale fruit juice preservation process called Solar Assisted Pervaporation (SAP). SAP is a batch process that allows for fruit juices or purées to be solar dried in water vapour-permeable membrane pouches in rural and remote areas. Traditional methods for measuring water vapour flux through membrane materials are not directly applicable for assessing the performance of SAP pouches, and so a new method was developed, considering a constant boundary condition on the inside of the pouch (i.e. water activity of 1.0). This paper presents a statistical validation of the method and then illustrates how it can be used to characterise membrane performance, identify a two-level factorial regression model for drying flux as a function of temperature, relative humidity and air velocity and their interaction effects, and estimate an operational window for drying juices in SAP pouches in an indirect solar dryer. The repeatability of the method is high with a precision of 95% or more. The regression model and operational window can be used to optimise the design of an indirect solar dryer specifically suited for SAP pouches filled with fruit juices.