Ultrafiltration to Increase the Consistency of Fruit Pulps: The Role of Permeate Flux

Ultrafiltration is a well-known operation, widely used in food processing, especially to concentrate selectively liquid compounds. However, so far, it has been mainly used to change concentration and/or clarify liquids with low viscosity. Ultrafiltration has seldomly been applied to viscous fluids. In this study, it was used to increase the consistency of fruit pulps, without changing their taste and organoleptic properties. This paper reports the findings achieved in experimental runs carried out on a pilot plant, equipped with four ultrafiltration tubular membranes (total surface area = 0.8 m2). Raw fruit pulps, namely, apple, apricot, and pear, were used to study the influence of the operative parameters on the permeate flux and organoleptic properties of the final products (retentate and permeate). The flow rate was in the range of 3.0–5.1 m3/h, at 50 °C. The influence of temperature on the permeate flux was checked, with one run with apple pulp at 20 °C. As expected, the findings show that high flow rate and temperature improve the permeate flux. Membranes show different performance in permeate flux for the tested pulps. This is probably due to their different chemical and physical composition, which could be responsible for different fouling of the membrane and, as a consequence, a different resistance to the permeate flow. The final products have the same taste as the raw ones, and each of them can be used as it is or as an ingredient. These results have a technological relevance, and, besides, the study shows a methodology for future applications of ultrafiltration.