Clarification and the concentration of apple juice using membrane processes: A comparative quality assessment

The aim of this study was to evaluate the potential of integrated membrane processes for the clarification and the concentration of apple juice, taking into account the impact on the product quality. The fresh apple juice, with a total soluble solids (TSS) content of about 12°Brix was previously clarified by combined application of fining agents (gelatin and bentonite) and ultrafiltration (UF) through 10 kDa or 100 kDa molecular weight cut-off (MWCO) membranes on laboratory scale. The clarified juice was then concentrated by osmotic distillation (OD), membrane distillation (MD), coupled operation of OD and MD or by conventional thermal evaporation up to 65°Brix. The effect of different clarification and concentration processes on formation of 5-hydroxymethylfurfural (HMF), retention of bioactive compounds (phenolic compounds, organic acids, glucose, fructose and sucrose) and their efficiency in preserving natural color and aroma (trans-2-hexenal, the most relevant compound in apple juice aroma) were evaluated in order to maintain a high quality product. The new membrane-based concentration techniques were very efficient since the product characteristics were very similar to that of the initial apple juice especially regarding the retention of bright natural color and pleasant aroma, which are significantly lost during thermal evaporation. Furthermore, among all the concentration treatments applied, only thermally evaporated samples resulted formation of HMF. Phenolic compounds, organic acids and sugars were very stable against all concentration processes, including thermal evaporation. Coupled operation of OD and MD reduced trans-2-hexenal losses drastically tending towards that of the initial juice and hence can be proposed as the most promising alternative to conventional thermal evaporation technique.