Partial removal of sugar from apple juice by nanofiltration and discontinuous diafiltration

Partial removal of sugars in fruit juices without compromising their bio-functional properties represents a high technological challenge. The current study was aimed at evaluating the separa-tion of sugars from phenolic compounds in apple juice by using three different spiral-wound nanofiltration (NF) membranes with molecular weight cut-off (MWCO) in the range of 200-500 Da. A combination of diafiltration and batch concentration processes was investigated to produce apple juice with reduced sugar content and improved health properties thanks to the preserva-tion and concentration of phenolic compounds. For all selected membranes permeate flux and recovery rate of glucose, fructose, and phenolic compounds, in both diafiltration and concentra-tion processes, were evaluated. The concentration factor of target compounds as a function of the volume reduction factor (VRF) as well as the amount of adsorbed compounds on the membrane surface from mass balance analysis were also evaluated. Among the investigated membranes a thin-film composite membrane with a MWCO of 200-300 Da provided the best results in terms of preservation of phenolic compounds in the selected operating conditions. More than 70% of phe-nolic compounds were recovered in the retentate stream while the content of sugars was reduced of about 60%.