## 3 Discussion  
Membrane filtration processes are often used for aroma, colour, sugar or ethanol correction in wine. They are pressure-driven operations that are influenced by transmembrane pressure, temperature, operating time, membrane type, number of used membranes and module arrangement, MWCO, velocity of the feed, membrane fouling, concentration polarization and osmotic pressure that occur on the membrane surface [19; 33]. The higher the pressure, the sooner the membrane fouling and concentration polarization occur, leading to decrease of permeate flux and higher retention of most compounds [13; 34]. Further,  
Figure 3: Principal component analysis (PCA) biplot of aroma profiles of initial wine (W) and retentates obtained by reverse osmosis (R) and nanofiltration (N). Abbreviations: 1–2.5 MPa with cooling; 2–3.5 MPa with cooling; 3–4.5 MPa with cooling; 5–2.5 MPa without cooling; 6–3.5 MPa without cooling; 7–4.5 MPa without cooling; 8–5.5 MPa without cooling. pressure increase leads to higher interaction of water with hydrophilic active layer of the membrane, resulting in higher permeability of water than other compounds [22]. On the other hand, temperature increment (without cooling regime) resulted in a loss of bioactive compounds in retentates due to thermal degradation or decreased viscosity of the feed at higher temperatures [18; 29]. [PERSON] et al. [35] stated that permeate flux during filtration of \(n\)-hexane solution was several times higher than water flux because \(n\)-hexane has three times lower viscosity than water. Reverse osmosis and nanofiltration membranes were permeable for ethanol and its content decreased during red wine concentration in all retentates comparing to the initial wine. Concentrates obtained in this study could be used for production of low alcohol wine. Besides health, there are several reasons for lowering or removing ethanol from wine, like social impacts and alcohol abuse or financial impacts and jurisdiction taxes for excessive alcohol content in wine [36]. Reverse osmosis and nanofiltration proved to be more effective for ethanol removal than thermal processes, such as traditional evaporation process, that results in high thermal degradation of wine components and aroma loss [37]. On the other hand, RO and NF membranes retained sugars and their concentration increased during both processes comparing to the initial wine. This ability could be used for sugar correction in wine or grape juice [14; 25]. Higher pressure and lower temperature resulted in higher retention of most compounds (sugar, SO\({}_{2}\), acids and ethanol), meaning that optimal operating conditions will depend on desired final product. Different transmembrane pressures (2.5, 3.5, 4.5 and 5.5 MPa) and temperature regimes (with and without cooling) did not have the same effect on all volatile compounds. Besides processing parameters, the retention of individual aroma compound depended also on membrane characteristics, molecular weight, volatility and activity coefficient of each compound [38] and on the wine non-volatile matrix [17]. Reverse osmosis membranes have greater ability to retain low molecular weight compounds due to their small pore size and MWCO value. This can cause higher energy consumption and severe membrane fouling than nanofiltration process [18; 39]. However, the results in this study showed that the reverse osmosis and nanofiltration membranes are permeable for acetic acid due to low molecular weight of this compound (60.05 g/mol). Acetic acid is the main component in wine volatile acids group. It is produced during alcoholic or lactic fermentation as secondary product. In small amounts it contributes the wine aroma, but higher concentrations lead to bitter and sour aftertaste or vinegar-like aroma and then it is considered as wine fault [6]. Therefore, the ability of RO and NF membranes to permeate acetic acid could be used for acetic acid correction in wine.