## 3 Discussion
The pH of wine (especially pH 3.2) usually favours the acetic acid permeability [40] and around 60% of the initial concentration of acetic acid can permeate through the membrane depending on the processing conditions [25]. This is the reason why total volatile acids content (Tables 1 and 2) was lower in all obtained retentates than in initial wine. The retention of acetic acid decreases with lower transmembrane pressure and higher temperature [25; 40]. It has been reported that acetic acid can be separated from other organic matrices (monosaccharide or xylose) using RO and NF membranes [23; 24; 41]. The content of rest of carboxylic acids in retentates increased during wine concentration, meaning that the RO98 pHt and NF M20 membranes are not permeable for higher acids. Their retention depended on acid chemical properties and operating conditions. Slight loss of these compounds occurred when cooling was not applied and at lower pressures. Reverse osmosis membranes showed greater ability to retain acids than nanofiltration ones. Higher alcohols at concentrations below 300 mg/L contribute to the desirable wine aroma, but if their concentration exceeds 400 mg/L, a negative effect was observed [30]. In this study, during reverse osmosis and nanofiltration processes, higher pressures were suitable for higher alcohol retention, but the temperature increase (without cooling regime) resulted in significantly lower retention of alcohols, depending on their vapour pressure and volatility [17]. In global, higher pressure and lower temperature resulted also in higher retention of carbonyl compounds, terpen and esters, but operating conditions did not affect each compounds equally. [PERSON] et al. [42] stated that the rejection of a compound depends on volatility of the compound and polarity of the membrane. The polyamide membranes showed higher retention of volatile compounds than cellulose acetate membranes. The retention of individual aroma compounds in that study were explained through polarity and hydrophobicity of the membranes. Stronger polar membrane would attract polar organic compounds on the surface and the permeability would be higher. On the other hand, hydrophobic membranes showed greater rejection for high polar organic compounds. For example, hexanol has a hydrophobic character and it is attracted to the hydrophobic part of membrane, which increases its permeability, especially with higher temperatures. This is consistent with our finding, because its concentration in retentates was not significantly higher than in initial wine, and during processes without cooling, its concentration was significantly reduced. Further, a notable loss of several esters was observed during both membrane processes used in this study, such as ethyl hexanoate, ethyl octanoate or ethyl decanoate. A previous study [43] stated that loss of certain esters during wine dealcoholisation could occur mostly due to their hydrophobicity. However, the retention of aroma compounds depends on several other factors, such as wine non-volatile matrix as mentioned before. Polyphenols represent major non-volatile component in red wine and they interact with the aroma compounds through hydrogen bonding, increasing their stability [17; 44]. Regarding volatile phenols, three compounds were monitored. The highest concentration among them was measured for 2,4-Di-T-butylphenol and its concentration increased during both processes, reverse osmosis and nanofiltration. However, increased temperature at without cooling regime resulted in a slight loss of 2,4-Di-T-butylphenol, but its retention increased at higher pressures. Further, 4-ethylphenol and 4-ethylguaiacol are low molecular weight compounds (122.2 g/mol for 4-ethylphenol and 152.2 g/mol for 4-ethylguaiacol) and they permeate through the membranes. The NF membranes showed slightly higher permeability for both compounds than reverse osmosis ones, as expected.