## 2 Results
### Aroma Compounds Retention
The content of ethyl octanoate, methyl palmitate and ethyl linoleate did not significantly change (\(p<0.05\)) during pressure increase in NF retentates obtained without cooling. Comparing two types of membranes, RO membranes showed higher ability to retain esters than NF ones. The highest total concentration of esters was found in RO retentate obtained at 5.5 MPa with cooling (4.31 mg/L). At those operating conditions during NF process, the total concentration of esters was 2.59 mg/L, that was still higher than the concentration in initial wine (1.83 mg/L). Tables 4 and 5 also present the content of volatile phenols. One of them, 2,4-Di-T-butylphenol, had a high concentration in initial wine, 1.11 mg/L, and it increased with the pressure increment during membrane filtration. At both temperature regimes at 2.5 MPa, nanofiltration retentates contained slightly higher concentrations of this phenol than reverse osmosis ones. Higher pressures were more favourable for reverse osmosis and the highest concentrations were measured at 5.5 MPa with cooling (1.71 mg/L). Temperature increase (without cooling processes) resulted in lower concentration of 2,4-Di-T-butylphenol in RO and NF retentates, comparing to the ones obtained with cooling. For 4-ethylphenol and 4-ethylguaiacol, higher pressure and lower temperature resulted also in higher retention in RO and NF retentates. The concentrations of 4-ethylphenol and 4-ethylguaiacol in the initial wine were 624.8 and 20.9 \(\mathrm{\SIUnitSymbolMicro}\)/L, respectively, but after concentration processes their content was significantly lower, with the exception of RO retentate at 5.5 MPa with cooling, where the retention of 4-ethylphenol was high (647.2 \(\mathrm{\SIUnitSymbolMicro}\)/L). Both processes resulted in removing 4-ethylguaiacol from retentates, except at 5.5 MPa with cooling where 14.3 and 12.9 \(\mathrm{\SIUnitSymbolMicro}\)/L was found in RO and NF retentates, respectively. In this study, all aroma compounds identified in initial wine and retentates obtained by reverse osmosis and nanofiltration processes at different operating conditions were divided according to their main flavour note. Odour descriptions have been presented in Table 3. There were eight groups of different flavour notes: fatty, green, floral, citrus, fruity, smoky, faint and other. The last group included acetic acid (vinegar aroma), 1-butanol (fusel oil aroma), methionol (sulphhorous note), ethyl 4-hydroxybutanoate (caramellic aroma) and ethyl pentadecanoate (honey aroma). For each sample, total sum of concentrations in each group was calculated and principal component analysis was made. Principal component analysis (PCA) is a multivariate statistical analysis method for representing complex and large dataset with reduced dimensionality, increased interpretability and minimized data loss [32]. In Figure 3, it can be observed that the principal component 1 (PC1), accounting 76.76% of total variance, separated the samples according to the operating condition used during filtration (membrane type, pressure and temperature regime). All NF retentates (except the one obtained at 5.5 MPa with cooling), RO retentates at 2.5 and 3.5 MPa without cooling and initial wine are located on the negative side of PC1. Principal component 2 (PC2; 12.07% of total variance) contributed to the separation of samples according to the dominating flavour note, from citrus to smoky. On the positive side of PC2, all NF retentate could be found. Initial wine and RO retentates are located on negative side of PC2, except for RO retentates obtained at 4.5 MPa at both temperature regimes. It can be observed that NF retentates had similar aroma profile that differed from the RO retentates and initial wine flavour composition. Slight difference can be observed in NF retentates at 4.5 and 5.5 MPa with cooling with more intense citrus note than the rest of NF retentates. Aroma profile of RO retentates depended on applied pressure and temperature.