## 3 Results
### Characterization of Permeates and Retentates
#### 3.2.2 FTIR  
In this study, FTIR-ATR was also used to compare samples based on their spectral differences in the 3750-700 cm\({}^{-1}\) spectral region. This method provided an alternative, cost-effective technique for the analysis of food samples (including honey and fruit juice), especially when coupled with chemometrics [39; 40], which makes it possible to discriminate between samples of various composition and concentration [41]. In order to facilitate band characterisation as well as comparison between samples, Figure 3 presents the ATR-FTIR spectra with an indication of the relevant band maxima. In Table 2, the exact position of bands juxtaposed with specific oscillations associated with respective functional groups are presented. Additionally, all the measured spectra were normalised at the wave number of 1050 cm\({}^{-1}\) to facilitate interpretation of the results as well as to emphasise the differences between the same. All the measured spectra revealed, within the analysed range, relatively strong oscillations, which to a large extent made their analysis feasible and allowed respective oscillations to be associated with specific functional groups, which in turn corresponded to changes in particular substances. Respectively, the bands with maxima at approx. 1250 cm\({}^{-1}\) correspond to stretching oscillations of the C-O group in carboxylic acids. Bands with maxima at approx. 1338 cm\({}^{-1}\) correspond to the oscillations of deformation \(\delta\) (-OH) groups in the C-OH group [41]. Deformation C-H oscillations also correspond to bands with maxima at approx. 915 cm\({}^{-1}\), which can also originate from acids. The aforementioned bands reveal only slight differences after application of UF and DF factors, naturally with the exclusion of the RUF sample for which the level of oscillation of the band with the maximum at 915 cm\({}^{-1}\) was significantly higher. In the case of bands with the maxim at approx. 1338 and 1247 cm\({}^{-1}\) respectively, a significantly higher level of oscillation was observed in samples subjected to the UF process, which evidenced the higher level of the aforementioned factors in those samples. A characteristic area corresponding to the stretching oscillations of -OH groups is the broad band with the maximum at approx. 3330-3350 cm\({}^{-1}\)[42]. The bands evidence the presence of small amounts of water molecules in the samples, but above all can originate from group carbohydrates and organic acids. Another characteristic area present in all the analysed samples contains bands corresponding to symmetrical and asymmetrical oscillations of -CH2 and -CH3 groups in the region of 2850 to 2950 cm\({}^{-1}\). The lowest intensity of these bands was observed in the DF samples. It should be emphasised that the reason for the significant broadening of the band corresponding to the stretching oscillation of the OH group stems from the strength of the hydrogen bonds present in dimers of carboxylic acids [41]. Carboxylic acids are characterised by a somewhat irregular character of absorption in the 3300-2500 cm\({}^{-1}\) area, with the broad O-H band overlapping with the C-H stretching oscillation bands. Oscillations of the -OH group belonging to various acidic fractions also correspond to deformation oscillations of the group in the region with the maximum at approx. 1640 cm\({}^{-1}\)[43; 44]. A region of very intensive bands within the range from 1060 to 930 cm\({}^{-1}\) can be associated with the C-O stretching oscillations in the C-OH group or C-C stretching in the carbohydrate structure. In turn, bands with the maxima at approx. 918 cm\({}^{-1}\) are the bands of the C-H deformation oscillations (as mentioned above). Another interesting region is very characteristic in terms of the assessment and characteristics of samples such as various types of juice; bands in the range from 890 to 760 cm\({}^{-1}\) (in our case with the maxima at approx.