# Removal of Volatile Phenols From Wine Using Crosslinked Cyclodextrin Polymers
## 2 Results and Discussion
### Batch Adsorption of Volatile Phenols by CD Polymers in Wine and Comparison with CD Addition  
Three concentrations of the CD polymers were used in batch adsorption tests, with the percentage of volatile phenols removed shown in Table 2. Not surprisingly, with increasing amounts of polymer addition, the amount of volatile phenol being adsorbed also increased. Following the addition of \(\rm\beta\)-CD-HDI and \(\rm\gamma\)-CD-HDI polymers (5% _w/v_), the residual concentrations of guaiacol, 4-methylguaiacol, 4-ethylguaiacol and 4-ethylphenol were 55% and 54%, 55% and 51%, 43% and 46%, and 23% and 37%, respectively. When comparing these adsorption results with CD addition, it is worth mentioning that the concentrations of CD added were close to the limit of solubility (according to the supplier), which is a limiting factor for CD functionality. Nevertheless, the reduction in volatile phenol levels following addition of CDs was positively correlated with CD concentration, with 23 and 34% removal of guaiacol, 25 and 37% removal of 4-methylguaiacol, 42 and 43% removal of 4-ethylguaiacol, and 68 and 52% removal of 4-ethylphenol following addition of \(\rm\beta\)-CD and \(\rm\gamma\)-CD (at 20 g/L), respectively (Table 3). 4-Ethylphenol was consistently the most readily removed volatile phenol. The ranking of volatile phenols by the efficiency of removal (irrespective of polymer type) was: 4-ethylphenol \(\rm>4\)-ethylguaiacol \(\rm>4\)-methylguaiacol \(\rm>guaiacol\). It has been established that hydrophobicity and size are among the most influential factors in determining the extent of encapsulation of a guest molecule by CD [23; 27]. The log P-value is the logarithm of a compound's partition coefficient, and it is often used to describe the hydrophobicity of a compound, with higher log P values correlating with higher hydrophobicity. As expected, 4-ethylphenol has the highest hydrophobicity of the four volatile phenols studied, followed by 4-ethylguaiacol, with guaiacol being the lowest. When comparing the guaiacol-based volatile phenols, it is clear 4-methylguaiacol and 4-ethylguaiacol have a greater tendency to be encapsulated within the CD cavity due to the higher hydrophobicity granted by the alkyl group. This was apparent by 2D nuclear magnetic resonance (NMR) analysis (Figure 4). The cross-peaks indicate the close spatial correlation between protons. Spectra were cropped to specifically show interactions between the aromatic and alkyl protons of the volatile phenols with the CD cavity. It is obvious that the methyl and ethyl groups facilitate the insertion of 4-methylguaiacol and 4-ethylguaiacol into the CD cavity.  
Polymerisation appeared to enhance the binding capability of the CD cavity, with greater removal of volatile phenols achieved following addition of 5% \(w/v\) of CD polymer (Table 2), compared with addition of 20 g/L of the corresponding CD (Table 3); in the current study, the relative proportion of CD within 5% \(w/v\) of the CD polymer is approximately 20 g/L, taking molar ratios into account. The observed differences in binding affinity likely reflect changes in host-guest inclusion interactions due to modification of the polarity or reactivity of polymerised CD molecules and/or the orientation of encapsulated volatile phenols, as a consequence of cross-linking with HDI.