In recent years, the growth of the functional foods industry has increased research into new compounds with high added value for use in the fortification of traditional products. One of the most promising functional food groups is those enriched in antioxidant compounds of a lipophilic nature. In spite of the numerous advantages reported for such antioxidant molecules, they may also have disadvantages that impede their use in functional foods, although these problems may well avoided by the use of encapsulant agents such as cyclodextrins. This explains the recent increase in the number of research papers dealing with the complexation of different guest molecules possesing important antioxidant properties using natural and modified cyclodextrins. This paper presents a review of the most recent studies on the complexes formed between several important types of antioxidant compounds and cyclodextrins, focusing on the contradictory data reported in the literature concerning to the antioxidant activity of the host/guest molecule complexes, the different complexation constants reported for identical complexes, the bioavailability of the antioxidant compound in the presence of cyclodextrins and recommendation concerning the use of natural or modified cyclodextrins. Moreover, the use of cyclodextrins as antibrowning agents to prevent enzymatic browning in different foods is revised. Finally, we look at studies which suggest that cyclodextrins act as ''secondary antioxidants," enhancing the ability of traditional antioxidants to prevent enzymatic browning.

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In this paper, the interaction between pterostilbene and cyclodextrins (CDs) is described for the first time using steady-state fluorescence. It was seen that pterostilbene forms a 1:1 complex with all of the natural (alpha-, beta-, and gamma-CDs) and modified (HP-beta-CD, methyl-beta-CD, and ethyl-beta-CD) CDs tested. Among natural CDs, the interaction of pterostilbene with beta-CD was the most efficient. However, all of the modified CDs showed higher complexation constants (K(F)) than beta-CD. The highest K(F) was found for HP-beta-CD (17520 +/- 981 M(-1)), in which its value showed a strong dependence upon pH in the region where the pterostilbene begins the deprotonation of its hydroxyl group. Moreover, the values of K(F) decreased as the system temperature increased. To obtain information on the mechanism of pterostilbene affinity for CD, the thermodynamic parameters of the complexation (DeltaH degrees , DeltaS degrees , and DeltaG degrees ) were studied. Finally, a comparison of the K(F) values obtained for three types of stilbenes revealed that both the stoichiometry and the K(F) values of the complex are dependent upon the structure of the guest molecule. While the trans-resveratrol-HP-beta-CD and pterostilbene-HP-beta-CD complexes showed a 1:1 stoichiometry with a higher K(F) value for the trans-resveratrol-HP-beta-CD complexes, trans-stilbene showed a 1:2 stoichiometry.

The complexation of pinosylvin, a potent antimicrobial and antifungal stilbenoid, by cyclodextrins (CDs) is described for first time in this work. Steady-state fluorescence was used to demonstrate that natural (alpha-, beta-, and gamma-CD) and modified (HP-beta-CD, methyl-beta-CD, and ethyl-beta-CD) CDs are able to complex pinosylvin following a 1:1 stoichiometry. However, substantial differences in the strength of the complexation exist between the CDs tested. Although among natural CDs the interaction of pinosylvin with beta-CD was more efficient than with alpha- and gamma-CD, the results show that the complexation constants (K(F)) were higher for all of the modified CDs than for natural CDs, the highest K(F) being that determined for HP-beta-CD-pinosylvin complexes (12112 +/- 761 M(-1)). Moreover, deprotonation of the hydroxyl group of pinosylvin led to a sharp fall in the K(F) values with respect to those observed for the complexes formed between the protonated structure of this stilbenoid and the CDs. Moreover, a pK(a) value is reported for the first time for pinosylvin. Furthermore, when the temperature of the system was increased, a significant drop was observed in the complexation constant values. From these K(F) values and to throw light on the mechanism of pinosylvin affinity for HP-beta-CD, three thermodynamic parameters, DeltaH degrees , DeltaS degrees , and DeltaG degrees , were calculated. The results show that the complexation of pinosylvin by HP-beta-CD is a spontaneous and exothermic process with negative values for entropy changes. Finally, to gain information on the effect of the structure of different compounds belonging to the stilbenoid family on the K(F) values, the complexation of other molecules such as (E)-resveratrol and pterostilbene was studied and compared with the results obtained for the HP-beta-CD-pinosylvin complexes.

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