Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Entropy-driven complex formation of malvidin-3-O-glucoside with common polyphenols in ethanol–water binary solutions
Introduction
The colour of red wine is one of its most important quality parameters, which determines the sensorial evaluation significantly. It plays a key role in the decision-making process of the consumer, who usually tends to prefer wines of deeper colour and hue. The colour in red wine is mainly due to its anthocyanin content. Anthocyanins are widely distributed glycosylated polyhydroxy and polymethoxy derivates of the 2-phenylbenzopyrylium (flavylium) cation, which do not only account for the colour of grapes and wine but also for the colours of other fruits, vegetables, flowers, and other plants and the respective products derived from them [1].
Several cofactors, such as phenolic acids, flavonoids, metal ions, or even other anthocyanins, increase the colour of anthocyanins through a process called “copigmentation”. Copigmentation has been identified by several researchers as being one of the most important parameters responsible for the colour of young red wines. According to literature, copigmentation may account for up to 50% of the colour in red wine [2]. During copigmentation loose associations are formed between the anthocyanin and the cofactor, the latter also often being referred to as the copigment. The anthocyanin–copigment complex results in visually enhanced colour perception often accompanied by hyperchromic, hypsochromic and/or bathochromic shifts in spectroscopic properties [2], [3], [4].
Several studies have been conducted to investigate the influence of various cofactors on copigmentation in model solutions as well as in wine. It has been shown that the magnitude of copigmentation strongly depends on the nature of the cofactor, its structure, and other parameters, such as pH, transition-metal and ethanol concentration [3], [4].
During red wine-making, the grapes are crushed, inoculated, and then fermented on the skin to extract the anthocyanins from the grape skin with the aid of the alcohol, which is being formed during fermentation. Due to the importance of colour for quality assessment of the wine it seems to be of overriding importance to understand the processes occurring during the first steps of wine-production. Only a full understanding of these process will ultimately help us to adapt the winemaking technology in a way that extraordinary colour stability will be achieved.
Thus, we wanted to re-build the processes occurring during red wine fermentation. The change from a pure aqueous environment to a binary solvent with 12–14 vol.% alcohol content presumably leads to the desolvation of the interacted species. It is important to identify the solvation shell and determine the thermodynamic parameters, which could play an important role in this desolvation and hitherto on copigmentation and colour stability (Fig. 1).
Section snippets
Chemicals (Fig. 1)
Anthocyanin: malvidin-3-O-glucoside: 1 MW = 494.87. Colourless polyphenols: 2a: caffeic acid, MW = 180.16; 2b: catechin, MW = 290.28; 2c: ellagic acid, MW = 302.20; 2d: rutin (quercetin-3-O-rutinosid), MW = 610.53 and 2e: procyanidin B2 (epicatechin-(4β-8)-epicatechin, MW = 610.53). All polyphenol standards were purchased from Extrasynthese (Genay, France) and used without further purification.
Sample preparation
The interaction between malvidin and the copigment was investigated by means of the Job's method. Detailed
Fluorescence lifetime of malvidin emission as a function of ethanol content of the solvents
In all cases the fluorescence decay shows a biexponential character. This property is probably due to the fact that the fluorescence emission comes from two different moieties of the malvidin skeleton: one fluorescent moiety consists of two aromatic rings forming a chroman-like structure, while the other moiety is a dimethoxy-phenol (syringol) structure. These two moieties have slightly different characteristics with regard to their hydrophobicity. This is because of the electron withdrawing
Acknowledgements
S.K.-M. wishes to thank the Hungarian Academy of Sciences for a Bolyai János Research Fellowship. This work was funded in part by DAAD (Deutscher Akademischer Austauschdienst) and the MÖB (Magyar Ösztöndíj Bizottság). Financial support of the EU (Grant GVOP-3.2.1-2004-04-0200/3.0) and the National Office for Research and Technology (NKTH, DD-KKV-06-311, OMFB-01486/2006) is highly appreciated.
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