Phenolic compounds are abundant in the plant kingdom. They are derived from phenol, which is the basic structure of all phenolic constituents and when containing one or more phenolic rings, they are generally referred to as polyphenols. These phenolic compounds play an essential role in the grapevine’s defence mechanisms against biotic and abiotic stressors. They are also strongly linked to wine quality, colour and flavour.
Grape phenolic compounds
The main grape polyphenols of oenological importance are the non-flavonoids, including the group of phenolic acids, and within the flavonoids group, the anthocyanins and tannins are the major role-players. It is important to understand the reactivity of phenolic compounds and the interaction phenomena that explain the mechanisms for change during the fermentation and post-fermentation stages. These mechanisms include condensation reactions, co-pigmentation and various tannin interactions. Below we look at the main phenolic reactions taking place during fermentation.
The fate of phenolic compounds during the fermentation phase
A. Production of phenolic acids
The tartaric esters of hydroxycinnamic acids (caftaric, coutaric and fertaric acid) in grapes, which have not taken part in must oxidation phenomena, see a 30% drop in concentration during fermentation. They undergo changes due to the metabolic activity of microorganisms (yeast and bacteria).
During alcoholic or malolactic fermentation, the formation of vinylphenols from coumaric and ferulic acids and their reduction to ethylphenols by Brettanomyces is the cause of characteristic olfactory defects. In the presence of anthocyanin molecules, these vinylphenols react to give pyranoanthocyanins. Furthermore, vinylphenols do not accumulate during ageing provided there is a sufficient quantity of monomeric anthocyanins.
B. Evolution of anthocyanins and tannins
- Diffusion of anthocyanins and tannins during fermentation
In conventional red vinification, the extraction of compounds from the skins and seeds occur during alcoholic fermentation, specifically at the maceration stage. The anthocyanins start to diffuse as soon as the grapes are crushed and their concentration peaks after four to five days of fermentation. The extraction of flavonols, monomeric flavanols (catechin and epicatechin), and low molecular mass oligomers from the skins, occur in parallel to extraction of the anthocyanins.
At this stage, the coloured and colourless forms are in equilibrium depending on the pH and addition reactions with sulphites take place. When their concentration is sufficient, a fraction of the anthocyanins interact in the phenomenon of self-association. With the extraction of other colourless phenolic compounds and when their concentrations are suitable, the anthocyanins take part in co-pigmentation phenomena.
The diffusion of skin proanthocyanidins and procyanidins from seeds, is slower and is favoured by the increase in ethanol. However, the highest molecular procyanidins from seeds and those with the highest relative galloylation rate, remain in the pomace. This diffusion continues gradually, depending on the technical means used, until the separation of the solid phase when the wine is racked and the press wine is obtained.
- Influence of the metabolism of fermentation microorganisms
During alcoholic fermentation, yeasts produce various carbonyl compounds which can react with anthocyanins and lead to the formation of vitisins. In addition to their resistance to bleaching by SO2, these vitisins are red-orange pigments of interest, because they are coloured irrespective of the pH, while anthocyanins in the coloured form represent only 5 to 25% of the anthocyanins at wine pH.
Acetaldehyde is also competitively involved in condensation reactions that lead to the formation of anthocyanin or tannin compounds linked by an ethyl bridge. When this polymer includes anthocyanins, its colour is purple and it is not bleached by sulphites. Once an anthocyanin molecule is incorporated in the polymerisation with flavanols, it is less reactive towards acetaldehyde and the polymerisation kinetics are slowed down, leading to the formation of polymeric pigments that remain more soluble. In pigments formed through the acetaldehyde pathway, the constituent anthocyanin molecules take part in intramolecular co-pigmentation phenomena, giving red wine its typical purple-red colour at the end of fermentation.
- Involvement of anthocyanase activity
Yeasts may have anthocyanase activity, an enzyme responsible for the release of glucose molecules from monomeric anthocyanins, and can in this way be responsible for the formation of less stable aglycone anthocyanins, leading to a loss of colour. In general, the winemaking strains of Saccharomyces cerevisiae are low producers of anthocyanases. In contrast, non-Saccharomyces grape flora are potentially a more important source of anthocyanase activity: the genera Candida, Hanseniaspora and Pichia show the most activity. In extreme cases, colour loss can reach 30 to 70%.
- Adsorption of phenolic compounds on the cell walls of fermenting yeasts and lees
Yeast cell walls have the ability to adsorb phenolic compounds during fermentation. The profile of the anthocyanins and their derivatives are modified in different ways depending on the yeast strain. Overall, the anthocyanins bound to the yeast cell walls represent 2 to 6% of the total anthocyanin loss with the removal of lees, even though release can be seen after long ageing on lees. Tannins also interact with the cell walls of dead yeast cells, particularly mannoproteins, but most of the polyphenols pass through the cell wall to interact with the intracellular constituents.
During ageing and storage, as the content of monomeric anthocyanins involved in various reaction mechanisms is reduced, the formation of yellow to brown pigments resulting from the oxidation reactions of flavanols and proanthocyanidins will become predominant, while the proportion of purple-red pigments is reduced at the same time.
If anthocyanins are absent or present in small quantities (as with white and rosé wine) the acid-catalysed cleavage leads, after rearrangement and oxidation, to the formation of yellow to brown pigments. This is particularly the case with polymers formed through glyoxylic acid or oak-derived compounds, such as furfural, hydroxymethylfurfural and vanillin.
In red wine, at the start of ageing and for as long as the quantity of anthocyanosides is sufficiently high, vinylflavanols and vinyl-tannins, resulting from cleavage of the ethyl bridge, have the capacity to react relatively quickly with anthocyanins to form flavanyl-pyranoanthocyanins; these pigments are orange in colour, like vitisins and pinotins. These conversion reactions probably explain the change from the ruby red colour of young red wines towards the brick red colour of older wines.
In old wines with reduced anthocyanoside content, vinylflavanols and vinyl-tannins can react preferentially with vitisin A, the predominant pyranoanthocyanin at this stage, to form A-type portisins blue-coloured pigments. They are present in very low concentrations, but their contribution to the colour of old wines with high pH should not necessarily be neglected.
- Today, we have the analytical means to study phenolic compounds in detail.
- The structure of each of the polyphenolic constituents is characteristic of each grape variety and there are family resemblances within groups of grape varieties.
- Within the grape bunch, the phenolic compounds are compartmentalised both at the level of the different tissues (skin, pulp, seed and stalk) and at the sub-cellular level.
- These different locations influence their extractability, which nevertheless remains strongly dependent on the technology used during the preparation of the grapes for wine production.
- Their fate in wine depends on a large number of parameters.
- Once in solution, the phenolic compounds that come into contact react by various reaction pathways: chemical, biochemical and physico-chemical.
- The kinetics of the chemical reactions in particular are very slow and affect only a fraction of the constituents extracted, so the impact on the wine composition depends on time and temperature.
- Dihydroxylated phenolic compounds are the structures that preferentially participate in oxidation phenomena. Those catalysed by the enzymatic pathway quickly cause changes in the must composition.
The strong potential for chemical and physicochemical reactivity of flavonoids gives rise to a diverse range of new polyphenolic constituents in wine, leading to changes in colour and taste. Knowledge of the chemical structures and the conditions for their formation, as well as those for the formation of supramolecular structures, are important to choose the appropriate winemaking practices, supported by the availability of effective oenological products like enzymes and tannins.
– For more information, contact Elda Binneman at Anchor Oenology, firstname.lastname@example.org.