Wine tannins and polysaccharides – frenemies?

by | Mar 1, 2021 | Oenology research, Winetech Technical

It can happen to the youngest and most experienced of winemakers alike – what we thought would be our boldest vintage for decades, has come out lighter in colour than we expected. Rather than the intense red and violet that we anticipated, we’re greeted with a wine that’s pale red, pitiful orange or even brown. Head hung in shame, we wonder what we could have done differently to get the colour we’re after. Don’t let wine tannins leave you bitter – let’s see how we can make your red wine blush again!

About tannins

Polyphenols are important contributors to wine. They play important roles in astringency, colour, mouthfeel and taste. Tannins are a group of polyphenols consisting of bitter and astringent compounds readily found in nature. They’re present in tea, coffee, cacao, and in plants, they’re found in the bark, leaves, fruit and seeds. The tannins in plants play an important role in survival by acting as a defence mechanism when eaten. The tannins bind to, and precipitate, proteins on the tongue causing a dry, unpleasant and astringent taste deterring any hungry and unsuspecting victim. Similarly, grape tannins react with compounds found in wine including proteins, polysaccharides and colour compounds such as anthocyanins. However, in this case, the tannins should complement the wine rather than deter the drinker. The magic of wine tannins operates through the positive influence on the mouthfeel, taste perception and complexity of the wine – they also have an important antioxidant role.

About anthocyanins

Anthocyanins are key colour polyphenols in red wine and are named according to their first detected source. An example is pelargonidin, an orange-coloured anthocyanin first isolated from the Pelargonium flower. Anthocyanins are produced during grape ripening at the start of véraison where they accumulate in the skins of the berry. In winemaking, the extraction of anthocyanins occurs during maceration and peaks before the end of fermentation, and gradually declines thereafter. Tannins, however, continue to be extracted well after fermentation. These anthocyanins are extremely sensitive to pH and need to be stabilised by other compounds or they will degrade thereby destroying their colour. The question then arises: how can anthocyanins be stabilised to preserve colour?

On stabilising colour

Winemakers have a few tricks up their sleeve when it comes to stabilising anthocyanins. These include keeping the wine at a pH of between 3.2 – 3.6 and controlling the amount of sulphur dioxide added, which can have a bleaching effect. These actions maintain the dark red/violet colours of the all-important anthocyanins. Anthocyanins can also be stabilised by other phenolic molecules such as tannins, which act as co-pigments allowing them to associate and form a ‘stack’ of molecules in the must or wine. These stacks then condense and associate further becoming even more stable as they get larger. Great, the poor wine colour problem is solved. Not so fast … what happens if the tannins are so stable that they don’t even leave the grape?

Explaining the problem

Tannin transfer from the grape skin and seeds to the final wine can be hindered by the tannins forming strong associations with polysaccharides in the grape cell wall, as well as soluble polysaccharide polymers present in the must and wine. In a dramatic turn of events, the same associations of stacking and condensation responsible for stabilising the final wine colour now hinder these compounds from being released from the grape berry. Alas, the content of these phenolic compounds in the wine is diminished, bearing no awards for the winemaker and no pleasure for the wine drinker.

Presenting the solution

Fret not! Once again, our winemaker has covered all bases to ensure enough polyphenol extraction: from intricate pressing, cold soaks, longer macerations to vigorous punch-downs and pump-overs. These are all methods adopted to improve the extraction of tannins and other polyphenols, but perhaps the greatest tools the winemaker possesses are macerating enzymes. These enzymes offer many advantages including accelerated settling, clarification and increased juice yield. But of interest to our winemaker, are the enzymes that degrade the associations of polyphenols with polysaccharides in the grape cell wall, encouraging the release of the phenolic compounds from the grapes and into the wine.

Research in this area of enzymes is active and much is still to be understood. Most enzymes are isolated from a diverse range of fungi. Enzyme preparations often contain pectinases – which break down the scaffolding between cells in the grape berry cell wall, as well as cellulases and proteases – which break down other structural components in the berry cells. The result of adding these enzymes is an increase in anthocyanins and other polyphenolics such as tannins, thereby improving colour, aroma and mouthfeel in the wine. Enzymes are usually dissolved in distilled water and added to the wine must soon after crushing. However, the degradation of the cell wall is a tough job and it requires the cooperation of several pectolytic enzymes to be effective.

What the latest research says

A study by Osete-Alcaraz et al. in 2020 aimed to determine the optimal form of addition of these enzymes for deconstructing the polysaccharide-tannin association in the grape cell wall. But first, they needed to determine whether the grape tannins already in solution could somehow inhibit the action of any added enzymes on the cell wall. The result was conclusive: the enzymes function so quickly by breaking down the cell wall that any tannins in solution could not interfere. This paved the way to determine what the best approach for adding enzymes is, namely individually, together or sequentially.

The findings showed that when pectin-lyase and polygalacturonase (both members of the broader family of pectinases) are added to artificial grape must individually, or sequentially, their action is effective to a certain degree. But when added to the must together, a significant increase in the content of tannins released into solution is observed. These enzymes attack the polysaccharide-pectin associations with tannins in grapes, and when working together, they efficiently free the tannins from clutches of the cell wall through pore formation and pectin-linkage breakdown. However, if the pectin molecules contain methyl groups (which on average 70% of grape pectin does) the activity of polygalacturonase is limited and another enzyme, pectin methylesterase (another member of the pectinase family) is necessary to remove the methyl groups. In the study, these enzymes also prevented the free tannins from re-joining with polysaccharides already in solution, which is good news as we can expect the improved colour, aroma and mouthfeel that we’re after.

When this combination of pectinase enzymes is incorporated into a maceration regime along with other maceration enzymes, the results can be impressive. There is an increased release of polyphenols from the grape cell wall along with a decrease in tannin-polysaccharide associations reforming in solution. This is good news since you can expect improved aromas and mouthfeel, and an emphasis on bolder, more stable colour. The growing topic of maceration enzymes is continuously providing new and advantageous strategies for producing fragrant, bolder and more complex wines. Through careful selection of pectinase enzymes, every red wine will have its chance to glow!

Abstract

Polyphenols are crucial contributors to wine astringency, colour and flavour. Tannins are phenolic compounds involved in wine stabilisation that contribute to mouthfeel attributes and complexity through bitterness and astringency. These tannins have a stabilising function for the anthocyanin colour compounds in red wine, which are extremely sensitive to degradation. The same association of tannins with anthocyanins that preserves colour also limits the tannin transfer from grape skin and seeds to the wine during maceration. To improve tannin extraction, enzyme preparations are utilised to assist in releasing tannins from polysaccharides in the cell wall. Recent research has evaluated the optimal addition of these enzymes to prevent tannin-polysaccharide associations from occurring during winemaking. The findings showed that when the pectinase enzymes pectin-lyase and polygalacturonase are added to the wine individually or sequentially, their action is limited. But when these two enzymes are added to the must together, the highest release of tannins into solution is observed.

At the time of writing, the author of this article was an Honours student at the Department of Viticulture and Oenology, Stellenbosch University. This article forms part of a science communication assignment where students were tasked to write a popular article based on a recently published scientific article.

References

Claus, H. & Mojsov, K., 2018. Enzymes for wine fermentation: Current and perspective applications. Fermentation 4, 1 – 19.

Kennedy, J.A., Saucier, C. & Glories, Y., 2005. Grape and wine phenolics: History and perspective. ASEV Phe. Imp. 1, 1 – 10.

Osete-Alcaraz, A., Gómez-Plaza, E., Martínez-Pérez, P., Weiller, F., Schückel, J., Willats, W.G.T., Moore, J.P., Ros-García, J.M. & Bautista-Ortín, A.B., 2020. The impact of carbohydrate-active enzymes on mediating cell wall polysaccharide-tannin interactions in a wine-like matrix. Food Research International 129, 1 – 9.

– For more information, contact James Duncan at dugald@sun.ac.za.

 

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