Definition: Micro-oxygenation 2

Interview with Michel Moutounet (MM) by Montserrat Daban and Míriam Cañas from Percepnet (P).

Michel Moutounet is Director of the Biopolymers and Aromas Research Unit at the Institute of Vine Products, INRA. His research aims to deepen knowledge about grape and wine polymers and study the relationships between their chemical and technological properties. In particular, he spoke in the context of Firavi's technical conferences on polyphenols, which have a decisive influence on the colour of wines and give them many of their organoleptic properties.

(P) A few weeks ago, we published an article by Guillem Roig and Stéphane Yêrle in ACE Magasin d'Œnologie on ten years of micro-oxygenation. You have mentioned on several occasions that you developed this process, and since then Patrick Ducourneau has come to you with his idea of developing a device that would allow micro-quantities of oxygen to be added to wine in a controlled manner. What can you tell us about this process?

(MM) Yes, we exchanged views and ended up creating the micro-oxygenation device. It's a long story, as it took us four or five years to develop the device. It is based on preliminary studies on gas exchange in wines in barrels. We studied the presence of dissolved oxygen in wine in barrels. That's the principle behind the creation of the device. It's been around for fifteen years now. In fact, thanks to micro-oxygenation, we claim to be able to deliver oxygen under conditions similar to those found in wine in barrels.

(P) But that doesn't mean that micro-oxygenation speeds up the process?

(MM) No, not to make it faster. Unfortunately or fortunately, that's not the case, nor is it the objective. The objective was: taking France as an example, we consider that a maximum of 10% of wines are aged in barrels. Around 30% of wines are table wines that are consumed very young. These are not likely to evolve significantly during ageing. So that leaves 60% of wines that are in vats and deserve to be aged. And it is this 60% that interests us, so that we can develop them during ageing with this form of oxidative evolution, by controlling the amount of oxygen added.

(P) According to your group, research has demonstrated the effectiveness of controlled oxygen additions in stabilising colour and developing wine structure, as well as their positive effects on aromatic expression, eliminating herbaceous or vegetal characteristics and enhancing varietal fruit characteristics. However, what is the influence of micro-oxygenation on astringency?

(MM) With regard to astringency, it has always been observed, particularly in very astringent wines, that micro-oxygenation for an average of three to four months reduces astringency. I do not believe this is due to precipitation, but rather to a change in the evolution of phenolic compounds, particularly tannins. Micro-oxygenation promotes reactions, particularly condensation reactions that promote the production of ethanal and acetaldehyde, which create supramolecular combinations and organisations of tannins that make them seem less astringent.

(P) However, what do we know about the ability of wines subjected to micro-oxygenation to evolve after time in the bottle? What will be the long-term consequences of micro-oxygenation on wines? Or, conversely, will these wines not remain in the bottle for long?

(MM) The idea is that, for us, there is no distinction between wines that deserve barrel ageing and those that deserve micro-oxygenation. Based on our experience to date, wines aged in barrels and those treated with micro-oxygenation generally react in the same way in the bottle.

(P) Are you sure there won't be any surprises?

(MM) Absolutely not. The most important thing to know is that the amount of oxygen consumed during micro-oxygenation is a maximum of 50 ml per litre. We need to ensure that this amount of oxidation is properly supplied and consumed. That is what we have to guarantee. Then, if the reactions of the wines are sometimes different, we are studying this to find out the reasons for certain interactions.

(P) It seems that we are moving towards molecular oenology, but there are delicate balances. Perhaps we do not know enough about yeast metabolism or what happens during fermentation with amino acids or other reactions. When you change one parameter, the others are affected. So, can it happen that one wine evolves well and another behaves differently?

(MM) First of all, the main consumers of oxygen at the molecular level are phenolic compounds. However, in certain circumstances, particularly when wines remain cloudy and there are still dead yeasts present, these yeasts retain their ability to consume oxygen even after fermentation. And this capacity to consume oxygen, in terms of speed, is greater than that of phenolic compounds. So, at certain times, the oxygen supplied is consumed by these yeasts. So, either we need to be able to stop adding oxygen (which is difficult to control, but we are working on it), or we need to separate the ageing phase from the chemical phase in the absence of microorganisms. It's not very complicated, we are still studying it. We can separate the biomass from the microorganisms in a wine so that ageing can take place with chemical reactions. For example, either the wine is heat-treated, and if this is not sufficient in the presence of oxygen, tangential filtration is used to eliminate the microorganisms. This is what we are working on today. We are not yet at the application stage, but we are already working at the experimental level, in two-thousand-litre vats.

(P) What is your opinion on the need to reach a consensus on oenological practices in the two major blocs of producing countries, those of the New World, now in the WWTG, and the European countries, which advocate a more traditional approach to viticulture and winemaking? For example, certain oenological practices, such as the addition of wood chips to increase the aroma content of ageing, are now the subject of intense debate. How do you see this being applied?

(MM) This has been the subject of heated debate for three or four years, but the question is still being asked. Personally, I think that at the international level, all producers should have the same technological opportunities. So how can this be achieved? Either all producers have the opportunity, since it is already done in other countries, for example Australia or Chile (they have an undeniable advantage today). So what should be done? Either European producers who wish to do so should have access to it, or we should be able to find the means (but this is complicated at the political level) to ban products that do not comply with authorised practices. At the moment, we are in conflict with the principles of fair trade, as laid down by international organisations. My opinion on this has been clear for a long time. In fact, I have two opposing positions: one is the technologist's position, i.e. if we have the opportunity to work with chips, which is favourable for a technologist, it is much easier than with barrels, because we will have a greater chance of success.On the other hand, as a consumer, if one day I visit the beautiful Penedès region, where I am amazed by the vineyards and cellars, and I am given a tasting of wines aged in barrels, I find them very good, I am interested, and I go home and give my friends a tasting, then every year I order a few bottles because I liked them. I remember this pleasure because it is visual, I have tasted the landscapes, and then if I am suddenly told that these wines to which wood chips are added are for tourists, as a consumer I am very disappointed.What interests me about barrels is that each one behaves differently, and for the chemist in me, it's a specific reactor; each barrel is a particular reactor. That's why, in oenology, it's a blending technology. This diversity is a palette for the oenologist. They can search through this or that barrel, as if it were a vat, to create the blend. A blend that corresponds to his own culture, his own experience, his past. He knows these wines, their reaction to this or that barrel, and he combines different barrels to create his great vintages. In the trade, there are heat-treated chips, more or less heavily toasted, with different dimensions, and here too we can imagine gaining ever more experience. For the technologist and the oenologist, and even the consumer, it is true that the great danger in this business is to end up with a formula for wine. That is the problem, but this technology is a function of costs. Technological changes are always a function of economics. We treat wines with this or that product because if there is a precipitate at the bottom of the bottle, the wine will not sell. So we try to solve this problem with chemistry. And that's where the idea of chips comes in, because barrels are so expensive that we think, "If we put the wine in contact with wood, it will be less variable and cost less."

(P) What do you mean when you talk about determining the sensation of astringency? Are these the factors that influence its perception? What information does astringency provide? Are you looking for mechanisms to influence this astringency?

(MM) Yes, that's right. It's more about how it works and which molecules are specifically involved, always at the molecular level.

(P) On that subject, according to physiologists, flavour is the sum of taste, smell and touch perceptions (including astringency, temperature, dryness or acidity, etc.). So, according to Jordi Llorenç, a physiologist at the University of Barcelona, if you want to conduct an experiment that only affects taste, you have to hold your nose. Do you agree with this statement? It seems that for astringency, this is essential, but not for taste.

(MM) I think taste is a matter of training for the taster. For the studies I presented in Australia, expert tasters met every day for three months to participate in the study. This resulted in a panel of experts that is reliable. They are now able to distinguish between smell, flavour and astringency.

(P) Why do you believe in the theory that only different areas of the tongue are capable of recognising the main flavours, when this theory has long been questioned?

(MM) I'm not a specialist in localisation. I think that statistically there are areas in the mouth where one flavour is perceived better than another.

(P) One last question about the size of polyphenols, the tannins that interact with proteins. Why is astringency greater in young wines, where the phenols are simpler and have not had time to polymerise, than in older wines?

(MM) I think that for a long time in oenology, it was thought that during ageing, tannins did indeed polymerise. But I believe the explanation lies in the fact that the polymers produced during the ageing and maturing phase are different from those of tannins produced by biosynthesis. We call them native tannins. They are linked by bonds resulting from the plant's biosynthesis. So it is a 4,8 or 4,6 bond that makes the polymer. Then, when other polymers are created in the wine, their bonds in numerical terms are not the same. Then there are oxidation phenomena that give other bonds in numerical terms. Finally, anthocyanins are involved in these polymers. This modifies the structure of these molecules. We still have epicatechins and catechins, but the bond is different.During his lecture, Moutounet also taught us that when the amount of ethanol increases, there is a tendency to reduce the characteristics of adhesion and the sensation of astringency, and even this sensation is modified by the presence of more acidic polysaccharide fractions or by the addition of anthocyanins from the grape skin.

Excerpt from "Rubes Editorial".