Extend the life of good wines, so that their aroma does not degenerate into rotten vegetables or raisins. This is what was achieved by identifying the molecules responsible for its aromatic decline
A few years ago, Grenache wines inevitably evolved towards a high oxidation state. The wine lost its reddish and bluish colors because its natural colorants (anthocyanins) coagulated in the form of a black precipitate, and in a few months a clean amber liquid with sherry-like aromas remained, appreciated only by consumers accustomed to this type of product, which today It would fall into the category of dessert wines.
Things improved with the introduction of technology that made it possible to limit the wine’s contact with the air, and with the discovery that, in order to stabilize color, colorants had to condense with tannins, which was favored in the presence of small amounts of oxygen. .
Since then we have learned to make Grenache wines with a very stable color, but the aroma… is something else. The aroma resisted to lengthen the life of good wine.
The smell of cooked or stale vegetables when they get old
Many wines, more often in the Mediterranean area, develop aromatic notes during aging that are reminiscent of cooked vegetables, raisins, honey or rancid. Other times the aroma of the wine fades as it ages, losing the freshness and fruit it had in its first year.
On some occasions this degeneration is a consequence of the action of oxygen, on others this relationship is not so clear. In any case, when it happens, the wine loses its quality.
This is a big problem for much of Mediterranean viticulture because for a wine to reach the highest levels of recognition (and price) it is essential that it ages well.
We discover the molecules involved in aroma degradation
More than 20 years ago we discovered that the molecules causing these aromatic problems were, fundamentally, two odorants derived from amino acids: methional, which is derived from methionine, and phenylacetaldehyde, which is derived from phenylalanine.
Methional has a strong raw potato odor and phenylacetaldehyde has a honey odor. Both are powerful aromas. Concentrations of micrograms/liter are enough for them to be appreciated. These molecules are found in numerous natural products and foods and have the ability to mix with other odorants to produce different odours.
Other characteristics of these components, which explain the complexity of their behavior, are their ability to combine with other chemical substances, sometimes in a reversible way, and the multiple chemical and biochemical pathways through which they can be formed.
Although they are oxidative aromas, we have discovered at least two ways in which they can be formed without the direct assistance of oxygen. One is in the alcoholic fermentation itself in which they are biochemical intermediates in the processes of amino acid synthesis. If sulfur dioxide (SO₂) is present in its intracellular formation, which is the most common antioxidant and antiseptic in wine, they form an association with it that prevents its transformation, and accumulate in the form of adducts with SO₂. These adducts are odorless and go unnoticed. Of course, when the wine ages, the SO₂ will gradually disappear and these components will reappear, imparting to the wine the notes of oxidative degradation that spoil it, even though there has been no contact with oxygen.
The second is due to a chemical reaction of amino acids known as Strecker degradation. This reaction takes place between amino acids and molecules with two carbonyl functions (C=O) on adjacent carbon atoms (alpha-dicarbonyls). Well, some of these dicarbonyls can be produced in significant amounts during fermentation or can come from grape oxidation processes.
We have been able to verify how, during aging, these dicarbonyls react slowly with the amino acids, forming the Strecker aldehydes and giving the notes of oxidation, again even though there has been no contact with oxygen.
Why some of the best wines are varietal blends
These molecules can also be formed during the oxidation of wine. During it, some polyphenols, when oxidized, form precisely dicarbonyls of the type that give rise to the Strecker reaction with amino acids. What we have discovered is that each type of polyphenol has a different tendency to give the reaction, from those that do not give it at all to those that give it a great intensity. This explains why the wines of some varieties age poorly and why some of our best wines are blends of varieties: the polyphenols of one complement those of the other, preventing the aforementioned reaction.
In the control of these three routes we risk the longevity of the wine. And that is where what we have learned is essential to make wines that last longer. We look for yeasts and fermentation conditions in which lesser amounts of these aldehydes are formed, no dicarbonyls accumulate, and trace amounts of amino acids remain. In addition, we optimize the varietal mixture to minimize production during oxidation and we are testing other strategies that we hope to be able to patent. Our tests, for the time being on a laboratory scale, confirm that we can completely avoid the formation of aldehydes responsible for aromatic degradation and make wines, if not eternal, much longer lasting.
This article has been published in ‘The Conversation‘.
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Eddie is an Australian news reporter with over 9 years in the industry and has published on Forbes and tech crunch.