The elusive and misunderstood world of tartaric acid

[originally from Konpira Maru newsletter 24th August 2022]

Vitis vinifera is full of biological mysteries but one of the most peculiar is that of tartaric acid (TA). It is the major acid in grapes and is one of the core components influencing taste and chemical stability. Yet it remains profoundly misunderstood despite its importance to wine and the unimaginable glory of unraveling its secrets. It certainly deserves more, playing a crucial role in a number breakthroughs in our understanding of how molecules look and behave, and was even worked on by the great Louis Pasteur himself.

One of the major mysteries is the fact TA is synthesized from the breakdown of vitamin C, via the activity of 5 or 6 enzymes. Vitamin C is a critical molecule for grapes in many ways but the actual role of TA still can only be speculated. So why did plants maintain a cryptic pathway through evolutionary time that involves half a dozen enzymes turning an important substance into an apparently fairly useless one? This remains to be eludicated and, consequently, TA’s unknown origins and unknown biological importance have traditionally relegated it to the sad role of “secondary metabolite”, the etymological pigeon hole for all ambiguous cellular molecules. Fortunately, in the current era of (long overdue) inclusivity, the term secondary metabolite is out and the term “plant specialised metabolite” is in!! 

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Tartaric acid: so celebrated but so misunderstood. The molecular equivalent of E.LO.’s “Livin’ Thing”.

The wines of the 2022 vintage abound in natural acidity and the ensuing wines will no doubt be celebrated for it. The standard reason attributed to this is that low average temperatures during the growing season produce wines with more acid. This was first postulated by Gerber (1897) and continues to dictate modern thinking.. However, whilst Kliewer (1964) showed that low temperature early in berry development (green berry stage) can lead to increased TA production, he and other authors since, found very little evidence that it alters tartaric acid levels at harvest. It appears that the fate of TA has been conflated with malic acid levels that reduce with increased respiration, so high average nighttime temperatures can signficantly drop their levels. Therefore, low temperatures can lead to wines high in malic acid, a much weaker acid than tartaric. Furthermore, in most reds this is further transformed to lactic acid by malolactic fermentation, a weaker acid again. So this doesn’t explain the phenomenon at all.

What is known about tartaric then? The first clue was actually found by Gerber(!!); temperature above 35C in some circumstances could reduce final tartaric levels. How could this be?  What is very much known is the role in potassium (K+) in forming with potassium bitartrate an insoluble salt, which drops out of grape juice (and wine), decreasing the TA levels, and increasing the pH. Grapevines have been shown to use K+ as a heat stress defence mechanism, therefore grapevines experiencing hot temperatures and low soil moisture would be expected to have higher K+ levels at harvest.  In fact the importance of this is so great that weak potassium absorbing capacity is being targeted for producing genetically engineered grapevines capable of maintaining acidity in warm climates. This has important implications for the industry as it will see further reduction in wines with natural acidity due to an increase in extreme events not necessarily increasing average temperature. This would be exacerbated for continental regions that do not have the temperature buffering effect of the ocean or mountains. So Gerber possibly actually got it right, but a century of scientists and wine enthusiasts took the wrong message from it.

The frequency of extreme weather events is increasing year on year since 1960 as boorish alarmist left-wing rhetoric interacts with ocean currents

Finally, if we look at the extreme temperature data from our vineyard in Whitlands (temperatures greater than 35C) we can see that for vintage 2022 there are no incidences of this occurring. 

The incidence of days over 35C in Whitlands, Victoria. No surprises 2019 was a bushfire year.

So vintage 2022 was a low potassium year which possibly best evidence leads us to surmise that lead to the lovely natural acidity seen in our wines this vintage.