Correcting faults at harvest: increasing alcohol potential, correcting acidity


(based on Emile Peynaud’s Connaissance et Travail du Vin)




It is not always possible to make quality wines in marginal climates, either if so warm and/or humid that the vines produce too much fruit and leaves, or so cold that desired ripening levels are not always achieved. 

If in a region with blending of different varieties, this can be compensated for in some cases, but not always - and where there is mono-cepage (single variety), it is sometimes necessary to find measures to improve the harvest grapes - or at least to mitigate the faults.

There are several available techniques for increasing sugar levels in grapes (subject to local rules), such as adding sugar or concentrated grape must, or for lowering acidity (again subject to usually stricter rules) such as adding tartaric acid. Generally speaking, techniques that take things away are better than those that add them (and adding things almost invariably results in higher yields).

Increasing sugar levels:
Allowable techniques under EU regulations:
- Passerillage sur souche (drying out the grapes on the vines, so for example Tokaj)
- passerillage hors souche (drying out the grapes after picking, so for example drying the grapes on straw as in parts of Italy)
- sur-maturation artificelle/deshydration (allowing the grapes to get over-ripe or dehydrate)
- extraction of the must at a specifically controlled temperature (cryo-extraction)

Technique of removing something from the wine to enrich it:
- evaporation at atmospheric pressure (EPA)
- evaporation sous vide (ESV)
- Can use inverse osmosis (reverse osmosis) to enrich must also

Additive Techniques:
- Chaptilisation
Allowed in EU up to a maximum of 2% of alcohol (or reduction of 20% of volume). But to avoid inbalance in final wine, best to stick to 1degree, maximum 1.5.

Chaptal first wrote about this technique in 1801 in his book 'Art de Faire les Vins'. Today EU is divided into five zones and each has different rules on chaptilisation. Certain AOCs allow it or don't allow it, and some southern parts of Europe don't allow it, or only in exceptional circumstances. Some northern countries allow it every year. In France have to declare it in writing.

Can use various tyoes of sugar (they all break down into the same thing glucose and fructrose) - so saccharose, cane sugar, beet sugar. Cane sugar is used for white wines. And for liqueur de tirage for sparkling wines.

In theory 17g/l of sugar = 1 degree of alcohol. So for a 225l barrel of white wine, add 4kg of sugar. Often red wines need more because lose alcoholic degree later through evaporation during racking etc. Generally it is the free run wine that is enriched, not the press.

Must dissolve the sugar before addition and add it through a controlled remontage (pumping over). Must be done at the start of fermentation, as vat is heating up and the cap is just forming - and best to do in one fell swoop to ensure the nutrients are there to allow a speedy and effective fermentation. Possible to do afterwards, but run risk of lactic bacteria fermenting the sugar instead of yeasts, and turning it into acetic acid/piqure lactique..

- Addition of concentrated grape must or rectified grape must

Concentrated must is usually prepared by adding sulphur (mutage) to stop the ferment and then heating up the must to remove most of the water through evaporation. With this technique you have other elements of the must, not just sugar (so including acidity in form of tartaric acid). Chaptilisation slightly lowers acidity, concentrated grape must slightly raises it. If AOC, have to use same origin for the rectified must as the wine itself.

It is not allowed to use both techniques of taking something away and adding it.

Correction of Acidity

Tartaric acid is the only allowable product for increasing acidity levels in must or wine.

Allowed to be added in grapes, musts, and young wines undergoing fermentation. But not allowed in finished wines (or those that have been enriched to increase alcohol).

In hot regions, it is sometimes very useful to ensure balance, bright colour and ability to age. Acidification often makes a wine seem hard, or at least less supple, so should always be last resort.

If pH higher than 3.6, can perhaps consider acidification. Basically want to slightly bring down pH without hugely affecting overall acidity. Try to avoid adding citric acid, as lactic bacteria can transform this into volatile acidity (and diacetyl).

For a (starting) total acidity of between 3-3.5g can add up to 50g tartaric acid per Hl

If under 3g, can add up to 100g tartaric acid

In all cases, maximum acidification allowed is 1.5g/l of tartaric acid (2.5g/l for finished wine)

Only products allowed CaCO3 (calcium carbonate) and KHC03 (potassium bitartate) or potassium salts. Again, this only has an effect on tartaric acid within the wine.

Problem with using calcium carbonate is that you can get tartaric crystals forming in the wine afterwards (but need less of it, so cheaper). So best to use only for v slight bringing down of acidity, or in non-fermented must. Potassium bicarbonate can be used in a young wine after fermentation. Best to do in two application, and to think of it as first step in a process of deacidification that will continue with malolactic fermentation (something which can be difficult to get started in wines that have a v low pH/high acidity).

It is not allowed to both acidify and de-acidify the same must. Also not allowed to both enrich and acidify the same must.

Can also sometimes add oenological tannins (usually tannin of chestnut trees or oak) in powdered form. But far better results from leaving wine in contact with pips and to ferment with stalks. Almost invariably, addition of tannins will not improve the quality of a wine.