Winemaking
Adjusting the must
The family of interventions that change what the grapes delivered — enrichment, acidification, concentration — and where each is permitted.
The short answer
Must adjustment covers the interventions that correct the composition of the juice before or during fermentation: raising potential alcohol, adjusting acidity, and concentrating. All are regulated, and which are permitted depends on where the wine is made.
Overview
Adjustment exists because grapes are an agricultural product and seasons are not uniform. A producer in a cool region facing an unripe vintage and one in a hot region facing an acid-stripped one have opposite problems.
This page describes what the interventions are and why they exist. It does not provide dosages or procedures, which are winery matters and not consumer information.
Step by step
1 Enrichment
Raising potential alcohol, by adding sugar, concentrated grape must or rectified concentrated grape must. Permitted only in cooler EU zones and capped.
2 Acidification
Adding tartaric or another permitted acid where acidity is too low. Generally permitted in warmer zones and not in cooler ones.
3 Deacidification
Removing acid using carbonates, or biologically through malolactic fermentation. The cool-region counterpart to acidification.
4 Concentration
Removing water to raise the concentration of everything else, by reverse osmosis, vacuum evaporation or cryoextraction, within limits.
5 Nutrient addition
Supplying nitrogen and other nutrients the yeast need, where the must is deficient. Routine, and the main defence against stuck fermentation.
The permitted set differs by jurisdiction and changes. Anyone needing the current position for a specific country should read the regulation rather than a summary.
Sources
Sources consulted
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Yeast converting sugar into alcohol and carbon dioxide — the step that makes wine wine.
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How red wine is made
Fermenting grape juice in contact with the skins, which is where colour and tannin come from.
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How white wine is made
Pressing the juice off the skins before fermentation, then fermenting the clear juice.