Viticulture
Ripening: several clocks running at once
Sugar, acid, tannin, colour and aroma each mature on their own schedule — which is why the picking decision is hard.
The short answer
Ripening is not one process. Sugar rises, acidity falls, phenolics develop and aroma compounds appear and degrade, all on partly independent timetables. The grower’s problem is that they rarely peak together.
Overview
Almost every argument about wine style — alcohol levels, green tannin, over-extraction, picking early — comes back to this single fact.
The trade vocabulary distinguishes "sugar ripeness" from "physiological" or "phenolic" ripeness. The distinction is real; the terms are imprecise and are used differently by different people.
How it works
The mechanism, at the scope it has actually been established.
Sugar accumulates steadily after véraison, driven by photosynthesis and by translocation from the vine’s reserves. It also concentrates passively as berries lose water late in the season.
Malic acid is respired by the berry, and respiration accelerates with temperature — particularly warm nights. Tartaric acid is comparatively stable, so the acid profile shifts as well as the total falling.
Tannins polymerise and seeds lignify, and both are associated with the softening of harsh, green-tasting phenolics. This runs on its own schedule and is not predictable from sugar.
Anthocyanin synthesis has its own temperature optimum, and very high temperatures are associated with reduced colour accumulation — one reason extreme heat is not simply "more ripeness".
Aroma compounds each behave differently: methoxypyrazines fall with light exposure, terpenes and thiol precursors accumulate, rotundone accumulates late and is associated with cooler conditions, and some compounds degrade if picking is delayed.
Berry water content changes the whole picture. Late-season dehydration raises sugar concentration without any additional photosynthesis, which produces high potential alcohol without the corresponding flavour development.
Why it matters in the glass
It is why "picked at optimal ripeness" is a claim about a compromise rather than about a moment, and why the same vineyard supports very different wines depending on who is picking it.
What people get wrong
2 widely repeated claims this page corrects.
Oversimplified
“Hot regions make high-alcohol wine because heat puts more sugar in the grapes.”
Warmth accelerates sugar accumulation, so the potential is there. What actually reaches the bottle depends on when the producer picks, and picking date is a decision, not a climate.
Oversimplified
“That capsicum note means the grapes were not ripe. It is a flaw.”
It comes from methoxypyrazines, which do fall as fruit ripens and gets more sunlight. But a degree of it is part of the expected character of several varieties and regions, and calling it a fault is a stylistic judgement, not a technical one.
"Phenolic ripeness" has no agreed measurement. It is assessed by tasting berries, chewing seeds and looking at skins, which is skilled practice rather than analysis, and different producers reach different conclusions from the same fruit.
Sources
Sources consulted
- UC Davis Department of Viticulture and Enology — University of California, Davis
- The Australian Wine Research Institute — AWRI
- Research on climate change and viticultural suitability — Peer-reviewed climate and viticulture literature
More in the vineyard
Viticulture
Climate and wine style
The largest single influence on what a wine tastes like — larger than grape, producer or winemaking, and the reason the same variety is unrecognisable in two places.
Viticulture
Climate change and wine
Warming is already reshaping where wine is grown and what it tastes like.
Viticulture
Harvest and picking decisions
When to pick — the single most consequential decision a producer makes each year.