Science
Why wine is like this
Wine attracts more confident causal explanation than almost any other subject, and most of it was never demonstrated. This section explains what is actually known — and says so plainly where the honest answer is that nobody knows.
Every relationship here is classified by what kind of claim it is. An established mechanism has been demonstrated and may be stated plainly. A measured association has been observed without a demonstrated mechanism covering the whole path, and is never written as a cause. A sensory tendency describes perception rather than the liquid. A traditional belief is recorded as what people believe, alongside what the evidence shows.
23 of these 34 pages carry an explicit statement of what is not settled. That is a feature of the subject, not a gap in the writing.
Start here
The questions people actually ask, answered without assuming any chemistry. 2 topics.
Science
Why can Chardonnay taste buttery?
Malolactic fermentation, diacetyl, lees and oak — four contributors that usually travel together, only one of which is the actual cause.
Science
Legs and tears on the glass
The drops that run down the side of the glass are a surface-tension effect driven mostly by alcohol. They say nothing about quality.
The mechanisms
How each thing works, at the scope it has been established — and what it does not explain. 19 topics.
Science
Acidity: the acids in wine and what each does
Tartaric, malic, lactic, citric and acetic acid — where they come from, how they differ, and why pH and total acidity are not the same measurement.
Science
Cool climate and warm climate
The single most useful distinction in wine — what it actually changes in the grape, and where the shorthand breaks down.
Science
Minerality: what the evidence actually shows
A word almost every taster uses, no agreed chemical definition, and no demonstrated route from vineyard rock to matching flavour.
Science
Oxygen: friend and enemy at every stage
Oxygen is needed in some quantities at some moments and damaging in others. What matters is how much, when, and to which wine.
Science
Sulfur dioxide and sulfites
What SO₂ does in wine, why free and bound fractions are different, why every wine contains some, and what the label is actually telling you.
Science
Tannin: what it actually is
The phenolic compounds behind astringency — where they come from, what they do in the mouth, and why they soften with age.
Science
Where wine aromas come from
Four origins — the grape, fermentation, oak and ageing — and why a tasting note is not a list of chemicals.
Science
Alcohol: what ethanol does in the glass
Ethanol contributes body, carries aroma, changes how sweetness and bitterness are perceived, and is felt as warmth — several distinct effects that get bundled into one word.
Science
Colour: where it comes from and how it changes
Anthocyanins in black grape skins, their reaction with tannin over time, and why reds fade towards brick while whites deepen towards gold.
Science
Oxidation: the chemistry, the fault and the style
The same reactions that ruin a forgotten bottle also produce Oloroso, Madeira and Vin Jaune. Intent is what separates them.
Science
Reduction: sulfur compounds and struck-match aromas
Low-oxygen conditions can leave volatile sulfur compounds in a wine — some transient and prized, some genuinely faulty.
Science
Smoke taint
How wildfire smoke reaches wine, why it can hide until after fermentation, and what growers can and cannot do about it.
Science
Soil and geology: what actually reaches the glass
Parent rock, soil and root-zone properties are three different things — and the ones that matter act through water and warmth, not flavour.
Science
Sugar in wine: from grape to residual sweetness
Where grape sugar comes from, what happens to it in fermentation, and why residual sugar and perceived sweetness are two different things.
Science
Tertiary aromas: what develops with age
The dried fruit, nut, leather, mushroom and honey characters that appear in bottle — and why they are possibilities rather than guarantees.
Science
The human factor: how much of a wine is decisions
A corrective to over-crediting geography. Picking date, extraction, oak, blending and ageing move a wine further than most site differences do.
Science
What actually makes a wine good?
Balance, intensity, complexity, typicity, length and the absence of faults — and where objective assessment stops and preference begins.
Science
Why Sauvignon Blanc smells the way it does
Two compound families pulling in different directions — pyrazines for grass and pepper, thiols for passionfruit — and how climate and winemaking shift the balance.
Science
Old World and New World: a shorthand past its usefulness
A distinction that once tracked real differences in regulation and style, and now mostly tracks neither.
Going further
Named compounds, measurement, and the places where the evidence runs out. 13 topics.
Science
The chemistry of alcoholic fermentation
What yeast actually do to sugar, why the schoolbook equation is a simplification, and where the flavour compounds come from.
Science
Yeast: the organisms doing the work
Saccharomyces cerevisiae, the non-Saccharomyces species that start most spontaneous ferments, and what strain choice does and does not determine.
Science
Astringency: why tannin is a feeling, not a taste
Astringency is a tactile sensation carried by the trigeminal nerve, not a taste carried by taste buds — which is why it behaves so differently from bitterness.
Science
Diacetyl and the buttery note
A bacterial by-product of malolactic fermentation, and the actual explanation for buttery Chardonnay.
Science
Esters and fermentation fruit
The compounds behind banana, pear-drop and tropical notes in young wine — made by yeast, and short-lived.
Science
Methoxypyrazines and green character
The compounds behind green pepper in Cabernet and grassiness in Sauvignon Blanc — and why sunlight on the fruit matters more than heat.
Science
pH and total acidity are not the same thing
One measures acid strength, the other measures acid quantity — and a wine can be high in both, or high in one and not the other.
Science
Rotundone and peppery Syrah
A single compound that accounts for black-pepper aroma — and which around a fifth of people cannot smell at all.
Science
TDN: the petrol note in Riesling
A compound that accumulates as Riesling ages, associated with sunlight and water stress in the vineyard — and divisive rather than faulty.
Science
Terpenes and floral aromatics
Grape-derived compounds behind the rose, lychee and citrus-blossom character of Muscat, Gewürztraminer and Riesling.
Science
Thiols: passionfruit, grapefruit and box hedge
Sulfur-containing compounds released by yeast from odourless grape precursors — the chemistry behind Marlborough Sauvignon Blanc.
Science
Typicity and quality are different questions
Whether a wine tastes like what it says on the label, and whether it is any good, are separate judgements that appellation systems frequently merge.
Science
Volatile phenols: smoke, spice and sticking plaster
One compound family behind three very different things — oak toast character, Brettanomyces spoilage, and smoke taint.
Elsewhere on WineHQ
Myths
43 things people believe about wine
Widely repeated claims, checked against the evidence — with what is genuinely true underneath each one.
How it's made
From vine to glass
Every production route, laid out by colour, method and style.
Viticulture
The vineyard
The vine's year, the climate that shapes it, and the hazards that decide a vintage.
History
How wine got this way
Eight thousand years of evidence, with the folklore separated out.
24 compounds are named across these pages. Each is described by where it comes from and, separately, what it is associated with — because those are different kinds of statement.