Section 2 · Factors Affecting the Style, Quality and Price of Wine

Common Elements in Winemaking and Maturation

Once grapes arrive at the winery, a new set of decisions begins. This chapter covers the winemaking processes and maturation choices that are common to virtually all styles of wine, from alcoholic fermentation to oak ageing to bottling. Later chapters (8–10) cover the specifics of white, red, rosé, sweet, and sparkling winemaking, but this chapter lays the foundation you need for all of them.

The Winemaking Process: A Bird’s-Eye View

Winemaking Process Flow

Grapes Arrive Sort / Destem / Crush Alcoholic Fermentation sugar + yeast = alcohol + CO2 Malolactic (optional) Maturation tank, barrel, amphora Clarify / Stabilise / Blend Bottling Cultured or wild yeast Oak, steel, concrete Cork, screwcap

At its simplest, winemaking follows this sequence:

Not every wine goes through every step, and the order can vary. But this is the general framework. Let’s look at each element in detail.

Alcoholic Fermentation

Alcoholic fermentation is the defining process of winemaking, it is what transforms grape juice into wine. The chemistry is straightforward:

The equation: Sugar (glucose + fructose) + Yeast → Alcohol (ethanol) + Carbon Dioxide (CO₂) + Heat. This is the fundamental reaction. The yeast consumes the sugar and produces alcohol and CO₂ as by-products. The CO₂ is usually allowed to escape (unless you’re making sparkling wine), and the heat must be managed to avoid damaging the wine.

How Fermentation Works

Yeast cells consume the sugars naturally present in grape juice. As they do, they multiply, producing alcohol, CO₂, and a wide range of other by-products (esters, aldehydes, higher alcohols) that contribute to the wine’s aroma and flavour complexity.

Fermentation typically takes between one and four weeks, depending on the temperature and style of wine. It stops naturally when either all the sugar has been consumed (producing a dry wine) or when the alcohol level reaches about 15–16% (at which point the alcohol itself becomes toxic to the yeast and kills it).

The winemaker can also stop fermentation deliberately before all the sugar is consumed, by chilling the wine rapidly (to stun the yeast), by filtering the yeast out, or by adding spirit (as in fortified wine production). Stopping fermentation early leaves residual sugar in the wine, producing a sweet or off-dry style.

The Role of Yeast

Yeast is the engine of fermentation. The winemaker’s choice of yeast is a significant stylistic decision:

Cultured (Selected) Yeast

Commercially produced yeast strains, selected for specific characteristics. The winemaker adds them directly to the grape juice. This gives:

  • Reliability: cultured yeasts are predictable. They start fermenting quickly and finish cleanly.
  • Control: different strains produce different aroma profiles. A winemaker can choose a strain that enhances tropical fruit aromas (for Sauvignon Blanc, for example) or one that is neutral (to let the fruit character dominate).
  • Lower risk: less chance of stuck or faulty fermentation.

Wild (Indigenous / Ambient) Yeast

Natural yeasts that are present on grape skins, in the winery environment, and in the vineyard. Instead of adding cultured yeast, the winemaker allows fermentation to start spontaneously with whatever yeasts are naturally present.

  • Complexity: multiple strains of wild yeast may be active during fermentation, each contributing different flavour compounds. Proponents argue this produces more complex, terroir-expressive wines.
  • Risk: wild yeast fermentations are less predictable. They may start slowly, get stuck (stop before all sugar is consumed), or produce off-flavours.
  • Association with quality: many prestigious estates (especially in Burgundy, the Rhône, and among natural wine producers worldwide) prefer wild yeast fermentations.
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Exam Tip

Know both approaches and their trade-offs. Cultured yeast = reliable, clean, controllable. Wild yeast = potentially more complex but higher risk. The exam may ask you to explain why a producer might choose one over the other.

Temperature Control During Fermentation

Fermentation generates heat. If not controlled, this heat can damage the wine, killing the yeast (causing a stuck fermentation) or producing cooked, stewed flavours. Temperature control is therefore essential.

Cool Fermentation (12–18°C)

Used primarily for white and rosé wines. Cool temperatures slow down fermentation, which preserves delicate, fruity, floral aromas. Wines fermented cool tend to be fresh, fruit-forward, and aromatic. This is standard practice for most commercial Sauvignon Blanc, Riesling, Pinot Grigio, and similar styles.

Warm Fermentation (20–30°C)

Used for red wines. Higher temperatures are needed to extract colour, tannin, and flavour from the grape skins (reds are fermented with their skins in contact with the juice). Warmer fermentation also produces different aroma compounds, less primary fruit, more complex, spicy, savoury notes.

Very high temperatures (above ~35°C) risk killing the yeast and producing unpleasant cooked flavours.

Temperature control equipment: Most modern wineries use stainless steel tanks with cooling jackets: double-walled tanks through which cold water can be circulated to control temperature precisely. This was one of the most important technological advances in 20th-century winemaking, particularly in warm climates where fermentation temperatures could previously be very difficult to manage.

Malolactic Fermentation (MLF)

Despite its name, malolactic fermentation is not a true fermentation, it is a bacterial conversion. Bacteria (usually Oenococcus oeni) convert malic acid (sharp, green-apple-like) into lactic acid (softer, rounder, dairy-like). CO₂ is also produced, but in tiny quantities.

What MLF Does to the Wine

When MLF Is Used

MLF Usage by Wine Style
Red wines
Almost always. Virtually all red wines undergo MLF. The softening of acidity and the textural improvement are desirable in reds. MLF is standard practice in Bordeaux, Burgundy, the Rhône, Rioja, Barossa, essentially everywhere reds are made.
Full-bodied white wines
Often. Oaked Chardonnay is the classic example. MLF gives Meursault, Puligny-Montrachet, and many California Chardonnays their characteristic creamy, buttery quality. White Burgundy and barrel-fermented Chardonnay from around the world typically undergo partial or full MLF.
Light, aromatic white wines
Usually not. Wines where you want to preserve freshness, crispness, and primary fruit aromas, like Riesling, Sauvignon Blanc, Albariño, Muscadet, are deliberately prevented from undergoing MLF. The winemaker suppresses it by keeping the wine cool, adding sulphur dioxide, or filtering out the bacteria.
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Exam Angle

The key exam concept is: MLF softens acidity and adds creaminess. If a question describes a wine as having "buttery" or "creamy" character, MLF is likely involved. If a wine is described as "crisp," "zesty," or having "green apple acidity," MLF has probably been avoided. Always connect the process to the sensory outcome.

Fermentation and Maturation Vessels

The vessel in which wine is fermented and/or aged has a significant impact on the final wine. Each type of vessel contributes differently to the wine’s flavour, texture, and development.

Stainless Steel

The workhorse of modern winemaking. Stainless steel is inert: it does not interact with or add any flavour to the wine. This makes it ideal when the winemaker wants to preserve pure fruit character and freshness.

  • Temperature control: easy to control with cooling jackets
  • Hygiene: easy to clean, reducing risk of bacterial contamination
  • Cost: durable and reusable indefinitely; cost-effective over time
  • Airtight: minimal oxygen exposure, preserving freshness

Used for: Most white wines, rosés, and many reds (especially for fermentation, even if the wine is later aged in oak).

Oak Barrels

Oak is the most important alternative to stainless steel. It affects wine in three distinct ways:

  • Flavour contribution: oak imparts specific aromas and flavours to the wine, including vanilla, toast, cedar, clove, coconut, caramel, and smoke. The specific flavours depend on the origin and treatment of the oak.
  • Tannin contribution: oak adds tannins to the wine. Oak tannins are different from grape tannins, they tend to be finer and silkier, adding structure without harshness.
  • Controlled oxygen exposure: oak barrels are slightly porous, allowing tiny amounts of oxygen to reach the wine over time. This micro-oxygenation softens tannins, stabilises colour, and promotes complexity. It is why barrel-aged wines develop more complex, evolved flavours than tank-aged wines.

Key Oak Variables

Oak Decisions and Their Effects
Origin: French vs American
French oak has a tighter grain, contributing more subtle, elegant flavours: toast, spice, cedar, and fine tannin. American oak has a wider grain, contributing more overt flavours: vanilla, coconut, dill, and sometimes a sweeter, more obvious oak character. French oak is more expensive. Some regions use other origins, Eastern European (Hungarian, Slavonian) oak is increasingly popular, offering a profile between French and American.
Size
The standard French barrel (barrique) holds 225 litres (Bordeaux) or 228 litres (Burgundy pièce). Smaller barrels have a higher ratio of wood surface to wine volume, so the oak influence is stronger. Larger vessels (500L demi-muids, 1,000–10,000L foudres or Slavonian botti) have a lower surface-to-volume ratio, providing gentler oak influence and more oxygen exchange. Traditional Barolo and Barbaresco producers often use large Slavonian oak botti for this reason.
Toast Level
Barrels are heated (toasted) during manufacturing to shape the staves. Light toast emphasises vanilla and raw wood character. Medium toast brings out toast, caramel, and spice. Heavy toast adds smoke, dark chocolate, and coffee notes, but can mask fruit character if overdone.
New vs Old
New oak imparts the most flavour and tannin. After one use, a barrel’s flavour contribution drops dramatically. By the third or fourth use, a barrel is effectively neutral: it still provides the benefit of micro-oxygenation but adds almost no oak flavour. This is why the percentage of new oak used is such an important stylistic decision. A top Bordeaux château might use 80–100% new oak for its grand vin; a producer wanting a more fruit-forward style might use 20–30% new oak or all older barrels.
The cost of oak: A new French oak barrique costs roughly €800–1,000. American oak barrels are cheaper (roughly $400–600). For a small producer ageing wine in 100% new French oak, barrel costs alone can add several euros per bottle. This is a significant component of why premium, oak-aged wines cost more than unoaked wines.

Oak Alternatives

For producers who want some oak character without the cost of barrels, there are cheaper alternatives:

Oak alternatives are widely used for everyday wines. They are generally prohibited in the production of premium appellation wines in Europe (e.g., you cannot use oak chips in Bordeaux Grand Cru wine).

Concrete

Concrete tanks (sometimes egg-shaped) are experiencing a revival. Like stainless steel, concrete is largely inert (it does not add flavour), but it is slightly porous, providing gentle micro-oxygenation similar to old oak barrels. Concrete also has natural insulating properties, maintaining a stable temperature. Some winemakers believe concrete promotes a distinctive textural quality. Used by producers in Bordeaux, the Rhône, and increasingly in the New World.

Amphora (Clay)

Clay vessels are the oldest winemaking containers, dating back thousands of years to Georgia’s qvevri tradition. Modern winemakers are rediscovering them for their unique properties: clay is porous (like oak, allowing micro-oxygenation), adds a subtle earthy character, and is seen as the most "natural" vessel. Amphora-aged wines are associated with the natural wine movement and with producers seeking to express terroir without any oak influence.

Ageing on Lees

Lees are the dead yeast cells and other solid particles that settle to the bottom of a wine vessel after fermentation. Far from being waste, lees are a powerful winemaking tool that significantly affects wine texture and flavour.

Types of Lees

What Lees Contact Does

When wine is kept in contact with fine lees over time, the yeast cells gradually break down (a process called autolysis). This releases compounds into the wine that affect it in several important ways:

Bâtonnage (Lees Stirring)

Some winemakers regularly stir the fine lees back into suspension, a technique called bâtonnage (bâton is French for "stick"). This increases the interaction between the lees and the wine, accelerating the effects described above. It is particularly common in the production of barrel-fermented Chardonnay (white Burgundy being the benchmark) and some traditional-method sparkling wines.

Where lees ageing matters most: Champagne (minimum 12 months on lees for non-vintage, 36 months for vintage, top producers go much longer), Muscadet sur lie, white Burgundy (Meursault, Puligny-Montrachet), barrel-fermented Chardonnay globally, and some Sauvignon Blanc styles (like barrel-fermented Fumé Blanc).

Sulphur Dioxide (SO₂)

Sulphur dioxide is the most important preservative in winemaking. It has been used for centuries and is almost universal, even most organic wines use some SO₂. Understanding its role is important for the exam.

What SO₂ Does

When SO₂ Is Added

SO₂ may be added at multiple points: to the grapes shortly after harvest (to prevent premature oxidation and wild yeast activity), during winemaking (after fermentation, to protect the new wine), and at bottling (to ensure stability during storage and transport).

Natural wine and SO₂: The "natural wine" movement advocates for minimal or no SO₂ addition. Wines made without SO₂ can be vibrant and expressive, but they are also less stable, they are more vulnerable to oxidation and spoilage, and they tend to have a shorter shelf life. This is a trade-off that divides the wine world, and the exam may ask you about the risks and benefits of low-SO₂ winemaking.

Clarification and Stabilisation

Before bottling, most wines are clarified (made visually clear) and stabilised (protected against haze, deposits, or refermentation in bottle). These processes are routine but represent important winemaker decisions.

Racking

Transferring wine from one vessel to another, leaving the sediment (lees) behind. This is the gentlest form of clarification and may be done several times during maturation. Each racking also exposes the wine to a small amount of oxygen, which can help the wine develop and soften tannins.

Fining

Adding a fining agent to the wine. The agent binds to specific unwanted particles (suspended solids, tannins, proteins, or colour compounds) and settles to the bottom, where it is removed. Common fining agents include:

  • Bentonite: a clay that removes proteins (preventing haze in white wines). The most common fining agent.
  • Egg whites: traditionally used for red wines. They soften harsh tannins and clarify the wine gently. Classic in Bordeaux.
  • Casein (milk protein), removes browning and some off-flavours from white wines.
  • PVPP (polyvinylpolypyrrolidone), a synthetic agent that removes browning compounds.
  • Isinglass (fish protein) and gelatin: used for gentle clarification.

Note: the use of animal-derived fining agents (egg whites, casein, isinglass, gelatin) means that many wines are not technically vegan, even though no animal products remain in the finished wine. Some producers now use plant-based or mineral alternatives to produce vegan-certified wines.

Filtration

Passing the wine through a filter to remove particles. Filtration can range from coarse (removing only large particles) to sterile (removing virtually all yeast and bacteria). Sterile filtration is important for wines with residual sugar, because any remaining yeast could cause refermentation in bottle.

The debate: Some quality-focused producers choose to bottle their wines unfined and unfiltered, believing that fining and filtration strip out flavour, texture, and complexity. These wines may throw a natural sediment in bottle and may appear slightly hazy, but proponents argue they are more complete and expressive. This is more common among top Burgundy, northern Rhône, and natural wine producers.

Cold Stabilisation

Chilling the wine to near-freezing temperatures for a period (usually a few days to a couple of weeks). This causes tartrate crystals (harmless potassium bitartrate) to form and precipitate out. If this is not done, the crystals may form later in the bottle, alarming consumers who mistake them for glass shards or sugar. Cold stabilisation is cosmetic, the crystals are completely harmless and have no effect on flavour.

Blending

Blending is one of the winemaker’s most important tools. Most wines are blends of some kind, if not of different grape varieties, then of different vineyard parcels, fermentation vessels, or maturation treatments.

Why Blend?

Types of Blending

Bottling

Bottling is the final step in winemaking. While it might seem straightforward, several decisions at this stage affect the wine’s quality and longevity:

Closure

The choice of closure affects the wine’s ageing trajectory:

  • Natural cork: traditional, allows micro-oxygenation (slow oxygen exchange through the cork), which aids development. Risk: cork taint (TCA contamination), which affects roughly 2–5% of bottles sealed with natural cork.
  • Screw cap (Stelvin): creates a near-perfect seal with virtually no oxygen transmission. Eliminates cork taint. Ideal for wines meant to be drunk fresh. Now also used for some ageworthy wines (widespread in New Zealand and Australia). The debate about long-term ageing under screw cap vs cork continues.
  • Synthetic cork: no cork taint risk, but may allow more oxygen transmission than natural cork. Used mainly for wines intended for early consumption.
  • Glass stopper (Vino-Lok): inert and elegant, but expensive. Niche use.

Timing

When the wine is bottled affects its final character. Wine that spends more time in barrel before bottling absorbs more oak character and undergoes more micro-oxygenation. Wine bottled earlier retains more primary fruit character. Many appellation laws specify minimum maturation periods before bottling is permitted (e.g., Rioja Reserva requires a minimum total of 36 months, with at least 12 in oak).

Bottle Ageing

After bottling, wine continues to evolve. In the sealed environment of the bottle, the wine undergoes slow chemical changes:

  • Tannins polymerise (join together in longer chains), becoming softer and silkier
  • Primary fruit aromas evolve into secondary and tertiary aromas (leather, earth, mushroom, dried fruit, tobacco)
  • Colour changes, reds evolve from deep purple to garnet to tawny; whites deepen from pale to gold to amber

Not all wines benefit from bottle ageing. Most of the world’s wine is made to be drunk within a year or two of release. But the finest wines from Bordeaux, Burgundy, Barolo, Riesling, vintage Port, and similar regions can develop beautifully over decades.

What the Exam Expects You to Know

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Short Answer Strategy

Winemaking questions are always about connecting process to outcome. Don’t just describe what a technique is, explain what it does to the wine you taste. "Barrel fermentation and ageing in new French oak barriques for 12 months, with regular bâtonnage, gives this Chardonnay its full body, creamy texture, and vanilla and toast complexity, while the controlled micro-oxygenation through the barrel’s pores softens the structure and integrates the oak."

Active Recall Prompts

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