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.

Two philosophies: At one end, the winemaker treats every step as a lever to pull toward a chosen style: cultured yeast, temperature control, additions, fining and filtering. At the other, the minimal-intervention or “natural” approach: no cultured yeast, little or no sulfur dioxide, often no fining or filtering, and the wine allowed to be whatever the grapes and the year make it. Most producers sit somewhere between the two, and the exam wants you to know what each choice does rather than which camp is right.

What a Grape Is Made Of

Every winemaking decision comes back to which part of the grape you are working with:

Oak adds tannin of its own, which is part of why barrel choice matters later on.

Oxygen: Friend and Enemy

The textbook puts oxygen right at the front of winemaking, because almost every choice in the winery is partly a choice about air.

Keeping Oxygen Out (Protective or Anaerobic Winemaking)

To keep fresh fruit character, winemakers pick at night, keep the fruit cool, and flush presses, pipes and tanks with carbon dioxide or nitrogen so the juice never meets air. Inert vessels (stainless steel, lined concrete) continue the protection. This is the default for fresh, fruity whites and rosés.

Letting Oxygen In

The counter-view is that a little oxygen adds complexity. Oak barrels let small amounts through, and the smaller the barrel the more oxygen per litre of wine, which is why barriques have a stronger effect than large casks (and why wines rarely stay in a barrique beyond a couple of years). At the far end, some styles are deliberately oxidised in partly filled vessels: Oloroso Sherry, Tawny Port, Rutherglen Muscat.

Too much, in a wine that was meant to be fresh, gives a stale, flat wine; in extreme cases, vinegar.

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 (the species that does the work is Saccharomyces cerevisiae) 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 will not start below about 5°C. It stops when the yeast run out of sugar (producing a dry wine), when the temperature climbs above about 35°C and kills them, or when the sugar or alcohol level gets too high for them, around 15–16% alcohol.

The winemaker can also stop fermentation deliberately before all the sugar is consumed, either by killing the yeast (a large dose of sulfur dioxide, or adding grape spirit as in fortified wine) or by removing them (chill the wine below 5°C so they stop, then filter them out). 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.
📝

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.

White Wine Fermentation (about 12–22°C)

Used for white and rosé wines. Cooler temperatures slow fermentation and hold on to the volatile floral and fruit aromas that would otherwise be blown off with the CO₂. Wines fermented at the cool end tend to be fresh, fruit-forward, and aromatic; this is standard practice for most commercial Sauvignon Blanc, Riesling, Pinot Grigio, and similar styles. Go too cold and the fermentation gives pear-drop aromas and loses varietal fruit, so the low end of the range is a floor, not a target.

Red Wine Fermentation (about 20–32°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. Pumping over the cap releases heat as well as mixing the skins.

Fermentation will not start below about 5°C. Above about 35°C the yeast die, the fermentation stops, and the wine picks up cooked flavours, so even red fermentations are kept under that ceiling.

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.

From Grapes to Must: Reception, Pressing and Adjustments

The unfermented juice (with or without skins) is called the must. Getting from grapes to must involves a handful of choices the exam expects you to know.

Grape Reception

A dose of sulfur dioxide usually goes on at reception to protect the fruit. Premium fruit passes over a sorting table to pull out damaged berries and MOG. Destemming and crushing are both optional: machine-picked fruit arrives without stems anyway, and crushing is done gently enough to split the skins and release free-run juice without breaking the seeds, which would release bitter oils.

Pressing

For white wine the grapes are pressed before fermentation; for red, the skins ferment in the juice and pressing comes after. Two kinds of press: the traditional vertical basket press, worked by screw or lever, and the modern pneumatic press, where an inflatable bladder squeezes the grapes gently against a perforated drum. Closed pneumatic presses can be flushed with inert gas to keep oxygen off the juice. The press runs in stages, and winemakers often keep the press fractions separate: early juice is closest to free-run, later fractions are coarser (and, in reds, darker and more tannic), so they can be blended in or left out.

Adjusting the Must

Where the law allows, winemakers correct the raw material:

  • Enrichment: adding sugar to raise the potential alcohol. One route is rectified concentrated grape must (RCGM), a neutral grape-sugar syrup; the other is chaptalisation, which uses sugar from another source such as beet. Both are strictly controlled or banned in many places, and overdoing it gives a thin, hard wine where the alcohol outruns the fruit.
  • Removing water from the must concentrates everything, including any faults, and cuts the volume of wine. Machinery also exists to remove alcohol from a finished wine.
  • Acidification: adding tartaric acid. Common in warm regions, and allowed in the warmer EU zones. One of the most routine adjustments in high-volume winemaking.
  • Deacidification: adding an alkaline agent to take acid out, for cool-climate must that is too tart.

Malolactic Fermentation (MLF)

Despite its name, malolactic fermentation is not a true fermentation, it is a bacterial conversion (the textbook calls it malolactic conversion, MLC, for that reason). 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. It is encouraged by keeping the wine warm and holding back sulfur dioxide, and blocked by cool storage, a dose of SO₂, or filtering the bacteria out.

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.
📝

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
The textbook splits oak by species. European oak (France, Hungary, Russia and elsewhere) has a tighter grain and contributes more subtle flavours: toast, spice, cedar, and fine tannin; the species is broadly similar across Europe, though the most prized forests are French. American oak is a different species with a wider grain, contributing more overt flavours: vanilla, coconut, dill, and a sweeter, more obvious oak character. French oak is the most expensive. Whatever the origin, hygiene matters: a tainted barrel ruins the wine in it.
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 both the oak flavour and the oxygen exposure are stronger. Larger vessels (500L demi-muids, and casks of 2,000 litres and more such as foudres or Slavonian botti) have a lower surface-to-volume ratio, giving gentler oak influence and slower 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 bend the staves, and the heat transforms the flavour compounds in the wood. 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 and are not normally used for premium ones. Some producers pair them with micro-oxygenation, bubbling tiny amounts of oxygen through the tank to mimic what a barrel does; the textbook notes this is still being researched.

Concrete

Concrete vats are an older technology back in fashion (sometimes now egg-shaped). They are usually lined with epoxy resin, which makes them inert like stainless steel. The thick walls regulate temperature naturally, which is their main practical advantage; the drawbacks are that they are harder to clean and cannot be chilled as precisely as a jacketed steel tank. Used by producers in Bordeaux, the Rhône, and increasingly in the New World. Glass is the other inert option, for storage rather than fermentation.

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

Letting the lees settle and racking the clear wine off them is also the first and simplest of the three clarification tools (the others are fining and filtration, below). A centrifuge does the same job in minutes, at a price.

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 (at least 12 months on the lees, inside a minimum 15 months from tirage to release for non-vintage and 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).

Why it has to be monitored: once SO₂ has reacted with oxygen or other compounds it is “bound” and no longer protects the wine, so winemakers measure the free SO₂ and top it up. The yeast that run fermentation tolerate it better than spoilage yeasts and bacteria do, which is what makes it so useful. There are legal upper limits, it can trigger allergic reactions in a small number of people (hence “contains sulfites” on labels), and too much makes a wine smell acrid and taste harsh.

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). The textbook gives three clarification tools, sedimentation (settling and racking), fining and filtration, and four areas of stability. 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 makes the tiny haze-forming particles (proteins, tannins, colour compounds) clump together faster than they would on their own, so they settle out and can be removed. Because those same compounds would otherwise form a haze later in the bottle, fining doubles as a stabilisation step. 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. Two kinds: depth filtration uses a thick bed of material that traps particles as the wine passes through, and can handle very cloudy wine, even gross lees. Surface filtration uses a fine membrane that stops anything bigger than its pores; it is more expensive and clogs easily, so it is used after depth filtration. If the pores are small enough to stop yeast and bacteria, that is sterile filtration, which matters 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.

The Four Kinds of Stability

“Stable” means the wine will behave predictably in the bottle. The textbook names four things to guard against:

Tartrate Stability (Cold Stabilisation)

Tartrate crystals (harmless potassium bitartrate) can form in a bottle, clear in white wine and purple-stained in red, and a cold cellar speeds them up. Consumers mistake them for glass or sugar. To prevent it, the wine is chilled to below 0°C for a short time so the crystals form in the tank, then they are removed, for example by filtration. Cold stabilisation is cosmetic: the crystals are completely harmless and have no effect on flavour.

Microbiological Stability

Yeast and bacteria left in a wine can referment it or spoil it. Clean equipment is the first line of defence. Only fortified wines are safe by nature; dry, high-acid wines that have been through malolactic conversion resist spoilage well; the wines most at risk are low in alcohol, low in acid or sweet, and those get sulfur dioxide plus sterile filtration before bottling.

Oxygen Stability

Air picked up at bottling ages a wine before it reaches the shelf. Bottles are flushed with carbon dioxide or nitrogen before filling so the wine meets as little oxygen as possible.

Packaging

The container is a winemaking decision too, and the textbook compares three:

Packaging Options Compared
Glass bottle
Inert, strong and rigid, so it is the only option for wines meant to age. Heavy to ship, which is why lighter-weight glass is spreading and why some wine now travels in bulk to be bottled in the market it is sold in.
PET (plastic) bottle
Light and unbreakable, but it lets oxygen in, so the wine loses freshness within months. For wine that will be drunk soon.
Bag-in-box
The bag collapses as wine is drawn off, so an open box keeps far better than an open bottle. The bag still lets some air through, though, so the wine should be drunk within months of packaging, not years.

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

Blending can happen at any stage, from the must right up to the eve of bottling.

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 slow oxygen exchange through the cork (the amount varies from cork to cork), which aids development. Risk: cork taint (TCA contamination), which affects a small and declining proportion of bottles sealed with natural cork.
  • Technical cork: made from agglomerated cork pieces. Cheap versions suit wines drunk within a couple of years; premium versions treated to minimise TCA can suit long ageing.
  • Screw cap (Stelvin): oxygen ingress is low but can be chosen by liner, so it is not strictly “zero”. Eliminates cork taint. Some research suggests wines keep their fruit longer under screw cap than under cork. Widespread in New Zealand and Australia, including for ageworthy wines; consumer acceptance still varies by market, which is why some producers keep cork for export.
  • Synthetic cork: no cork taint risk. Older types let in too much oxygen for anything beyond early drinking; premium long-life versions now exist.
  • Glass stopper (Vino-Lok): inert and elegant, but expensive. Niche use.

The textbook’s framing: closure is a stylistic winemaking choice, because each one sets how much oxygen the wine will see over its life.

Timing

When the wine is bottled affects its final character. Many wines, including some premium ones such as Australian Riesling, are bottled within a few months of harvest from an inert vessel, to keep their primary fruit. Wine that spends longer in barrel before bottling absorbs more oak character and undergoes more oxygen exposure. Long pre-bottling maturation only pays off for wines with the tannin, acidity or alcohol to carry it and flavours that develop well. 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 of release. But the finest wines from Bordeaux, Burgundy, Barolo, Riesling, vintage Port, and similar regions can develop beautifully over decades. The chemistry of why is still poorly understood; what is understood is the storage that lets it happen: cool (about 10–15°C), dark, a steady temperature, constant humidity, and cork-sealed bottles on their side (Chapter 3).

What the Exam Expects You to Know

📝

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

Close this page and try to answer these from memory: