4.1 Must Adjustments, Oxygen, SO2, and Vessels

Once grapes leave the vineyard, winery decisions determine whether potential quality is preserved, enhanced, or lost. WSET Level 3 LO1 Range 3 expects you to link must adjustments, oxygen and SO2 management, vessel choice, and the two core conversions — alcoholic fermentation (AF) and malolactic conversion (MLC) — to finished style, quality level, and price.

---

Must Adjustments: Enrichment, Acidification, Deacidification

Must is the juice (and for reds, juice plus skins) before or during fermentation. Natural composition varies with climate, vintage weather, and ripeness. When the balance is outside the winemaker’s target, legal adjustments may be used.

Enrichment (Chaptalisation and Related Techniques)

Enrichment adds sugar (or concentrated grape must) before or during AF to raise potential alcohol. It is most relevant in cool climates or cool vintages where grapes struggle to reach sufficient sugar. Enrichment does not add flavour by itself — it mainly raises alcohol, which affects body and perceived sweetness balance. Many warm regions ban or tightly restrict enrichment because grapes already reach high sugar.

Related options include adding rectified concentrated grape must (RCGM) or concentrated grape must, which may be preferred where chaptalisation with sucrose is restricted. Exam scenarios often ask: cool vintage, low sugar, desired mid-weight dry wine → enrichment is the logical tool where law allows.

Acidification

Acidification (typically with tartaric acid) raises total acidity and lowers pH. It is common in warm/hot climates or hot vintages where acid falls and pH rises as grapes ripen. Benefits include fresher taste, better microbial stability (lower pH), and brighter colour in reds. Over-acidification tastes hard and green; skilled producers dose carefully after analysis.

Deacidification

Deacidification reduces excess acidity in cool climates or underripe fruit. Methods include chemical deacidification (e.g. calcium carbonate) or biological routes such as encouraging MLC (malic → lactic acid, softer). Deacidification protects drinkability when raw acidity would dominate; it can also reduce ageing potential if structure is stripped too far.

Exam link: A warm-climate entry-level white that tastes soft and flat may have had insufficient acidification (or grapes picked too late). A premium cool-climate producer may refuse enrichment to keep alcohol moderate and authenticity high — a quality and marketing choice that often supports a higher price.

---

Oxygen: Friend and Enemy

Oxygen is essential in small, controlled amounts and damaging in excess.

Useful roles of oxygen

• Early must/juice aeration can help yeast health and avoid stuck ferments
• Limited oxygen during red fermentation supports colour stability and softer tannin polymerisation
• Controlled oxygen during maturation (e.g. barrel) builds complexity and integrates oak

Damaging roles of oxygen

• Enzymatic browning of white juice (polyphenol oxidase) → dull colour and lost freshness
• Oxidation of aroma compounds → bruised apple, sherry-like notes unwanted in fresh styles
• Favouring acetic acid bacteria and spoilage organisms if SO2 and hygiene are weak

Management tools include inert gas blanketing (CO2, N2, argon), closed transfers, early SO2, cool temperatures, and choosing inert vs oxygen-permeable vessels. Hyperoxidation of white must (deliberate early oxygen) is a specialised technique that can stabilise colour and remove unstable phenolics before AF — opposite of the “protect at all costs” approach used for aromatic whites.

---

Sulfur Dioxide (SO2)

SO2 is the winemaker’s primary antioxidant and antimicrobial. It exists as free and bound forms; free SO2 (especially molecular SO2 at low pH) is the active fraction.

Key roles

1. Antioxidant — binds oxygen and inhibits oxidative enzymes
2. Antimicrobial — suppresses wild yeast and bacteria, protecting desired fermentations and finished wine
3. Antioxidasic — limits enzymatic browning in white must

Timing matters. SO2 is commonly added at grape reception/crush, after AF (and after MLC if used), and at bottling. High free SO2 at tasting can smell sharp or matchstick-like and suppress aroma — a fault or poor finishing if excessive. Low SO2 plus poor hygiene risks spoilage. Many premium producers minimise doses through healthy fruit, low pH, cleanliness, and inert handling, supporting “low intervention” marketing without abandoning SO2 entirely.

Exam scenario: Pale, aromatic Sauvignon Blanc with intense passionfruit → expect protective handling + moderate SO2 + cool stainless AF. Oxidised, dull supermarket white with brown tint → likely oxygen exposure and/or inadequate SO2 protection at critical stages.

---

Vessels: Stainless Steel, Oak, Concrete

Vessel choice is one of the clearest style and price signals on the exam.

Stainless Steel

Inert, easy to clean, excellent temperature control. No flavour addition and minimal oxygen ingress when sealed. Ideal for fresh, fruit-driven whites and rosés, and for fermenting or storing wine before oak ageing. Relatively high capital cost but long life and low flavour risk — widely used from entry-level to premium aromatic styles.

Oak (Barrels and Large Casks)

Oak allows slow oxygen ingress and can extract flavour compounds (vanillin, toast, spice) and tannins, especially from new small barrels. Effects depend on species (American vs French), toast level, barrel age, and size (barrique vs foudre). New oak is expensive and labour-intensive → strongly associated with higher price wines. Neutral large oak gives micro-oxygenation with little flavour — classic for some traditional reds and whites seeking texture without overt oak aroma.

Concrete

Increasingly used eggs, tanks, and amphora-like shapes. Provides thermal inertia, some micro-oxygenation without oak flavour, and distinctive convection currents in egg shapes that keep lees in suspension. Signals artisanal or premium positioning more often than cheapest bulk production.

---

Alcoholic Fermentation (AF) Overview

Alcoholic fermentation is the conversion of grape sugars to ethanol + CO2 (plus heat and secondary metabolites) by yeast. Key controllable factors:

• Yeast — cultured Saccharomyces cerevisiae for reliability; wild/indigenous yeasts for complexity but higher risk of stuck or spoilage ferments
• Temperature — cool AF (≈12–16°C) preserves volatile aromatics in whites; warmer AF (≈20–32°C) aids extraction and body in reds
• Nutrients — yeast needs nitrogen and micronutrients; deficiency risks H2S (rotten egg) and stuck ferments
• Vessel and cap management (reds) — shape extraction profile

AF ends when sugar is depleted (dry wine) or when stopped deliberately (sweet wine methods — Section 4.3).

---

Malolactic Conversion (MLC) Overview

Malolactic conversion (often called malolactic fermentation) is the bacterial conversion of sharper malic acid to softer lactic acid + CO2, typically by Oenococcus oeni. Effects:

• Softens acid taste; slightly raises pH
• Can produce diacetyl (butter/cream) especially in some Chardonnays
• Improves microbiological stability by removing malic acid that bacteria could later consume in bottle

Reds: MLC is almost universal after or during AF. Whites: optional — blocked for crisp aromatic styles; encouraged for fuller, creamy Chardonnay-style wines. Blocking tools include SO2, low temperature, sterile filtration, and lysozyme in some regimes.

Price/quality link: Barrel AF + MLC + new oak on Chardonnay is a classic mid-premium to premium recipe. Cool stainless AF with blocked MLC and no oak is the recipe for many high-volume fresh whites — lower cost, different style, not automatically lower quality if fruit is excellent.