Fermentation Basics for Wine
Understanding alcoholic and malolactic fermentation for Level 1 wine exams
Learning Objectives
- Explain the chemistry of alcoholic fermentation in simple terms
- Describe the role of yeasts in wine production
- Understand how temperature affects fermentation outcomes
- Define malolactic fermentation and its effect on wine style
- Identify which wine styles typically undergo MLF
What Is Alcoholic Fermentation?
Alcoholic fermentation is the biochemical process at the heart of winemaking. Yeasts consume the natural sugars in grape juice (primarily glucose and fructose) and convert them into ethanol (alcohol) and carbon dioxide. The equation is simple: sugar + yeast → alcohol + CO₂. This process generates heat, which must be controlled by the winemaker. Fermentation stops naturally when all the sugar has been consumed, producing a dry wine, or it can be halted early — by chilling the wine or adding alcohol — to leave residual sweetness.
Wild Yeasts vs Cultured Yeasts
Yeasts are present naturally on grape skins and in the winery environment. Wild (indigenous) yeasts start spontaneous fermentation and can add complexity and terroir character to the wine, but they are less predictable and can fail or produce off-flavours. Many winemakers prefer cultured commercial yeasts, which are reliable, start fermentation quickly, and produce clean results. Some winemakers combine both approaches, using wild yeasts to start fermentation and cultured yeasts to complete it.
Temperature Control
Fermentation temperature has a profound effect on wine style. Cool fermentation (12–16°C for white wines) is slower and preserves delicate floral and fruity aromas — this is why Riesling and Sauvignon Blanc benefit from cold fermentation. Warm fermentation (25–30°C for red wines) extracts more colour, tannin, and body. High temperatures (above 35°C) risk killing the yeasts, causing a stuck fermentation. Temperature-controlled stainless steel tanks give winemakers precise control.
Malolactic Fermentation (MLF)
Malolactic fermentation is a secondary fermentation carried out by bacteria (Oenococcus oeni), not yeast. It converts the sharp, green-apple-tasting malic acid into softer, dairy-tasting lactic acid, reducing the wine's perceived acidity and adding a creamy, buttery texture. MLF is almost universal in red wine production and common in fuller-bodied whites like Chardonnay, but is typically avoided in fresh, aromatic whites like Riesling and Sauvignon Blanc where high acidity is desirable.
Key Vocabulary
Exam Question Examples
Why might a winemaker choose not to carry out malolactic fermentation on a white wine?
Approach
State that MLF reduces acidity. For wines where high acidity is desirable (e.g. Riesling, Sauvignon Blanc, sparkling wine base wines), preserving the natural malic acid keeps the wine fresh, crisp, and suitable for ageing or secondary fermentation.
What is a stuck fermentation and what might cause it?
Approach
Define it as fermentation that stops before all sugar is converted. Causes: temperature too high (yeast dies), nutrient deficiency, high alcohol killing the yeast, or low initial yeast health. Mention that the winemaker must restart it by adding nutrients, cooling the wine, and re-inoculating.
Quick Summary
- 1.Fermentation converts grape sugars into alcohol and CO₂ via yeast
- 2.Wild yeasts add complexity; cultured yeasts offer reliability
- 3.Cool temperatures preserve aroma; warm temperatures extract colour and tannin
- 4.MLF converts sharp malic acid to soft lactic acid — used in most reds and oaked Chardonnay
- 5.Wines can be made dry (all sugar consumed) or sweet (fermentation stopped early)
Practice questions on this topic
Frequently Asked Questions
- Does all wine go through malolactic fermentation?
- No. MLF is used in almost all red wines and many fuller-bodied whites. However, winemakers producing crisp, aromatic whites — such as Riesling, Sauvignon Blanc, and Pinot Grigio — typically avoid MLF to preserve the wine's natural freshness and acidity.
- What is the difference between dry and sweet wine?
- A dry wine has had virtually all its grape sugars converted to alcohol during fermentation, leaving little or no residual sugar. A sweet wine retains some or all of its grape sugars, either because fermentation was stopped early or because naturally very sweet grapes were used.
Consolidate your knowledge
Use Vinlecta to practise exam-style questions on fermentation basics for wine and related topics under timed conditions.
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