Acetic acid bacteria spoil wine only where
Answer: C
Correct answer: Oxygen is available to them
Acetobacter and its relatives are strict aerobes: they oxidise ethanol to acetic acid and cannot do so without oxygen. This is why wine is kept in full vessels with minimal head space and why a part-finished bottle turns to vinegar - the remedy is the exclusion of air rather than anything added to the wine.
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Malolactic fermentation in wine is carried out by
Answer: D
It is a bacterial step, distinct from the yeast-driven alcoholic fermentation, and Oenococcus oeni predominates because it tolerates the low pH and the alcohol. Calling it a fermentation is slightly misleading, since it is a decarboxylation rather than a sugar fermentation.
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In malolactic fermentation, malic acid is converted to
Answer: A
Correct answer: Lactic acid and carbon dioxide
Lactic acid bacteria decarboxylate the dicarboxylic malic acid to the monocarboxylic lactic acid, releasing carbon dioxide. Since lactic acid is the softer of the two, the wine loses its sharp green-apple character and gains roundness - which is why the conversion is encouraged in most reds and in some whites, and suppressed where crisp acidity is wanted.
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The chief sensory effect of malolactic fermentation is that the wine becomes
Answer: B
Malic acid tastes markedly sharper than lactic acid, so converting one to the other softens the wine and gives a rounder mouthfeel. Diacetyl produced alongside can add a buttery note. No alcohol or sugar is generated by the step.
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Malolactic fermentation is generally encouraged in
Answer: C
It is standard for reds and for certain whites and sparkling wines, but many crisp white styles are deliberately prevented from undergoing it because the sharpness of malic acid is wanted. The choice is a stylistic one rather than a technical necessity.
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If malolactic fermentation occurs unintentionally after a wine has been bottled, the result is
Answer: D
Carbon dioxide released in a sealed bottle leaves the wine gassy and the bacterial growth makes it hazy, so the step is either completed and stabilised before bottling or prevented outright. Filtration and sulphur dioxide are the usual means of preventing it.
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Sparkling wine made by the traditional method acquires its bubbles from
Answer: A
Correct answer: A second fermentation taking place inside the bottle
In the traditional method a still base wine is bottled with sugar and yeast, and the carbon dioxide from the resulting fermentation dissolves under pressure because it cannot escape. The wine then rests on the spent yeast, which contributes the characteristic biscuit and bread notes, before the deposit is riddled into the neck and disgorged.
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Yeast lees left in contact with a wine after fermentation contribute chiefly by
Answer: B
As the dead cells break down they release mannoproteins and other compounds that give a rounder mouthfeel and added complexity, which is why some wines are deliberately aged on their lees. The yeast is no longer fermenting anything.
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A wine fermentation that stops before all the sugar is used is described as
Answer: C
A stuck fermentation leaves residual sugar together with a relatively low alcohol content, which is exactly the condition in which spoilage organisms thrive. Causes include excessive temperature, nutrient shortage and very high initial sugar.
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Zygosaccharomyces is a particular nuisance in sweet wines because it
Answer: D
Its osmotolerance lets it grow where the sugar concentration would stop most organisms, and its resistance to sulphur dioxide makes it hard to control, so a sweet wine can referment in the bottle. Sterile filtration is the usual defence.
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