The alcoholic fermentation of grape must is carried out principally by
Answer: A
Saccharomyces cerevisiae converts the sugars of the must to ethanol and carbon dioxide and tolerates the alcohol it produces better than its competitors. Oenococcus carries out the separate malolactic step, while Acetobacter and Lactobacillus are spoilage organisms in wine.
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The two main products of the alcoholic fermentation of sugar by yeast are
Answer: B
Correct answer: Ethanol and carbon dioxide
Under anaerobic conditions yeast converts each molecule of glucose to two of ethanol and two of carbon dioxide: \[ \mathrm{C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2}. \] The ethanol is what preserves the drink and the carbon dioxide is what carbonates it or, in a still wine, is simply allowed to escape.
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Grape must is best described as
Answer: C
Correct answer: The juice, and often skins, of crushed grapes before fermentation
Must is the starting material: crushed grapes, with the skins included for red wine and generally pressed off for white. Once fermentation is complete it is no longer must but wine, and the solid residue left after pressing is pomace.
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Red wine differs from white wine in that red wine is normally fermented
Answer: D
Colour and much of the tannin come from the skins, so a red must is fermented with them present and the alcohol formed helps extract the pigments. White wine is usually pressed first and fermented as clear juice, and generally at a cooler temperature.
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White wines are usually fermented at a lower temperature than red wines in order to
Answer: A
Correct answer: Retain delicate aromatic compounds in the wine
White wine is fermented cool, often between 12 and 18 degrees, because the volatile esters and terpenes responsible for its aroma are lost with the escaping carbon dioxide at higher temperatures. Red wine is fermented warmer partly because the extraction of colour and tannin from the skins requires it.
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Wild yeasts present on grape skins typically
Answer: B
Genera such as Hanseniaspora dominate the first hours but are killed once the ethanol reaches roughly four or five per cent, at which point the more alcohol-tolerant Saccharomyces takes over. Their early activity still contributes to the aroma of the finished wine.
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Sulphur dioxide is added to must and wine chiefly to
Answer: C
It acts both as an antimicrobial, selecting for the relatively resistant Saccharomyces, and as an antioxidant that protects colour and aroma. It is the standard means of preventing and controlling spoilage in winemaking.
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Which organism is the most economically damaging spoilage yeast of red wine?
Answer: D
Brettanomyces produces volatile phenols giving barnyard and medicinal off-odours, and the losses it causes in red wine are substantial. Saccharomyces is the fermenting yeast and Oenococcus performs a desirable step, while Lactobacillus is a bacterium rather than a yeast.
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The usual method of controlling Brettanomyces in winemaking is
Answer: A
Sulphur dioxide is the standard control, though strains differ in their sensitivity to it, which is why cellar hygiene and careful topping-up of barrels matter as well. Warmth, residual sugar and oxygen all favour the organism rather than hindering it.
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Wine turning to vinegar is the work of
Answer: B
Acetic acid bacteria oxidise the ethanol of the wine to acetic acid, and because they require oxygen the defect appears where wine meets air: in a partly empty vessel or a poorly sealed bottle. Keeping vessels full is the classical precaution.
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