Beer is a difficult medium for most microorganisms because it combines
Answer: A
Several barriers act together, so the spoilage flora of beer is unusually narrow. Removing any single one, such as brewing a low-alcohol or lightly hopped beer, widens the range of organisms that can grow and raises the stability problem sharply.
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The lactic acid bacteria that spoil beer are notable for being
Answer: B
Hop resins inhibit most Gram-positive bacteria, so the strains of Lactobacillus and Pediococcus that trouble brewers are precisely those that have acquired hop resistance. This is why hop rate influences which spoilage organisms matter.
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Pediococcus growing in beer is chiefly associated with
Answer: C
Correct answer: Sourness together with a buttery diacetyl note
Pediococcus is a lactic acid bacterium, so it sours the beer, and it also produces diacetyl, which carries a strong butter or butterscotch aroma detectable at very low concentrations. Some strains additionally form ropiness. The barnyard aroma belongs to Brettanomyces and the vinegar note to Acetobacter.
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Wild yeasts are a problem in a brewery mainly because they
Answer: D
A wild yeast may attenuate further than the pitching strain, leaving a beer that is drier, over-carbonated in the package and carrying flavours the brewer did not intend. Rigorous hygiene and pure pitching yeast are the defences.
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Beer may be stabilised against microbial spoilage after packaging by
Answer: A
Correct answer: Pasteurisation or sterile filtration
Once the beer is in its container the only remaining options are to kill the organisms with heat or to remove them physically through a membrane. Tunnel pasteurisation is the common industrial route; sterile filtration is used where the heat treatment would harm the flavour. Beer conditioned in the bottle deliberately forgoes both.
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Oxygen picked up at packaging harms beer chiefly by
Answer: B
Dissolved oxygen drives oxidation reactions that give cardboard and sherry-like staling flavours, and it also permits aerobic organisms that the anaerobic beer would otherwise exclude. Modern packaging works hard to keep pick-up to a minimum.
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Which organism is used in both winemaking and ale brewing?
Answer: C
The same species performs the alcoholic fermentation in both, though the strains used are quite different and have been selected over centuries for each purpose. The other three organisms are associated with malolactic fermentation or with spoilage.
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Wine and beer differ fundamentally in that the sugar for beer must first be
Answer: D
Grapes supply fermentable sugar directly, whereas barley stores carbohydrate as starch that yeast cannot use, so malting and mashing exist entirely to convert it. This extra enzymatic stage is the essential difference between the two processes.
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The alcohol content of a wine is usually higher than that of a beer because
Answer: A
More sugar in the starting material means more alcohol at the end, and ripe grapes are far sweeter than a typical wort. The same species of yeast is at work in each, and neither product is routinely diluted after fermentation.
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Which product is made by an aerobic process rather than an anaerobic one?
Answer: B
Acetic acid bacteria oxidise ethanol and need oxygen to do it, so vinegar production is deliberately aerated. All three alcoholic products are fermented with air excluded, since oxygen would both spoil them and divert the yeast from making alcohol.
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