IGCSE Biology Paper-4: Specimen Questions with Answers 260 - 261 of 279

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Passage

Yeast is used in making some types of bread and in brewing.

Question 260 (1 of 2 Based on Passage)

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Explain the role of yeast in bread-making.

Explanation

Baker՚s yeast is the common name for the strains of yeast commonly used in baking bread and bakery products, serving as a leavening agent which causes the bread to rise (expand and become lighter and softer) by converting the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker՚s yeast is of the species Saccharomyces cerevisiae.

Dough Rises

Question 261 (2 of 2 Based on Passage)

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Explain the role of yeast in brewing.

Explanation

Yeast are unicellular fungi that reproduce asexually by budding or fission. The reaction by which alcoholic beverages are produced is generally referred to as fermentation. The production of alcohol occurs best in the absence of oxygen. However, from the yeast՚s point of view, alcohol and carbon dioxide are waste products, and as the yeast continues to grow and metabolize in the sugar solution, the accumulation of alcohol will become toxic when it reaches a concentration between 14 - 18 % , thereby killing the yeast cells. This is the reason why the percentage of alcohol in wine and beer can only be approximately 16 % . In order to produce beverages (liquor) with higher concentrations of alcohol, the fermented products must be distilled.

Ethanol and Acetylaldehyde

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