9.33C - Production of ethanol by fermentation
Explain how yeast converts glucose into ethanol, why the conditions matter and how a water seal lets carbon dioxide escape while limiting air entry. Fermentation produces ethanol mixed with water.
Glucose becomes ethanol and carbon dioxide
Fermentation converts sugars such as glucose into ethanol and carbon dioxide under anaerobic conditions (without oxygen). Glucose is dissolved in water to make an aqueous solution.
Fermentation of glucose
The equation conserves 6 carbon, 12 hydrogen and 6 oxygen atoms. Ethanol stays dissolved in the water while carbon dioxide escapes as gas. Fermentation therefore does not produce pure ethanol.
Yeast is a microorganism containing enzymes, biological catalysts that speed up the reactions without being used up overall. Yeast is not itself one enzyme. Dissolved glucose can be taken up and processed by yeast cells.
Conditions that let yeast work
Dissolve glucose in water, let any hot solution cool, then add yeast. Keep it warm, for example around 30 °C. At low temperature fermentation is slow; excessive heat denatures enzymes, changing their shape, and can kill the yeast.
Exclude oxygen to favour anaerobic fermentation. With oxygen available, yeast can respire aerobically, converting glucose to carbon dioxide and water. Do not assume that every bubble proves ethanol has formed.
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