Edexcel IGCSE Biology: Double Science 复习笔记 5.1.3 Yeast in Food Production

• Microorganisms can be used by humans to produce foods and other useful substances
• One example of this is the production of bread using yeast
• Yeast is a single-celled fungus that can carry out both aerobic and anaerobic respiration

Yeast is a single-celled fungus, similar to the one shown in the diagram above

Making bread

• When yeast carries out anaerobic respiration, it produces an alcohol (ethanol) and carbon dioxide
• Yeast will respire anaerobically if it has access to plenty of sugar, even if oxygen is available
• This is taken advantage of in bread making, where the yeast is mixed with flour and water
• The yeast produces enzymes that break down the starch in the flour, releasing sugars that can then be used by the yeast for anaerobic respiration
• The carbon dioxide produced by the yeast during anaerobic respiration is trapped in small air-pockets in the dough, causing the dough to rise (increase in volume)
• The dough is then baked in a hot oven to form bread
• During baking, any ethanol produced by the yeast (as a waste product of anaerobic respiration) is evaporated in the heat
  • This is why bread doesn't contain any alcohol

   

• The yeast is killed by the high temperatures used during baking
  • This ensures there is no further respiration by the yeast

   

• Once cooled, the bread is ready to be eaten

The carbon dioxide produced by the anaerobic respiration of glucose is what makes bread dough rise

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