During brewing, yeast is initially given access to oxygen but later the vessel is sealed. Why?

• Aoxygen is needed to kill bacterial contaminants before fermentation begins
• Bwith oxygen, yeast respires aerobically and reproduces rapidly to build a large population; once the vessel is sealed, yeast switches to anaerobic respiration, producing ethanol as the desired product
• Coxygen prevents the formation of carbon dioxide which would burst the vessel
• Dyeast needs oxygen to convert ethanol into a stronger alcohol

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