Which substance is a product of a fermentation reaction? ?

Answer is:  a product of a fermentation reaction is ethanol (C₂H₅OH).
Fermentation is a metabolic process in which ethanol is produced from sugar in the absence of oxygen (anaerobic digestion).
Fermentation occurs in yeast, bacteria and in oxygen-starved muscle cells.
Fermentation is a way of extracting energy from molecules and it is the oldest metabolic pathway.

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