You've decided to try to make your own wine by adding yeast to a sweet grape juice. after allowing plenty of time for the yeast to grow, you find that although the sugar levels in the juice are decreased, there's no alcohol in the mixture. what could have been the reason fermentation did not occur?

Explanation:

Two possible explanations for this phenomenom are:

The juice was exposed to the air and the sugar were oxidized, explaining why the  sugar decreased but no alcohol was formed. Other possibility is that a contamination happened and insted of the yeast, bacteria developed in the juice. Bacteria perform a different type of fermentation: a lactic fermentation, in which they produce lactic acid from the sugar.

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Answer is to avoid experimental error