Question

Take a $500ml$ beaker filled up to $3/4$th with water. Dissolve $2 - 3$ spoons of sugar in it. Add half a spoon of yeast powder to the sugar solution. Keep it covered in a warm place for $4 - 5$ hours. Now smell the solution, could you get a smell?

Answer 1

Yeast are single-celled fungi that are very useful in the culinary and nutritional world. They're used to make bread, wine, and beer, and certain types can be used as a dietary supplement since they're high in vitamin B, selenium, and chromium. Yeast gets activated when dissolved in warm water and sugar, which work as the food supplement for yeast.

Complete Explanation:
Fermentation is a metabolic process that uses enzymes to cause chemical changes in organic substrates. It is known as the extraction of energy from carbohydrates in the absence of oxygen in biochemistry. It may also apply to any phase in which the action of microorganisms results in a beneficial improvement to a food or beverage in the food industry.
Say, for ex. Yeast, the most common Saccharomyces cerevisiae, brings out the incomplete oxidation of glucose under anaerobic conditions by the sets of reaction where pyruvic acid is converted to $C{O_2}$ and ethanol. These reactions are catalysed by the enzymes pyruvic acid decarboxylase and alcohol dehydrogenase. $2$ molecules Pyruvic acid is the end result of the process of glycolysis which occurs in cytoplasm of the cell in all the organisms, may it be aerobic or anaerobic.
Along with pyruvic acid, glycolysis also results in formation of $2$ molecules of ATP and $2$molecules of$NAD{H^ + } + {\text{ }}{H^ + }$. The reducing agent in the process of fermentation is $NAD{H^ + } + {\text{ }}{H^ + }$which is deoxidised to$NA{D^ + }$.
Hence, the net production of energy in fermentation (anaerobic process) is $2$ ATP, i.e., less than $7\%$ of the energy of glucose is released and not all of it is trapped in high energy ATP bonds.
One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules during ethanol fermentation. It's used to make bread dough rise by forming bubbles that expand the dough foam. The intoxicating agent in alcoholic drinks like wine, beer, and liquor is ethanol.

The smell is of alcohol produced due to fermentation.

Note:
As the concentration of alcohol exceeds about $13\%$, yeast poisons itself to death. Generally, it takes $90$ minutes, after that yeast cells divide continuously. But it gets activated within $10 - 15$ and rises and a frothy/bubbly layer is formed. Later after $4 - 5$ hours the yeast converts the sugar to alcohol by the process called “fermentation” which gives it an alcoholic smell.