What is the chemical reaction that takes place when an microorganism turns one substance into another?
This reaction is called fermentation.
We call chemical reaction to the process that leads to the transformation of one substance or set of substances into another different one. One very simple and common example of a chemical reaction is the combination of simple oxygen gas with simple hydrogen gas to conform the complex substance called water.
The law of conservation of mass states that matter is neither created nor destroyed during a chemical reaction. This mean that if you have, for example, 20 g of a substance and react it with another substance, you would still have 20 g of the first substance at the end of the reaction, just in a different form.