When considering the difference between decomposition and fermentation, it depends on the bacteria for decomposing. If putrefactive bacteria are in large quantities, then decomposition usually occurs, and if there are many fermentative bacteria, fermentation is the likely outcome.
Fermentation is a process involving microorganisms particularly yeast types. Enzymes required to facilitate this process may vary from those involved in decomposition. It is a metabolic process that converts sugar to acids, gases, or alcohol. It occurs in yeast and bacteria, and also in oxygen-starved muscle cells, as in the case of lactic acid fermentation. Decomposition is a term that relates to ecology. It is the microbial digestion and destruction of dead material which is consumed by the bacterial process. This putrefaction, also known as decomposition is very different from lactic acid fermentation. Both processes can produce some unique smells from the toxic compounds produced during putrefaction. During decomposition small organisms decompose in order to get their required energy for growth and development.