When animals and plants stop assimilation or metabolism, they lose their self-defense capability. Once they lose that capability, they soon become food for other living organisms. Living organisms have enzymes that break down food. They secrete these enzymes to decompose and ingest food. Enzymes are proteins produced within living organisms. Giraffes, cattle and horses can eat and live on plants because they directly or indirectly secrete cellulase, an enzyme that breaks down plants. Lions and tigers secrete enzymes that break down animals, but they cannot eat plants because they do not produce cellulase, the enzyme that breaks them down.
Most of the food we eat must be consumed before such microorganisms grow, or we will fall ill. However, when grain is abundant or we cannot finish a meal, leftovers are inevitable, and being too precious to discard, they must be stored somewhere. After a few days, even if the food smells and has gone bad, human hunger does not hesitate to taste it. If harmful microorganisms have grown at this point, an upset stomach soon follows, and one may even die. But with some microorganisms, the food has an odor yet causes neither stomach trouble nor death. When the enzymatic action of microorganisms produces something harmful that cannot be eaten, it is called "decay (腐敗)"; when it creates new substances that are not harmful and can be eaten, it is called "fermentation (醱酵)."
Not all microorganisms are harmful to our bodies. Kimchi, jang (fermented soybean paste), alcohol, vinegar and jeotgal (salted seafood) are representative fermented foods. Beneficial microorganisms that emerge during fermentation fight off and overcome other harmful decay microorganisms, preventing food from spoiling so that it can be eaten without causing illness. That is why fermentation is described as being "well ripened" or "well aged," while decay is referred to as "soured" or "rotted."
