Saponification is an exothermic (gives off heat) chemical reaction that occurs when fats or oils (fatty acids
) come into contact with lye (a base
.) Saponification literally means "turning into soap" from the root word, "sapo", which is Latin for soap. The by-products of the saponification reaction are glycerin and soap.
Oils + lye (dispersed into water) = soap + glycerin
Oils and fats each have what is called a saponification value, which is the amount of lye needed to completely neutralize them into soap with no lye left over. Each oil has a different value, which is why its important to always run your recipes through a lye calculator.