Saponification is a chemical reaction that involves the production of a metal salt or soap. The reaction involves the attack on a methyl ester, free fatty acid, triglyceride, or other glyceride by a hydroxide ion, -OH.
The hydroxide ion implies the presence of water in the system. If water could be eliminated, then there would be no soap formation. However, this is a practical impossibility. There is always some water present, and there will always be some soap formed when biodiesel is made.
The exception to this is when a solid (heterogeneous) catalyst is used which does not provide the free metal ions needed to form soap. These catalysts should provide biodiesel and glycerin that are free of soap. In reality, many of these supposed heterogeneous catalysts leach metals ions into the liquid and thus require some clean-up of the reaction products.
Soap must be removed from biodiesel after the reaction, and this can be done by either water washing, using a solid adsorbent mixed with the liquid, or by passing the liquid through a packed bed of ion exchange resin.
Soap is not formed when biodiesel is manufactured using a supercritical technology that does not require a catalyst. For more information on supercritical reactors, see Reactors for Biodiesel Production.