As mentioned previously, the presence of FFAs in the biolipid is detrimental to oil quality and function, including biodiesel production. Removal typically in­volves the reaction of these FFAs with an alkaline solu­tion. In the edible oil industry, usually only caustic soda is used for this reaction, but potassium hydroxide is also used by some producers. The acidity of the FFA comes from the H+ of the carboxyl group. This H+ of the functional group of the stearic acid reacts with the OH~ group of the caustic soda (NaOH) to produce soap and water. In addition to the removal of FFAs, other undesirable nonglyceride materials are also removed in this fashion such as phenol, oxidized fatty compounds, heavy metals and phospholipids.

Winterization

Most biolipids do not need dewaxing, as they contain little or no waxes. Only biolipids of higher melting temperatures, such as sunflower oil and rice bran oil, give a hazy appearance during winter season due to precipitation of dissolved waxes. Hence, they require being dewaxed. This is carried out by chilling the oil to 10—15 °C, followed by filtration of precipitated solids. The oil thus treated has a sparkling appearance, even in winter temperatures.