When oil has high content of free fatty acids, irritating smell will be produced, influencing oil flavor, and expediting the hydrolytic rancidity of the neutral oil, so we must try to remove free fatty acids. The removal can be through chemical process and physical process.
Physical refining is removing by steam distillation. Chemical refining is by neutralizing the FFA with alkali, which is often caustic soda, the chemical reaction creates soap stock, which settles down to the bottom of the tank and removed or separated by centrifugation. This process is called neutralization, or alkali refining. Alkali refining also has certain decolorizing ability.
Using caustic soda in neutralization has the following advantages:
1. low price and wide sources.
2. Strong alkalinity.
3. The soap-stock is well separated with oil.
4. Most FFA can be separately effectively.
5. High decolorizing ability.
There are also physic-chemical processes besides neutralization during alkali refining.
* Reacting most of the free fatty acis with caustic soda to form sodium soaps referred to as soap-stock. The soap-stock is oil insoluble flocculating jell, and can be separated out.
* Soapstock has strong absorption ability. Quite a lot of impurities like residual proteins, pigments, phosphatides, trace metals and oxidizing materials are absorbed and removed.
* In the case of crude cottonseed oil, free gosspyl can react with the alkaline solution and forms phenoxide, which is easy to be absorbed onto soapstock and removed, so that the color of cottonseed oil is lowered.
During alkali refining, part of neutral oil reactes with caustic soda, i.e. is saponified, and goes into the soapstock, resulting in increased refining loss. Meanwhile some neutral oil is also entrapped in soapstock and carried away. To reduce the loss of neutral oil and increasing refining rate, we shall choose the best operation conditions. For oil which has high acid value, re-refining with caustic soda is required.
Neutralization can be carried our either by batch process with neutralization pot neutralization pot or by continuous process with centrifuge. For small oil processing plant, batch type is usually adopted. Degumming is required before neutralization as the crude oils often have a high phosphatide content, which negatively affects the yield. Improper neutralization will result in problems in subsequent refining steps of bleaching and deodorizing.
The by-product soapstock is processed for the product acid oil containing free fatty acid and glyceride, which is mostly sold as liveststock feed. Acid oil is also used as a feedstock for soaps, biodiesel and fatty acid industrial production.