Crude vegetable oils, such as sesame, groundnut, cottonseed and sunflower oil, contain free fatty acids that give an off taste, shorten shelf life and cause smoking when frying. Chemical refining removes them with caustic soda. This step, called neutralisation, is one of the most important in an edible oil refinery.
How neutralisation works
Caustic soda reacts with free fatty acids to form soap, known as soapstock. The soapstock is heavier and separates from the oil, usually in a centrifuge. Along with the fatty acids, neutralisation also removes some colour, gums and impurities.
Working out the caustic dose
- Measure free fatty acid (FFA) content of the crude oil by titration.
- Calculate the theoretical caustic needed to neutralise that FFA.
- Add an excess, set by experience for each oil, to ensure complete neutralisation and help remove colour.
- Choose the caustic strength, usually expressed in degrees Baumé, to suit the oil and process.
Too little caustic leaves acid in the oil. Too much saponifies neutral oil and increases refining loss, which costs money. Many refineries also pre-treat with phosphoric acid to condition gums before neutralisation.
After neutralisation
- Washing with hot water removes remaining soap.
- Drying, bleaching and deodorising complete the refining.
- Soapstock can be split with acid to recover fatty acids for soap making or animal feed.
Caustic quality for edible oil
Use clean, consistent caustic soda. Food-related processes need low heavy metals, arsenic and iron; check the certificate of analysis for each batch. Prepare solutions with clean water in stainless steel or suitable plastic tanks.
Safety
Strong caustic solutions cause severe burns. Wear face protection, gloves and aprons, keep eyewash stations at the caustic preparation area, and always add caustic to water.
For refiners and food producers operating in Oman
Caustic soda flakes 99%, with batch certificates and a published daily price, are available within Oman from MBBT, Oman, at muscatchemical.com/caustic-soda.
