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What is soapstock and how is it removed during alkali refining?

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Three blocks of butter on a plate with a piece cut off, next to a butter knife and a blue checkered cloth in the background.
fat and oil processing Butter.

Soapstock is a byproduct of the refining process for fats and oils, consisting of an aqueous emulsion of soaps formed from free fatty acids, along with other impurities and oil.12

During alkali refining, fats are treated with an aqueous solution of sodium hydroxide or sodium carbonate at temperatures between 40 and 85 °C (104 to 185 °F). This treatment neutralizes free fatty acids, creating soap. The removal of the resulting soapstock depends on the system used:23

  • Batch (tank) refining: The soapstock emulsion settles to the bottom of the tank by gravity and is subsequently drawn off.2
  • Continuous refining: The emulsion is separated from the oil using centrifuges.2

After the initial separation, the refined fat is typically washed with water to remove any remaining traces of alkali and soapstock.2