b) Neutralization (chemical processing)

Caustic soda is used in this procedure to remove free fatty acids. It is vigorously mixed (a small excess, as determined in the lab) with the heated fat at relatively high temperature (say 60-80ºC) and left to stand for the aqueous phase to settle.

The aqueous solution that remains is known as foots or soapstock and, after separation, may be used for producing soap. Residual foots are eliminated from the neutral oil by washing it with hot water, along with settling or centrifugation.

Excess soapstock may constitute an environmental hazard and may de disposed of by taking advantage of the extremely low water solubility of calcium soap.

Thus an equimolar amount of calcium chloride may be directly added to the soapstock and prompt separation of the calcium soap by precipitation from a relatively pure saline (NaCl) solution will ensue. Calcium soaps are useful industrial ingredients, for instance as demoulding agents.

Even though the main purpose of the alkali treatment is the removal of free fatty acid, the process also produces a reduction in phospholipid and colouring matter content.