Citric Acid Dosing in Soap Noodle

Fatty acid and caustic react with each other to produce soap. The caustic is added in excess to create free alkali in neat soap.

Citric acid is added at the end of the soap noodle process for following two reasons

1) Create Chelating agent

2) Superfatting 

When citric acid react with the free alkali of neat soap, it creates sodium citrate, which is a chelating agent. It helps to attract, bind and trap metal ions which can lead the soap noodle to rancidation. Chelating agent also trap the calcium and magnesium ions present in hard water.

Instead of Citric acid, can we add sodium citreate directly?

Yes, you can. But citric acid is very cheap as compare to sodium citreate. That's why citric acid is commonly used. 

How it creates superfatting?

Citric acid consume the alkali from the neat soap which eventually let some of the fatty acid free which is known as super fating. Superfatting gives mildness affect when soap apply on the skin.

Instead of free alkali, can we left free acid in neat soap to create superfatting?

The reason of free alkali in neat soap is to provide sodium hydroxide which will react with citric acid to create sodium citreate which is a chelating agent.