Oils have other impurities such as Free-Fatty Acid (FFA), color and smell. 

If  FFA is high, the oil must be refined to remove the FFA.  FFA is an acid and it irritates the throat; thus making it un-suitable for human consumption.  The FFA can be removed by neutralizing with caustic solution.  Caustic reacts with FFA and produces Soap.  Soap is settled and drained.  Oil is subsequently washed and dried. 

Color and smell are not harmful for humans but most modern consumers demand colorless and odorless cooking or salad oil.  Oil is bleached with the use of bleaching clay at a certain temperature and pressure and then filtered to obtain pale yellow colored oil.  Oil is deodorized via steam distillation at a certain temperature and pressure. 

A Mini-Refiner will include equipment for Neutralizing the FFAs, Bleaching to remove color, and Deodorizing to remove smell (odor). The plant is equipped with electric heating arrangement, steam generator, and a vacuum system.  The plant is skid-mounted with electric control panel.  See more detailed coverage about refining process at

The oil quality changes as it goes through the three steps.  The following picture shows how soybean oil looks after Caustic Neutralization, Bleaching, and Deodorizing.  Different types of oils have different starting colors; not all oils are as dark as soybean oil.  Some oils can be darker than soybean oil but they can be refined to the same extent as shown below.

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