Unfortunately there is not a selective extraction procedure for fatty acids. These are low polarity compounds and will be extracted together with other components at similar polarity. A suitable solvent could be hexane, or petroleum ether, but the organic extract will contain also glycerides, phytosteroids, triterpenoids, etc., together with free fatty acids.

If you are interested only to the free fatty acids you may recover them by extracting the organic solution with an aqueous alkaline solution (i.e. pH 9-10) of a not very much strong alkaline compound and working at room temperature to avoid the possible hydrolysis of esters (sodium carbonate may work well). The alkaline solution will convert the free fatty acids in the respective sodium salts which remain in the aqueous solution and may be separated from the organic layer. The organic solution instead will contain the glycerides and other low polarity components. The free fatty acids could be recovered from the alkaline solution after acidification (free fatty acids will be reformed) and extraction with an apolar solvent i.e. hexane.

The original hexane extract now containing glycerides and other low polarity components may be further processed, i.e. by alkaline hydrolysis (NaOH in ethanol/water solution and heating). In this way all the ester components (i.e. mono-, di- and triglycerides) will be converted in glycerol and sodium salts of fatty acids and remains in the aqueous phase. Also these fatty acids salts could be converted in free fatty acids by acidification and could be separated as previously indicated with hexane. This could be useful if you want to know which kind of fatty acids are present in the glyceride fraction. The remaining low polarity components in the original hexane extract are, in genera,l called "unsaponifiable matter" and may contain several classes of organic compounds such as long chain alcohols, triterpenoids, sterols, liposoluble vitamins (i.e. tocopherols, vit. A), etc...

Unfortunately there is not a selective extraction procedure for fatty acids. These are low polarity compounds and will be extracted together with other components at similar polarity. A suitable solvent could be hexane, or petroleum ether, but the organic extract will contain also glycerides, phytosteroids, triterpenoids, etc., together with free fatty acids.

If you are interested only to the free fatty acids you may recover them by extracting the organic solution with an aqueous alkaline solution (i.e. pH 9-10) of a not very much strong alkaline compound and working at room temperature to avoid the possible hydrolysis of esters (sodium carbonate may work well). The alkaline solution will convert the free fatty acids in the respective sodium salts which remain in the aqueous solution and may be separated from the organic layer. The organic solution instead will contain the glycerides and other low polarity components. The free fatty acids could be recovered from the alkaline solution after acidification (free fatty acids will be reformed) and extraction with an apolar solvent i.e. hexane.

The original hexane extract now containing glycerides and other low polarity components may be further processed, i.e. by alkaline hydrolysis (NaOH in ethanol/water solution and heating). In this way all the ester components (i.e. mono-, di- and triglycerides) will be converted in glycerol and sodium salts of fatty acids and remains in the aqueous phase. Also these fatty acids salts could be converted in free fatty acids by acidification and could be separated as previously indicated with hexane. This could be useful if you want to know which kind of fatty acids are present in the glyceride fraction. The remaining low polarity components in the original hexane extract are, in genera,l called "unsaponifiable matter" and may contain several classes of organic compounds such as long chain alcohols, triterpenoids, sterols, liposoluble vitamins (i.e. tocopherols, vit. A), etc...

Thank you Alessandro for all these details, do you have a protocol with exact amounts to follow in the lab, please.