Sudan stains and Sudan dyes are synthetic organic compounds that are used as dyes for various plastics (plastic colorants) and are also used to stain sudanophilic biological samples, usually lipids.
Sudan III is a dye used for Sudan staining. Similar dyes include Oil Red O, Sudan IV, and Sudan Black B. They are used for staining of triglycerides in frozen sections, and some protein bound lipids and lipoproteins on paraffin sections.
In biochemistry laboratory, Sudan III is used for lipid determination. Sudan III solution has a red-brown color. For lipid testing, the Sudan III solution is added in the aqueous solution of lipids. It should create two layers; the aqueous layer at the bottom and the Sudan III solution on top. The test will be positive when the red (sometimes red-orange) solution of the Sudan Red will decolorize the aqueous solution. It only means that the lipid particles in the aqueous layer will dissolve the Sudan III dye. It will show negative test when the two layers will not change even when the mixtures are shaken.
