Which reagent would you use to test for lipids in supposedly fat free donuts?

To test for the presence of lipids in supposedly fat-free donuts, you would use Sudan IV or Oil Red O stain. These are both lipid-soluble dyes that will stain lipids bright red or orange, making them easy to detect visually.

To perform the test, you would:

Cut a thin section of the donut.

Place the section on a glass slide.

Add a few drops of Sudan IV or Oil Red O stain to the section.

Wait a few minutes for the stain to absorb.

Rinse the slide with water to remove excess stain.

Examine the slide under a microscope.

If lipids are present in the donut, they will be stained bright red or orange. The more lipids present, the more intense the staining will be.