What it is
Glycerol, sold as glycerin or glycerine, is a clear syrup that tastes faintly sweet and clings on to water, which makes it handy for keeping cakes moist, icing pliable and toothpaste from drying out. Most of it is made by splitting fats and oils apart under high heat and pressure, the same reaction that sits at the heart of soap-making and biodiesel production, with glycerol falling out as a valuable by-product. The snag for a vegan reader is that the fats fed into that reaction can be vegetable oils like palm and soya, or animal fats, and the finished glycerol is identical either way. European safety reviewers also describe two other routes: building it synthetically from propene, a gas from petroleum refining, and growing it by fermentation with yeasts or bacteria. So a single ingredient, glycerin, can trace back to a plant, an animal or an oil refinery.
How to tell
Glycerin is almost everywhere: in icing and fondant, chewy protein and cereal bars, moist low-fat cakes, toothpaste and mouthwash, cough syrups and the liquid in vapes. On a food label it shows up as glycerol, glycerine or the code E422, none of which reveal the source. The useful signal to look for is the phrase 'vegetable glycerine' or the letters VG, which is standard in cosmetics and e-liquids and tells you the glycerol came from plants. A plain 'glycerin' or 'E422' with no such note is genuinely ambiguous, and the surest way to settle it is to ask the manufacturer which feedstock they use. It is worth knowing that in 2022 European safety reviewers recommended tightening the official definition of E422 so that food-grade glycerol is made only from vegetable oils and fats, a change that both confirms non-plant glycerol has been circulating and points the whole category towards plants.