Glycerol is a sweet edible food substance. (it does however have negative optimality for use to prevent it being used as a food substance and has slightly less nutrition.

Zoning and providing better food alternatives (within range) can help to prevent it being selected to be used as a food.

Colony policy rule mods are also available to provide even better control over what food items will be used. I personally use the mod "Pawn rules" by Jaxe, which also helps to better control animal handler food selection.

Social supplements also provides better "alternative" food substances and animal treats that can also be further used to ensure a more applicable selection.

Note in the case of switching to produce a better animal training product like animal treats it will normally take some time for animal handlers to convert over. This can be more rapidly applied by removing the existing animal feeding items from their inventories after a better food alternative is available.