What it is
E491 (sorbitan monostearate, SMS) combines sorbitol (E420, plant) with stearic acid. The stearic acid is the variable bit — plant or animal.
What to do
Default plant in modern UK production; ask for premium products. The closely-related E492 (sorbitan tristearate), E493 (sorbitan monolaurate), E494 (sorbitan monooleate), E495 (sorbitan monopalmitate) all have the same consideration.