Ester Gum, CAS# 8050-31-5, is a wood rosin manufactured through chemical synthesis, available as Hard, yellow to pale amber-colored resin. Ester Gum is widely used as food thickening agent.It is widely accepted as safe food additive in many countries with E number E445.ster Gum, CAS# 8050-31-5, is a wood rosin manufactured through chemical synthesis, available as Hard, yellow to pale amber-colored resin. Ester Gum is widely used as food thickening agent.