Erythritol and resorcinol crystals, polarised light micrograph. Erythritol occurs naturally in fruits and fermented foods. It is used as a sugar substitute as it is very low in calories, does not affect blood sugar levels and does not contribute to tooth decay. Resorcinol is an organic phenolic compound. It is used to treat acne, dermatitis, eczema, psoriasis and other skin disorders. Magnification: x93 when printed 10 centimetres wide.