The hard, matted hair tuft in a trichobezoar is mainly due to gastric hydrochloric acid (HCl), not a specific enzyme.

Gastric acid denatures and oxidizes hair keratin, while gastric churning compacts the hair with mucus and food debris. This produces the firm, dark hair mass. Pepsin does not digest keratin effectively.

The surgical text confirms that ingested hair remains undigested in the stomach: Bailey and Love's Short Practice of Surgery, 28th ed., p. 542.