Birefringence is the optical property of double refraction, in which polarised light splits into two rays travelling at different velocities through an anisotropic material. In food science it is a diagnostic tool: native starch granules display a characteristic Maltese-cross birefringence under polarised light, whose disappearance signals gelatinization during cooking and processing. Microscopists use it to assess starch damage, crystallisation in fats and sugars, and fibre orientation in meat analogues. The technique is rapid, non-destructive, and requires only a polarising microscope, making it a practical quality-control method for detecting processing changes at the microstructural level.