The concomitant polymorphism of the sulfur‐containing essential amino acid DL‐methionine was revealed, and α‐ and β‐polymorphic forms were separated from aqueous solution through antisolvent crystallization with ethanol as antisolvent. The addition of different volume fractions of ethanol into the mother solution reduces the solubility of DL‐methionine in double‐distilled water and in turn proportionately generates a range of supersaturation levels. This induces the nucleation of a stable β‐polymorph at low supersaturation levels, a mixture of α‐ and β‐ polymorphs at intermediate supersaturation levels, and a metastable α‐form at high supersaturation levels. Needle‐shaped α‐polymorphs and platy‐shaped β‐polymorphs were well distinguished through in‐situ optical microscopic analysis and their structural confirmation was done by X‐ray diffraction analysis.

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Polymorphic control and crystal nucleation of and polymorphs of dl-methionine, an essential amino-acid, is attained from saturated aqueous solution through swift cooling crystallization process by generating wide range of supersaturation (0.02 < < 2.19) in the temperature range of 323-274 K. The type of nucleation is very much supersaturation dependent: while lower supersaturation range (0.02 < < 0.08) prefers the stable form, higher supersaturation range (1.20 < < 2.19) yields metastable form. In situ optical microscopy and X-ray diffraction analyses confirm the morphological and structural distinction of the grown polymorphs.