Control of polymorphism and crystal size of L-glutamic acid in the absence of additives

Slow cooling of a supersaturated solution of L-glutamic acid, with continuous or pulsed agitation during cooling, is sufficient to stabilise the α-polymorph for crystallisation times of 24 h at 45°C. Rapid cooling with agitation or slow cooling without agitation favours formation of the stable β-polymorph. A reduction in average particle size and crystal quality of the α-crystals was observed following agitation and there was a marked absence of β-inclusions on the surface of and inside the α-crystals under these conditions. Two hypotheses are presented to explain the stabilisation of the α-polymorph namely, (i) that agitation is sufficient to disrupt nucleation of the β-crystals on the surface of the α-crystals and (ii) that α-crystals formed with agitation during slow cooling are small and poorly formed and lack the necessary well-formed crystallographic facets on which the β-form can nucleate. That the β-polymorph is favoured during rapid cooling with agitation can be explained in terms of the reduced period of agitation.