Amino acid crystallisation behaviours during convective drying

The manufacture of amino acid (AA) particles in powder form is a growing industry with estimated annual production of more than two million metric tons for their applications as pharmaceuticals and functional foods. The importance of engineering the particle morphology, especially the particle shape and size, is well established as the morphology is directly related to the bioavailability and handling of the powder. Most of the current methods used to modify the morphology of AA particles are multi-step processes, whereas spray drying is a one-step process that could potentially improve the particle sphericity. The principle behind this technique is to induce rapid solidification of the solutes in the presence of a hot drying environment in seconds. However, there is limited understanding of the AA particle formation due to the complexity of crystallisation, drying and possibly polymorphism involved in spray drying. The aim of this research is to achieve control over particle size and morphology of amino acid particles by investigating the crystallisation behaviours of amino acid during convective drying, with a view to exploring the formation mechanisms of spray-dried AA particles. The behaviours of taurine and glycine as model AAs were examined through well-controlled single droplet drying (slow convective drying) and micro-fluidic jet spray drying (rapid convective drying) techniques. Controllable taurine size in the range of 20–350 μm was achieved in a preliminary study of cooling crystallisation, but the taurine particles remained elongated. An in situ study in single droplet drying indicated the high possibility of spray drying taurine at a low temperature without adverse effect. Subsequently, spherical spray-dried taurine and α-glycine with size below 100 μm were obtained without significant losses in crystallinity or polymorphic stability. Furthermore, the particle surface roughness and hollowness of spray-dried AAs were tuned by adjusting the drying temperatures. Surprisingly, post-drying humidity treatment of α-glycine microparticles led to the formation of a porous “coral-like” structure, which facilitated a fast dissolution rate. Additionally, an understanding of multicomponent formulations was gained from spray drying of mixed taurine and glycine solution, where dense AA microspheres compactly assembled by numerous nanocrystals with uniform component distribution were obtained. Importantly, the formation of amino acid particles was affected by the characteristic crystallisation behaviours of amino acids, including the surface nucleation, dissolution and recrystallisation as well as the crystallisation inhibition. As such, the formation mechanisms of crystalline AA microspheres with various morphologies were ultimately elucidated. This research contributes fundamental knowledge of AA crystallisation behaviours that provides an essential basis for in-depth understanding of the spray drying of amino acids. The newly obtained correlation between these behaviours and the particle formation mechanisms enables advanced particle control for improvement in powder handling. Additionally, this research demonstrates the innovative use of spray drying and post-drying humidity treatment to manipulate the geometric shape and tailor the surface morphology of amino acid particles. The use of these processes could be further applied in crystalline drugs and sugars for particle design. The spray-dried amino acid microspheres might also have potential uses as drug carriers or fast-dissolve food additives.