Spray drying of mixed amino acids: The effect of crystallization inhibition and humidity treatment on the particle formation

Abstract

This study focused on the spray drying of mixtures of amino acids by systematically investigating the influence of crystallization inhibition between taurine and glycine on particle formation. Two well-controlled drying techniques, namely Single Droplet Drying (SDD) and Micro Fluidic Jet Spray Drying (MFJSD), were used. An extended nucleation time, compared to that of pure amino acid, was first observed in the mixed AA sample dried from SDD, possibly as a result of crystallization inhibition. The spray-dried particles were dense microspheres, compactly assembled by numerous fine crystal grains of uniformly distributed taurine and glycine. Crystallization inhibition was also confirmed by the presence of the unstable polymorphic form of glycine in the spray-dried mixed AA particles obtained from MFJSD, and was verified to be strongly associated to the formation of nanocrystals. The spray-dried microspheres were likely formed in a two-stage process of rapid nucleation followed by inhibited growths of taurine and glycine. The microstructure of mixed AA particles, assembled by nanometer-scale grains with the increasing size towards the inner particle, was due to the crystallization behavior of amino acids during convective drying. In the post-drying humidity treatment, a significant segregation between taurine and glycine was in a clear contrast to the uniform distribution in those without treatment. Such segregation was possibly be due to various solubilities of taurine and glycine and their recrystallization in the humid condition. The recrystallization mechanism was further validated by a real-time observation of the morphological evolution on the particle surface. To the best of our knowledge, this is the first work that captured in real time the effects of post-humidity treatment on the morphological differentiation in spray-dried mixed amino acid particles. The understanding obtained here would advance the design formulation of amino acid particles.