Anomalous redispersibility behavior of glycerophosphate deyhydrogenase microparticles dried in an acoustic levitator or bench-top spray dryer.

The enzyme glycerophosphate dehydrogenase (GPDH) behaves differently when dried either as single droplets in an acoustic levitator or spray dried on a bench-top machine. The GPDH in particles dried in the levitator at a drying gas temperature of 60°C could not be redispersed in water, whereas spray drying at an outlet temperature of 92°C produced denaturation but the particles were redissolvable. One difference between the two processes is that the larger levitated droplets take longer to dry than the small spray dried droplets. The slow drying process of the levitated droplet/particle apparently causes denaturation that is sufficient to make the particles non-redispersible. This does not happen on spray drying.

Trehalose and sorbitol alter the kinetic pattern of inactivation of glutamate dehydrogenase during drying in levitated microdroplets.

A single-droplet acoustic levitator was used to determine the drying rate and the kinetics of inactivation of glutamate dehydrogenase in the presence of added trehalose or sorbitol. The solution was also spray dried under the same process condition of drying gas temperature on a bench-top machine. Both trehalose and sorbitol delay the point of onset of enzyme inactivation which lies after the critical point of drying. Both carbohydrates also reduce the apparent rate constant of inactivation calculated during the subsequent inactivation phase. The carbohydrates stabilise, therefore, the enzyme during droplet drying and particle formation mainly during the falling rate drying period. There is no difference between the stabilising effects of the two carbohydrates when examined as levitated single droplets. This suggests the importance of water replacement as a stabilising mechanism in the levitated droplets/particles. On spray drying, the trehalose stabilises the enzyme better than does the sorbitol at a drying gas (outlet) temperature of 60°C. This suggests glass formation with the trehalose but not the sorbitol during the very rapid drying process of small-atomised droplets in the spray dryer.

Slow motion picture of protein inactivation during single-droplet drying: a study of inactivation kinetics of L-glutamate dehydrogenase dried in an acoustic levitator.

A novel technique is presented to allow measurement of the kinetics of protein inactivation during drying of an acoustically levitated single droplet. Droplets/particles are removed from the acoustic field after various times during drying, and the state of the protein within them is analyzed. The influence of drying air temperature, relative humidity, buffer concentration, and the presence of a substrate on the inactivation of glutamate dehydrogenase is described. The kinetics of inactivation showed three distinct phases. The first phase of constant drying rate demonstrated little protein inactivation in the solution droplet. After the critical point of drying, a second phase was distinguishable when the surface temperature has risen sharply, but there is still only little inactivation of the protein in the solid particle. An onset point of rapid inactivation of the protein marked the start of the third phase that proceeded with approximately first-order rate kinetics. In the case of L-glutamate dehydrogenase, the evidence suggests that the residual moisture content of the solid and not the temperature alone determines the point of onset of protein inactivation.