Stability and surface activity of lactate dehydrogenase in spray dried trehalose

The stability of the model protein lactate dehydrogenase (LDH) during spray-drying and also on subsequent dry storage was examined. Trehalose was used as a carrier, The spray-drying temperatures T-inlet and T-outlet have a measurable effect on LDH inactivation. Low T-inlet produced the least process inactivation, but gave a high residual moisture content making the protein's storage stability poor. High T-inlet reduced residual moisture and improved storage stability, but at the cost of high process inactivation. As already found for other systems, addition of a surfactant (in this case polysorbate 80) could ameliorate process inactivation of LDH at T-inlet = 150 degrees C. Surfactant had, however, a deleterious effect on storage stability of LDH, the vital factor being the molar ratio of surfactant/protein in the dried product. By using electron spectroscopy it was shown that LDH has a 10 times higher surface concentration in the dried trehalose particles than expected for a homogeneous distribution. Surface tension measurements at the water/air interface proved that LDH is surface active, although the Gibbs equation appeared to be inapplicable. Calculations of spray-droplet formation time and drying time indicate than the extent of diffusion-driven LDH adsorption to the liquid/air interface is sufficient to account for the measured amount of LDH inactivation during spray-drying, The presence of 0.1% polysorbate 80 to the spray solution prevents LDH from appearing at the surface of the dried particles. As a negative control, the phosphatide Lipoid E 80 does not prevent the appearance of LDH in the surface according to electron spectroscopy and does not therefore prevent LDH inactivation during spray-drying at T-inlet = 150 degrees C.