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 me
asurable 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 impro
ved storage stability, but at the cost of high process inactivation. As alr
eady found for other systems, addition of a surfactant (in this case polyso
rbate 80) could ameliorate process inactivation of LDH at T-inlet = 150 deg
rees 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 t
he water/air interface proved that LDH is surface active, although the Gibb
s equation appeared to be inapplicable. Calculations of spray-droplet forma
tion time and drying time indicate than the extent of diffusion-driven LDH
adsorption to the liquid/air interface is sufficient to account for the mea
sured amount of LDH inactivation during spray-drying, The presence of 0.1%
polysorbate 80 to the spray solution prevents LDH from appearing at the sur
face 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 ele
ctron spectroscopy and does not therefore prevent LDH inactivation during s
pray-drying at T-inlet = 150 degrees C.