Short-time drying of heat-sensitive, biologically active pulps and pastes

Tomato concentrate and baker's yeast suspension were dried in the laboratory scale Mechanically Spouted Bed dryer with inert packing. Due to a large contact surface Provided by the inert particles, very fast drying process can be achieved, if the thickness of the wet coating formed on the surface of the inert packing is film-like. The diffusion resistance can be then considered negligible and drying can be performed at ''quasi-constant'' rate. Derived from hydrodynamic characteristics the contact time between hot air and the wet coating is very short, therefore gentle drying process can be performed. The quality of the dried yeast was controlled by counting the living cells (relative viability) and by measurement of the dough rise time. During continuous drying of thermoplastic tomato puree the ''case hardening'' periods could be shortened and tomato powder with insignificant quality loss was produced.