bioreactor studies of heterologous protein production by ...

inactive and useless. The problern is iotensified in the case of aembic fermentations due
to oxygen W e r rate limitation-.
Yeast ceh cm &O be immobilized by adsorption of cells ont0 solid supports, in
which only extemal surface area is used for ceIl retention. Such supports include glas,
ion-exchange resin, wood chips, P.V.C. chips and ceramic materiais (Moo-Young et ai..
1980; bptey, 1983). Because cells oniy reside on outer surface of the supports, the
interna1 mas W e r limitations can be overcome. The He time of the support rnadals
seems to be unlimited due to their inert property and strong mechanic nrength. However,
ceil leakage is substantial in these systems since the binding forces involved are m;iinly due
to Van der Waals' forces (Rutter and Vmcent, 1980) and hydrogen bonds. The
interactions between the microbial ce& and the supports can be infiuenced by
temperature, pH, hydrodynamic forces and ionic saength of the medium. Generaily, lower
cell concentration is achieved by this method. In order to increase ceii concentration,
some soft, highiy porous materials such as polymer foam, Cotton cloth and sponge have
ken developed to immobiüze yeast ceh ( Joshi and Yamazaki. 1984; Philipps, 1992;
Roostaazad, 1993). Unlike solid carries, both extemal and intemal silrfaces are availabk
for cefl retention in these materiais. Combined with their low cos6 easy avaiiability and
long life the, they could be attractive materials for immobilization.
In alcohol fermentation nsing immobibd yeast ce& packed-bed imrnobilized c d
bioreactors are usually employed (Corieu, et al, 1976; Kierstan and Bucke, 1977; Wada