52 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
islics are grown under defined, controlled con- 
ditions. 
In its simplest form, fermentation consists of 
mixing a micro-organism with a liquid broth 
and allowing the components to react. More so- 
phisticated large-scale processes require control 
of the entire environment so that fermentation 
proceeds efficiently and, more importantly, so 
that it can be repeated exactly, with the same 
amounts of raw materials, broth, and micro- 
organisms producing the same amount of prod- 
uct. Strict control is maintained of such vari- 
ables as pH (acidity/alkalinity), temperature, and 
oxygen supply. (See figure 17.) The newest mod- 
els are regulated by sensors that are monitored 
by computers. The capacity of industrial-sized 
fermenters can reach 50,000 gal or more. The 
one-shot system of fermentation is called batch 
fermentation— i.e., fermentation in which a 
single batch of material is processed from start 
to finish. 
In continuous fermentation, an improvement 
on the batch process, fermentation goes on 
without interruption, with a constant input of 
Figure 17.— Features of a Standard Fermenter 
Exhaust 
raw materials and other nutrients and an at- 
tendant output of fermented material. The most 
recent approaches use micro-organisms that 
have been immobilized in a supporting struc- 
ture. (See figure 18.) As the solution containing 
the raw material passes over the cells, the 
micro-organisms process the material and re- 
lease the products into the solution flowing out 
of the fermenter. 
In general, products obtained by fermenta- 
tion also can be produced by chemical synthe- 
sis, and to a lesser extent can he isolated by ex- 
traction from whole organs or oi'ganisms. A 
fermentation process is usually most competi- 
tive when the chemical process retjuires sex eral 
Figure 18.— Immobilized Cell System 
Solution with product out 
f ■ 
) 
Raw material solution in 
Typically, a solution of raw materials is pumped through a 
bed of immobilized micro-organisms which convert the 
materials to the desired product. 
SOURCE: Office of Technology AssessmenI 
