Page 307 - Biosystems Engineering

284 Cha pte r Ni ne

9.3 Unit Operations in Bioseparation
Various unit operations are involved in bioseparation: (1) disruption
of cells, (2) centrifuging, (3) thickening, (4) flocculation, (5) filtration,
(6) evaporation, (7) drying and crystallization, (8) chromatographic
techniques, and (9) membrane techniques.
9.3.1 Disruption of Cells
The content of microbial cells has high osmotic pressure and is con-
strained by a fragile, semipermeable membrane, which is protected
from rupture by a strong, rigid outer cell wall. Microbial cells are far
harder to break than most animal or plant cells. Many cell-breaking
methods have been developed, but the cheapest and most effective
ones should be selected. Cell rupture can be done using the following
methods: (1) mechanical, (2) liquid homogenization, (3) sonication,
and (4) other methods.

Mechanical Methods

Breakage by Impact and Shear Machines that can be used for cell rup-
ture include a stamp mill, a mortar and pestle (with powdered glass,
sand, or alumina), a colloid mill (Fig. 9.1), and a homogenizer, for
example. The colloid mill and pan crusher are described as follows.
Colloid mills are used when there is to be very little breakdown of
individual particles and when the task is to disrupt lightly bonded
clusters or agglomerates. Dispersion of the material might be impor-
tant at times, as in emulsion formation, which is basically a two-fluid
system (e.g., syrups, milk, ointments, and creams). A special class of
mill is used for dispersion and colloidal operations, which work on
the principle of high-speed fluid shear (Fig. 9.1). Although colloid

FIGURE 9.1
A colloid mill.