Handling of Antifoam Oils for Fermentation
What is Foam?
Foaming is the presistance of bubbles caused by fermenter
aeration and agitation.
Check out some foam
Click on the glass to read a story
about Andy's favorite foam!
Foam, Who Cares?
Foam is very important part of the fermentation process.
Severe foaming in a fermentation vessel may cause contamination
or loss of material, either of which will decrease yields and
invalidate experimental data. While there are reports of
satisfactory foam control by mechanical means, the great majority
of troublesome foam are treated with defoaming agents. In
addition to foam control, oils added to fermentations usually
participate in the nutrition of the culture and may sometimes be
added in excess for crude pH adjustment with the acids produced
by metabolism of the oil.
Detection
There are several methods for detecting foam. Manual control
by inspection and valve turning is poor because close attention
is too wasteful of man power, and errors lead to serious loss of
contents and to contamination.
Automatic oil addition depends
on reliable oil detection in the vessel. Mechanical devices such
as a paddle or a float could sence the foam level however,
electrical methods are better because there are no sterility
problems. One method is to use a conducting probe, insulated
from the wall of the tank, placed so that the maximum allowable
rise of foam completes a circuit and a current flow is detected.
Relays are actuated and can control the valves or pumps in the
oil system. One problem with the probes is that they are
sometimes shorted by collection of material across the insulation
at the point of entuy into the tank. This results in addition of
excess antifoam oil in amounts which may spoil the batch.
Another type of device senses the change in capacitance as
foam becomes the dielectric in the danger zone in the tank.
Again simple relay circuitry is used to signal the antifoam oil
controls. Unlike the probes the capacitance detector cannot be
fouled.
Foam Control!
Foam reduction can be performed by use of mechanical foam
breakers or the addition of antifoams. Mechanical methods are a
crude approach in the removal of foam. Therefore, antifoams are
extensivly used in industry. Control of foams by oil addition is
of large economic importance to the fermentation industry.
Excessive foaming causes loss of material and contamination,
while excessive oil additions may decrease the product formation. Antifoam oils may be synthetic, such as
silicones or
polyglycols, or natural, such as lard,soybean, linseed, tall,
and rape seed oils. Either will substantially change the
physical strusture of foam, principally by reducing
surface
elasticity. Some alcohols are suitable for anitfoame use,
those derived from fatty acids are best.
Antifoam
Control
The prime action of a defoamer is its ability to reduce
surface elasticity.Industrial antifoam systems usually operate
automatically from levelsensing devices. Methods for metering of
oil under aseptic
conditions are:
The metering of sterile oil is a rather specialized operation.
For large fermenters, timed delivery is sometimes adequate to
obtain
proper oil shot size. The foam detection apparatus trips a timer
which in turn opens a valve for the desired interval. With
fermentations, which metabolize the oils with beneficial effects,
this crude control may be desirable because of the tendency to
exceed the minimum amounts of oil required to break the foam.
An inexpensive device can be installed on a fermenter to record
antifoam shots using a
temperature chart.
Entry
There are many devices that are used in the delivery of
antifoam to the fermenter. Distribution devices can improve
efficiency over straight-pipe entry. The best method of entry
is the use of
wick devices.
Another way oil can be delivered is by use of a
disc feeder
placed inside the fermentor. Unfortunately, during sterilization
it traps moisture which collects under the oil and floats it out
when it's not needed. Another way to introduce oil is by a
blow-pot
principle that gives an exact shot of oil by forcing the contents
of a small chamber into the fermentor with air pressure.
So you think you know all their is to know about foam?
Try these practice questions, you just might see them on your next quiz.
Source: Handling of Antifoam Oils for Fermentations,
H. R. Bungay, C.F. Simons and P. Hosler.
Web pages created by Andrew Welch and Yvonne Thomas, 1997.