1.
Introduction
Foam in fermentation processes
Foam in fermentation process is defined as the entrapment of many air
bubbles or other gases in culture medium or fermentation broth. A general
definition of foam in fermentation process, determines foam to occur when gas
holdup in a gas-liquid dispersion is greater than 60% (Van’t Riet and Tramper,
1991; Schubert et al., 1993). Two main types of foam are common in
fermentations which are classified as unstable foams, which are short-lived,
transitory, containing a wide range of bubble sizes and stable foams, which are
usually smaller and uniform in size, long-lasting, rigid.
Fermentation is often accompanied by foam formation because of the
high foaming tendency of solutions containing biomaterials such as proteins
(Wilde and Clark, 1996). Foam never occurs in pure liquids even upon gas
introduction and stirring due to of lack of surface active components (Pahl and
Franke, 1995). Conditions that affect the degree of foaming during fermentation
include gas introduction, medium composition, cell growth, metabolite
formation, surface-active substance formation and indirectly, vessel geometry
(Taticek et al., 1991; Vardar-Sukan, 1992). The fermentation processes that
produce excessive foam are of mainly cell cultures and aerobic fermentations
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.2
which depend on the aeration. A little amount of foam doesn’t cause any
problems but in excess leads to problems while handling fermentation process.
Excessive foam and its consequences in fermentation process
Excessive foam formation in fermentation process remains as a major
technological challenge and require large attention and investigation. The foam
in a fermentor moves upwards because of its low density compared to the
liquid medium. If produced in excess, the foam touches the lid of fermentor and
tries to come out through the exhaust outlet filter. In this case the wet and sticky
media or the subsequent fouling of microorganisms can block the exhaust filter
and cause the pressure to rise drastically within the fermentor. High pressures
in the vessel can lead to slow cultivations or reduced productivity. If the filters
are completely blocked, the elevated pressure in the vessel can cause the filters
either to burst or trigger the opening of high pressure valve of the fermentor
and discharge of the fermentor content into the area surrounding the fermentor,
which can represent potential health hazard and certainly results in a large
cleaning task (Laven, 1990).
Sometimes foam builds up so rapidly that within a few seconds the
entire liquid contents turn to foam and may overflow before any remedial
action can be taken specifically in an aerobic fermentation. The presence of
small bubbles from foam also can interfere with electrode sensors and cause
false readings (Pandit, 1989; Vardar-Sukan, 1992). Microorganisms trapped in
INTRODUCTION
.3
foam experience oxygen and nutrient limitations (Vogel, 1983; Pandit, 1989).
Removal of cells from medium due to foam formation can cause autolysis
which releases microbial proteins that enhance foam stability (Stanbury and
Whitaker, 1984).
In general a process can run quite effectively with a certain amount of
foam but there are certain case where there should not be any foam to be
present in the fermentation process. It is very important that foam should be
detected and controlled to avoid such difficulties in fermentation.
Detection of foam
Manual observation of foam by the operator is the basic method of foam
detection. In order to avoid the difficulties with the manual observation, foam
detection sensors were developed that are inserted into the fermentor at a
height. There are several types of foam sensors to detect the presence and in
some cases, depth of foam. Conductive probes based on conduction of electrical
charge i.e., DC voltage between the probe and vessel by ions present in the
liquid (Brown et al., 2001), are sensitive to biofilm (Evans and Hall, 1971), which
causes a permanent short circuit due to the collection of material across the
probe’s insulation (Ghildyal et al., 1988). Capacitance probes have less biofilm
interference than conductive probes since the entire capacitance probe is
covered with insulation (Evans and Hall, 1971).
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Capacitance is sensed as the foam becomes dielectric (Ghildyal et al.,
1988). Both conductance and capacitance foam sensors are able to measure
liquid level (Getchell, 1983; Brown et al., 2001), possessing a user-adjustable
detection level rather than fixed point. Less common are admittance probes,
which use high-frequency signals with wide band measuring bridges and
amplifiers able to measure foam even with a biofilm present (PharmaTec, 2004).
Another alternative for foam detection is sonar level detection based on the
presence of a foam layer in some cases interfering with sonar feedback. All
antifoams tend to reduce the ability of gas to transfer into the liquid phase. For
these reasons, antifoaming agents have to be added in a well controlled way
and in small doses which is difficult to achieve without accurate sensing or
detection.
Foam controlling methods
In general foam controlling methods falls into three categories, physical,
mechanical and chemical methods. Physical methods are designed to prevent
foam by using ultrasound, thermal or electrical treatment. Mechanical methods
are designed to break the foam by mechanical devices like a centrifugal foam
breaker, external foam breaker and nozzle. Chemical methods are designed to
break the foam by adding chemical agents usually called as either antifoam or
defoamer. Strictly speaking antifoam is added before the start of the
fermentation and defoamer during the fermentation but the terms are
frequently used indiscriminately.
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Since the physical methods were not recommended for fermentations,
one has to depend on either mechanical or chemical methods. Both chemical
and mechanical methods offer advantages and disadvantages for defoaming in
fermentations. Both methods should be evaluated to determine which is most
efficient and cost effective for specific fermentation process. Chemical method
of foam controlling is recommended for most of the fermentations.
Chemical methods of foam controlling
Chemical methods are designed to break the foam by adding chemical
agents usually called as either antifoam or defoamer into the fermentor in
different ways. The general way of adding antifoam is by manual addition by
the operator or by automatic method of addition using a tube and pump.
The efficiency of chemical foam controlling methods depends on
parameters like detection of foam, selection of suitable defoaming agent and
addition methods of defoamer into the fermentor. Straight pipe entry of
antifoam or defoamer into the fermentor is the earliest method reported.
Bungay et al. (1960) described that the antifoam distribution devices like spray
distributor, diverter bars, wick device, disc feeder and blow pot can improve
defoaming efficiency over straight pipe entry in a fermentor.
All automatic methods of adding antifoaming agents on demand are
basically same that a fermentor vessel is fitted with a sensor probe and when
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the foam reaches, the circuit that was in connection with the sensor is closed
allowing the antifoaming agent to enter the fermentor. Once the foam is broken
then the circuit is opened allowing the solenoid valve to stop the entry of
antifoaming agent. So it was designed to add the antifoam in shots to minimise
the antifoam consumption. Despite of many advantages, chemical methods of
foam controlling have few problems which are described below.
Problems with the chemical methods of foam controlling
Foam sensing is a difficult task: Manual observation of foam by the
operator is tedious because of the requirement of continuous monitoring for the
presence of foam and especially in case of large fermentors. No operator can be
expected to watch the fermentor continuously for hours or days (PharmaTec,
2004). Often the foam may retain on the viewing glass which leads to false
finding of foam by the operator and triggering of unnecessary addition of
antifoam by the operator which further causes the disruption in the process.
The automatic foam detection methods which depend on the use of a
probe are prone to fouling by condensation and surface growth as the foam
leaves sticky deposits over the probe which obviously leads to the false
readings and causes overdosing of antifoam. The overdosing of antifoam causes
unfavourable conditions to the actively growing culture and may lead to poor
yield of the product of interest. The foam sensing probe requires proper
calibration other wise leads to false detection of foam which triggers
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unnecessary addition of antifoam. The foam sensing probe may fail at any time
during the fermentation thereby disrupting the process. So far these kinds of
experiences have convinced that the sensing of foam cannot be done reliably.
All sensor probes are somewhat unreliable because they give spurious signals
when they become fouled (Garry Montague, 1997).
Defoamer addition is often a problem: The defoamer can be added
into the fermentor through a pump and tubing which may sometimes leads to
the operational problems like failure of the antifoam addition pump and
damage of the antifoam supply tubing in the pump head by that not supplies
the required defoamer on demand and leads to the poor foam controlling.
Ready to use antifoaming agents (for example: hyclone antifoam-pre sterilized,
from Thermo Scientific) also have to be added to the fementor using a tube and
pump which may cause operational troubles.