If foam formation cannot be avoided, it can be destroyed mechanically by
foam breakers, physically by ultrasound, heat or electrical methods, or chemically by antifoam agents [33]. In industrial production, with a few exceptions,
mechanical foam breakers (e.g. steroid biotransformation [34]) are not used
because of their high power input demand, which is often higher than the
power input by the stirrer. Physical methods are not used either, because
ultrasound, heat or electric treatment can impair the viability of the microorganisms. Therefore, only chemical methods are considered in this review.
2.3.1
Foam Suppression by Chemical Antifoam Agents
For foam suppression during antibiotic production often antifoam agents
(AFAs) are used which can be metabolized (e.g. soy oil). For enzyme production, inert antifoam agents, which cannot be metabolized by the microorganisms, are preferred [35].
According to Ross [36] and Robinson and Woods [37], AFAs may affect foam
in two different ways: (1) The antifoam agent is dispersed into very small
droplets which penetrate into the foam lamellae and form a duplex film. This
film spreads on the lamellae. It bursts because of the strain caused by the extension of the duplex film. (2) The antifoam agent penetrates into the lamellae
and forms a mixed monolayer on the lamellae which has less cohesion than the
lamellae-stabilizing protein film in the absence of antifoam.
Antifoam agents destroy the surface elasticity and surface viscosity of the
foaming system. The antifoam agent must have, therefore, low surface tension
to spread on the foam lamellae. To be active at low concentrations, they must
also be insoluble in the foaming medium [38].
Vardar-Sukan [29] evaluated the efficiency of different natural oils with
unsterilized and sterilized model media consisting of soybean flour and sugar
beet cosette. Soybean was most efficient in unsterilized and cotton seed for
sterilized soybean model media. Sunflower oil was most efficient in unsterilized
and cotton seed in sterilized sugar beet model media. Their optimum concentrations are in the range 0.1 to 0.6% v/v.
In the following sections, a few examples of the effect of antifoam agents on
the properties of cultivation medium and foam are considered. Most of the
authors evaluated the effect of antifoam agents on the volumetric mass transfer
coefficients in bioreactors.
2.3.2
Influence of Antifoam Agents on the Oxygen Transfer Rate
Systematic investigations were carried out by Adler et al. [39–41] and König et
al. [26] with various cultivation media. The volumetric mass transfer coefficients k L a were determined by a steady state method with distilled water,
nutrient salt solution and various cultivation media in the presence and absence
of antifoam agents.Volumetric mass transfer coefficients are strongly enhanced
by increasing aeration rate. At low superficial gas velocities (< 2.5 cm s –1 ) the
200
K. Schügerl
foam breakers, physically by ultrasound, heat or electrical methods, or chemically by antifoam agents [33]. In industrial production, with a few exceptions,
mechanical foam breakers (e.g. steroid biotransformation [34]) are not used
because of their high power input demand, which is often higher than the
power input by the stirrer. Physical methods are not used either, because
ultrasound, heat or electric treatment can impair the viability of the microorganisms. Therefore, only chemical methods are considered in this review.
2.3.1
Foam Suppression by Chemical Antifoam Agents
For foam suppression during antibiotic production often antifoam agents
(AFAs) are used which can be metabolized (e.g. soy oil). For enzyme production, inert antifoam agents, which cannot be metabolized by the microorganisms, are preferred [35].
According to Ross [36] and Robinson and Woods [37], AFAs may affect foam
in two different ways: (1) The antifoam agent is dispersed into very small
droplets which penetrate into the foam lamellae and form a duplex film. This
film spreads on the lamellae. It bursts because of the strain caused by the extension of the duplex film. (2) The antifoam agent penetrates into the lamellae
and forms a mixed monolayer on the lamellae which has less cohesion than the
lamellae-stabilizing protein film in the absence of antifoam.
Antifoam agents destroy the surface elasticity and surface viscosity of the
foaming system. The antifoam agent must have, therefore, low surface tension
to spread on the foam lamellae. To be active at low concentrations, they must
also be insoluble in the foaming medium [38].
Vardar-Sukan [29] evaluated the efficiency of different natural oils with
unsterilized and sterilized model media consisting of soybean flour and sugar
beet cosette. Soybean was most efficient in unsterilized and cotton seed for
sterilized soybean model media. Sunflower oil was most efficient in unsterilized
and cotton seed in sterilized sugar beet model media. Their optimum concentrations are in the range 0.1 to 0.6% v/v.
In the following sections, a few examples of the effect of antifoam agents on
the properties of cultivation medium and foam are considered. Most of the
authors evaluated the effect of antifoam agents on the volumetric mass transfer
coefficients in bioreactors.
2.3.2
Influence of Antifoam Agents on the Oxygen Transfer Rate
Systematic investigations were carried out by Adler et al. [39–41] and König et
al. [26] with various cultivation media. The volumetric mass transfer coefficients k L a were determined by a steady state method with distilled water,
nutrient salt solution and various cultivation media in the presence and absence
of antifoam agents.Volumetric mass transfer coefficients are strongly enhanced
by increasing aeration rate. At low superficial gas velocities (< 2.5 cm s –1 ) the
200
K. Schügerl