1
Introduction
Foams play an important role in several fields of human life: in food technology, medicine, cosmetics, oceanography, environmental technology, fire extinguishing technology, etc. Therefore, foams were investigated very early by
natural scientists. Foam films and Plateau borders were characterized, and the
formation and structure of foams were described. The mechanical, optical and
electrical properties of foams and theories for foam stability were presented [1].
Most of the physicochemical investigations were carried out with surfactant
foams. However, in biotechnology, protein foams in combination with surfactants play a role. The properties of these protein and protein/surfactant
foams differ considerably from those of pure surfactant foams. Therefore, the
results evaluated by surfactant foams can only be partially applied to protein
foams. Since proteins adsorb at interfaces at very low concentrations, protein
concentrations of as little as 1 mg l
–1 can influence foaming [2]. Protein concentration in industrial cultivation media are far above this limit, because of the
high protein content of complex nutrient media and because the microorganisms produce proteins and excrete them into the cultivation medium. In
this chapter model protein foams and protein/surfactant foams formed in
cultivation media, and their effect on flotation of proteins and microorganisms,
are discussed. The results with model protein foams are compared with those of
cultivation foams.
Foam flotation during biological waste water treatments is not considered,
because of the undefined properties of these systems. Aqueous media have
unknown and often changing compositions and the mixed cultures consist of
uninvestigated and only partly identified organisms, respectively.
2
Characterization of Biological Foams
The main components of foam formation in cultivation media are proteins.
Therefore, several authors have applied solutions of particular proteins and
used them as model media to investigate the behavior of biological foams. In
this section foams formed by various protein solutions and foams produced by
cultivation media are characterized. In addition, the properties of foams suppressed by antifoam agents (AFAs) are described.
2.1
Protein Foams
The high foaming capacity of protein solutions is explained by the stabilization
of the gas/liquid interface due to the denaturation of proteins, in particular due
to their strong adsorption at the interface. According to Cumper et al. [3] the
adsorption process takes place in three main stages: (1) diffusion of the native
protein molecules to the interface and their adsorption, (2) uncoiling of the
polypeptide chains at the interface (surface denaturation), and (3) aggregation
194
K. Schügerl
Introduction
Foams play an important role in several fields of human life: in food technology, medicine, cosmetics, oceanography, environmental technology, fire extinguishing technology, etc. Therefore, foams were investigated very early by
natural scientists. Foam films and Plateau borders were characterized, and the
formation and structure of foams were described. The mechanical, optical and
electrical properties of foams and theories for foam stability were presented [1].
Most of the physicochemical investigations were carried out with surfactant
foams. However, in biotechnology, protein foams in combination with surfactants play a role. The properties of these protein and protein/surfactant
foams differ considerably from those of pure surfactant foams. Therefore, the
results evaluated by surfactant foams can only be partially applied to protein
foams. Since proteins adsorb at interfaces at very low concentrations, protein
concentrations of as little as 1 mg l
–1 can influence foaming [2]. Protein concentration in industrial cultivation media are far above this limit, because of the
high protein content of complex nutrient media and because the microorganisms produce proteins and excrete them into the cultivation medium. In
this chapter model protein foams and protein/surfactant foams formed in
cultivation media, and their effect on flotation of proteins and microorganisms,
are discussed. The results with model protein foams are compared with those of
cultivation foams.
Foam flotation during biological waste water treatments is not considered,
because of the undefined properties of these systems. Aqueous media have
unknown and often changing compositions and the mixed cultures consist of
uninvestigated and only partly identified organisms, respectively.
2
Characterization of Biological Foams
The main components of foam formation in cultivation media are proteins.
Therefore, several authors have applied solutions of particular proteins and
used them as model media to investigate the behavior of biological foams. In
this section foams formed by various protein solutions and foams produced by
cultivation media are characterized. In addition, the properties of foams suppressed by antifoam agents (AFAs) are described.
2.1
Protein Foams
The high foaming capacity of protein solutions is explained by the stabilization
of the gas/liquid interface due to the denaturation of proteins, in particular due
to their strong adsorption at the interface. According to Cumper et al. [3] the
adsorption process takes place in three main stages: (1) diffusion of the native
protein molecules to the interface and their adsorption, (2) uncoiling of the
polypeptide chains at the interface (surface denaturation), and (3) aggregation
194
K. Schügerl