3 Induced Assemblies of Food Proteins
This section focuses on fibrils, ribbons, spherulites, nanotubes and particulate
aggregates, which are the most regular macromolecular structures obtained from
food protein solutions. The structure (shape, size) of induced assemblies may be
changed by controlling parameters such as protein concentration, temperature and
time of heating, pH, presence of small solutes, etc. The structures of induced
assemblies are generic forms of proteins aggregation as long as the conditions for
protein aggregation are defined: these conditions may change depending on the
protein system [7]. For instance, under appropriate conditions, β-Lg is able to form
fibrils, ribbons, spherulites or particulate aggregates. Similar supramolecular
structures may be obtained with proteins having different structures and properties.
This means that the supramolecular structure of the induced assemblies is independent of the primary structure of the proteins; it results from identical aggregation
mechanisms occurring for different physicochemical conditions of the medium [8].
3.1 Fibrils
Amyloid-type fibrils are linear polymers of about 3–10 nm width and range in length
from hundreds of nanometres to micrometres (Fig. 2a) [23, 76]. Protein fibrils are
extensively studied and theoretical models have been proposed that explain and
predict their behaviour. Fibril formation follows a nucleation/growth mechanism
[77–81] where nucleation constitutes a lag phase in which no significant growth is
measurable. Fibril nucleation is usually the limiting step but the lag phase is reduced
by shearing, adding fibril seeds or increasing the temperature [18, 82, 83]. Fibril
growth is hierarchical and unidirectional. During this step, proteins form linear
β-sheet-organised aggregates in which β-sheet conformation is perpendicular to
the growth direction of the fibrils. Protein fibril formation requires specific physicochemical conditions allowing the proteins to expose hydrophobic patches to the
solvent and to conserve some net charge on the surface. For this reason, peptides
and intrinsically unfolded proteins such as κ-casein are well adapted to fibril formation. Fibrils of κ-casein are obtained at neutral pH and physiological temperature
(37
C) after reducing protein disulfide bonds [13, 14, 84]. Fibril formation from
globular proteins such as whey, egg white and soy proteins require a preliminary step
of unfolding and/or hydrolysis of the proteins in order to expose on the protein
surface residues prone to establish intermolecular β-sheets. Even if protein hydrolysis
was suggested to be an essential preliminary step for fibril formation [85, 86],
experimental evidence indicates that fibril formation also occurs under conditions
where an absence of protein hydrolysis is expected [19–21, 87]. In vitro, fibrils from
globular proteins are obtained under elevated temperatures, low pH and low ionic
strength [23, 85], in the presence of urea [21, 87] or at high concentration of alcohol
[20]. Arnaudov et al. [88] indicated that fibrils formed after a short heating time but
not after long heating periods are unstable and disintegrate on cooling. For numerous
76
S. Bouhallab and T. Croguennec
This section focuses on fibrils, ribbons, spherulites, nanotubes and particulate
aggregates, which are the most regular macromolecular structures obtained from
food protein solutions. The structure (shape, size) of induced assemblies may be
changed by controlling parameters such as protein concentration, temperature and
time of heating, pH, presence of small solutes, etc. The structures of induced
assemblies are generic forms of proteins aggregation as long as the conditions for
protein aggregation are defined: these conditions may change depending on the
protein system [7]. For instance, under appropriate conditions, β-Lg is able to form
fibrils, ribbons, spherulites or particulate aggregates. Similar supramolecular
structures may be obtained with proteins having different structures and properties.
This means that the supramolecular structure of the induced assemblies is independent of the primary structure of the proteins; it results from identical aggregation
mechanisms occurring for different physicochemical conditions of the medium [8].
3.1 Fibrils
Amyloid-type fibrils are linear polymers of about 3–10 nm width and range in length
from hundreds of nanometres to micrometres (Fig. 2a) [23, 76]. Protein fibrils are
extensively studied and theoretical models have been proposed that explain and
predict their behaviour. Fibril formation follows a nucleation/growth mechanism
[77–81] where nucleation constitutes a lag phase in which no significant growth is
measurable. Fibril nucleation is usually the limiting step but the lag phase is reduced
by shearing, adding fibril seeds or increasing the temperature [18, 82, 83]. Fibril
growth is hierarchical and unidirectional. During this step, proteins form linear
β-sheet-organised aggregates in which β-sheet conformation is perpendicular to
the growth direction of the fibrils. Protein fibril formation requires specific physicochemical conditions allowing the proteins to expose hydrophobic patches to the
solvent and to conserve some net charge on the surface. For this reason, peptides
and intrinsically unfolded proteins such as κ-casein are well adapted to fibril formation. Fibrils of κ-casein are obtained at neutral pH and physiological temperature
(37
C) after reducing protein disulfide bonds [13, 14, 84]. Fibril formation from
globular proteins such as whey, egg white and soy proteins require a preliminary step
of unfolding and/or hydrolysis of the proteins in order to expose on the protein
surface residues prone to establish intermolecular β-sheets. Even if protein hydrolysis
was suggested to be an essential preliminary step for fibril formation [85, 86],
experimental evidence indicates that fibril formation also occurs under conditions
where an absence of protein hydrolysis is expected [19–21, 87]. In vitro, fibrils from
globular proteins are obtained under elevated temperatures, low pH and low ionic
strength [23, 85], in the presence of urea [21, 87] or at high concentration of alcohol
[20]. Arnaudov et al. [88] indicated that fibrils formed after a short heating time but
not after long heating periods are unstable and disintegrate on cooling. For numerous
76
S. Bouhallab and T. Croguennec