2009_Book_FoodChemistry

1.4 Proteins 57
bridges, ionic or hydrophobic bonds. This can be
accomplished by: changing the temperature, adjusting
the pH, increasing the interface area, or
adding organic solvents, salts, urea, guanidine hyrochloride
or detergents such as sodium dodecyl
sulfate. Denaturation is generally reversible when
the peptide chain is stabilized in its unfolded state
by the denaturing agent and the native conformation
can be reestablished after removal of the
agent. Irreversible denaturation occurs when the
unfolded peptide chain is stabilized by interaction
with other chains (as occurs for instance with egg
proteins during boiling). During unfolding reactive
groups, such as thiol groups, that were buried
or blocked, may be exposed. Their participation
in the formation of disulfide bonds may also cause
an irreversible denaturation.
An aggregation of the peptide chains caused by
the folding of globular proteins is connected with
reduced solubility or swellability. Thus the part
of wheat gluten that is soluble in acetic acid diminishes
as heat stress increases (Fig. 1.29). As
a result of the reduced rising capacity of gluten
caused by the pre-treatment, the volume of bread
made of recombined flours is smaller (Fig. 1.30).
In the case of fibrous proteins, denaturation,
through destruction of the highly ordered structure,
generally leads to increased solubility or
rising capacity. One example is the thermally
caused collagen-to-gelatin conversion, which
occurs when meat is cooked (cf. 12.3.2.3.1).
The thermal denaturation of the whey proteins
β-lactoglobulin and α-lactalbumin has been wellstudied.
The data in Table 1.27 based on reaction
kinetics and the Arrhenius diagram (Fig. 1.31)
indicate that the activation energy of the overall
reaction in the range of 80–90 ◦ C changes. The
higher E a values at lower temperatures must be
attributed to folding, which is the partial reaction
that determines the reaction rate at temperatures
<90 ◦ C. At higher temperatures (>95 ◦ C),
the aggregation to which the lower activation energy
corresponds predominates.
Fig. 1.30. Volume of white bread of recombined flours
using thermally treated liquid gluten (wheat) (according
to Pence et al., 1953)
Fig. 1.29. Solubility of moist gluten (wheat) in diluted
acetic acid after various forms of thermal stress (according
to Pence et al., 1953)
Fig. 1.31. Arrhenius diagram for the denaturation of the
whey proteins β-lactoglobulin A, β-lactoglobulin B and
α-lactalbumin B (according to Kessler, 1988)