Food Chemistry

1.4 Proteins 57

bridges, ionic or hydrophobic bonds. This can be
accomplished by: changing the temperature, ad-
justing the pH, increasing the interface area, or
adding organic solvents, salts, urea, guanidine hy-
rochloride 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 confor-
mation 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 reac-
tive 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 di-
minishes 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 struc-
ture, generally leads to increased solubility or

Fig. 1.29.Solubility of moist gluten (wheat) in diluted
acetic acid after various forms of thermal stress (ac-
cording toPenceet al., 1953)

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 andα-lactalbumin has been well-

studied. The data in Table 1.27 based on reaction

kinetics and theArrheniusdiagram (Fig. 1.31)

indicate that the activation energy of the overall

reaction in the range of 80–90◦C changes. The

higher Eavalues at lower temperatures must be

attributed to folding, which is the partial reac-

tion that determines the reaction rate at temper-

atures< 90 ◦C. At higher temperatures (> 95 ◦C),

the aggregation to which the lower activation en-

ergy corresponds predominates.

Fig. 1.30.Volume of white bread of recombined flours

using thermally treated liquid gluten (wheat) (accord-

ing toPenceet al., 1953)

Fig. 1.31.Arrheniusdiagram for the denaturation of the

whey proteinsβ-lactoglobulin A,β-lactoglobulin B and

α-lactalbumin B (according toKessler, 1988)