2009_Book_FoodChemistry

58 1 Amino Acids, Peptides, Proteins
Table 1.27. Denaturation of β-lactoglobulins A and B
(β-LG-A, β-LG-B) and of α-lactalbumin (a-LA)
Protein n ϑ E a ln(k o ) ΔS≠
( ◦ C) (kJmol −1 )(s −1 ) (kJmol −1 )
K −1
β-LG-A 1.5 70–90 265.21 84.16 0.445
95–150 54.07 14.41 − 0.136
β-LG-B 1.5 70–90 279.96 89.43 0.487
95–150 47.75 12.66 − 0.150
α-LA 1.0 70–80 268.56 84.92 0.452
85–150 69.01 16.95 − 0.115
n: reaction order, δ: temperature, E a : activation energy,
k o : reaction rate constant, ΔS≠: activation entropy.
The values in Table 1.27 determined for activation
entropy also support the above mentioned
attribution. In the temperature range of 70–90 ◦ C,
S # is always positive, which indicates a state
of greater disorder than should be expected
with the predominance of the folding reaction.
On the other hand, the negative S # values at
95–105 ◦ C indicate a state of greater order than
should be expected considering that aggregation
predominates in this temperature range. Detailed
studies of the kind described above allow optimal
control of thermal processes. In the case of
milk processing, the data have made it possible,
for example, to avoid the separation of whey
proteins in heating equipment and to optimize
the properties of yogurt gels (cf. 10.1.3.3
and 10.2.1.2).
Figure 1.32 shows the denaturation of β-LG
in a diagram that combines the heating period
with the temperature (cf. 2.5.4.3) in the
form of straight lines of equal denaturation
degrees. This allows us to read directly the
time/temperature combinations required for
a certain desired effect. At 85 ◦ C/136 s for
example, only 60% of the β-LG-B are folded,
so that only 60% can aggregate, although 90%
would be potentially able to aggregate: at this
temperature, the folding determines the overall
reaction, as shown above. Conversely, 90% of
the protein is potentially folded at 95 ◦ C/21 s
while only 60% can be aggregated. At this
temperature, aggregation determines the overall
reaction.
Denaturation of biologically active proteins is
usually associated with loss of activity. The fact
Fig. 1.32. Lines of equal denaturation degrees of
β-lactoglobulin B. [The steeper lines correspond to the
folding (60%, 90%), the flatter lines to aggregation
(60%, 90%); at point a, 60% are folded and 90% can
be aggregated, corresponding to an overall reaction
of 60%; at point b, 90% are folded and 60% can be aggregated,
corresponding to an overall reaction of 60%;
according to Kessler, 1988]
that denatured proteins are more readily digested
by proteolytic enzymes is also of interest.
1.4.3 Physical Properties
1.4.3.1 Dissociation
Proteins, like amino acids, are amphoteric.
Depending on pH, they can exist as polyvalent
cations, anions or zwitter ions. Proteins differ
in their α-carboxyl and α-amino groups – since
these groups are linked together by peptide
bonds, the uptake or release of protons is limited
to free terminal groups. Therefore, most of the
dissociable functional groups are derived from
side chains. Table 1.28 lists pK values of some
protein groups. In contrast to free amino acids,
these values fluctuate greatly for proteins since
the dissociation is influenced by neighboring
groups in the macromolecule. For example, in
lysozyme the γ-carboxyl group of Glu 35 has a pK