Physical Chemistry of Foods

bonds. The overall result mostly is loss of the native conformation, but not
formation of a highly unfolded state. Several sugars and polyols tend to
stabilize the native conformation, when present at high concentrations. This
is similar to the increased stability usually observed in systems with a very
low water content (Section 8.4.2).

4. Specific reagents. Here solutes are meant that destabilize
   conformation at low concentration. A prominent example is reagents that
   reduce a sulfur bridge to 22 SH groups, such as mercaptoethanol and
   dithiothreitol. Note that this implies a change in configuration (primary
   structure). The reduction leads to further unfolding of a denatured protein,
   as observed by changes in hydrodynamic size. For example, serum albumin
   has an intrinsic viscosity of 3.7 ml?g
   ^1 in its native form, 16.6 in 8 molar
   urea, and 43.2 in 8 molar urea after breaking of all sulfur bridges.
   Variousdetergentsdenature proteins at concentrations in the range of
   1 to 10 millimolar. SDS (sodium dodecylsulfate) is often used, but
   detergents with a more apolar chain are even more effective. The apolar
   part strongly binds to hydrophobic regions in the protein molecule, thereby
   disrupting the native structure, but it appears that the ionic group is also
   essential. Detergent-denatured proteins tend to fully regain their native
   conformation after removal of the detergent by dialysis.


5. High pressure. Very high pressure treatments are used in food
   processing for several purposes, for instance to kill microorganisms, while
   very little chemical reactions occur. The main mechanism is that high
   pressures cause denaturation, or at least unfolding, of globular proteins. The
   unfolding occurs over a narrow pressure interval, indicating a cooperative
   transition between two states. The pressure needed greatly depends on
   temperature (Figure 7.7c) and pH (7.7d).
   The effect of pressure on a chemical equilibrium follows theprinciple of
   le Chatelier: an increase in temperature shifts the equilibrium in the direction
   of highest enthalpy for an endothermic reaction, an increase in pressure in
   that of smallest volume. The relation giving the temperature effect is
   obtained by differentiating Eq. (7.3) at constant pressure, resulting in

R

qlnK

qð 1 =TÞ



p

¼DN?UH ð 7 :4aÞ

whereDH>0 forT>Topt(Figure 7.5). For the pressure effect a similar
relation holds

RT

qlnK

qp



T

¼
DN?UV ð 7 :4bÞ