Industrial Use of Oil Cakes for Material Applications - The Economic Utilisation of Food Co-Products - page 142

7.2.3 Thermal Denaturation of Proteins
Protein denaturation is commonly defined as any noncovalent change in the
structure of a protein. This change may alter the secondary, tertiary, or quat-
ernary structure of the molecules. Modifications in the structure of proteins can
be caused by many factors, e.g., heat, pH, dielectric constant and ionic
strength. 14
When proteins are exposed to increasing temperature, losses of solubility
or enzymatic activity occur over a fairly narrow range. Depending upon
the considered protein and the severity of the heating, these changes may
be reversible or irreversible. As the temperature is increased, a number of
bonds in the protein molecule are weakened. The first affected are the long-
range interactions that are necessary for the configuration of the tertiary
structure. As these bonds are first weakened, and then broken, the protein
becomes more flexible. As heating continues, some of the cooperative
hydrogen bonds that stabilise the helical structure will begin to break and
hydrophobic groups are exposed. Molecular mobility and high temperature
result in protein aggregation (coagulation), making the denaturation
irreversible.
The denaturation phenomenon is often monitored by DSC analysis of
a protein solution or suspension. The corresponding endothermic peak,
which depends on the conformational state of the protein, is found around
70-90 1C. 15 Under low moisture conditions, similar to those used for a
thermomechanical transformation (i.e. extrusion, hot pressing), the use of
pressure-resistant DSC pans makes it possible to study the denaturation
process. 16 The denaturation temperature of sunflower helianthinins, the main
protein in sunflower, for example, decreased from 190 to 120 1C as the oil cake
moisture content increased from 0 to 30% (Figure 7.3). Exothermic aggre-
gation was then related to the decrease of the denaturation enthalpy but, more
recently, work on heat-induced denaturation of soy proteins stated that a
specific exothermic peak is observable at 220 1C. 17
Controlled denaturation, or the prevention of denaturation, is an important
aspect in the development of protein food applications. For the use of proteins
in nonfood applications such as surfactants, adhesives, coatings or plastics, it is
accepted that a certain degree of denaturation must occur in order to make
proteins processable, and to reach the required product properties such as
strength, water resistance or adhesion. 18

d n 9 r 3 n g | 5

d y y f n n 3 .

7.2.4 Glass Transition of Oilseed Proteins
The glass-transition temperature, T g , is the temperature at which an
amorphous solid, such as glass or a polymer, becomes brittle on cooling, or
softens on heating.
In the 1980s, with the intense development of theories on the glassy
state of foods, 19 almost all biopolymers have been tested to assess their T g .
Among oilseed proteins, only soy globulins were considered. 20 But for many