Chemistry Reference
In-Depth Information
behaviour of low moisturised protein systems.
25
Torque rheometry experiments
have been conducted on gluten
26
and soy protein isolate
27
showing the
occurrence of shear-induced crosslinking. Concerning the capillary rheology,
according to our knowledge, the only study has been performed on sunflower
protein isolate.
28
It shows that below the denaturation temperature, sunflower
protein has a shear thinning and pseudoplastic behaviour and the ''melt''
apparent viscosity follows the power-law model. Reduction of disulfide bonds
with sodium sulfite greatly enhances the mixtures flowing characteristics
(Figure 7.5).
As disulfide bonds are a key factor in the thermomechanical transformation
of protein-based materials, further explanations seem necessary.
Cysteine bridges are the only nonpeptidic covalent bonds in proteins.
Formed by the oxidation of cysteine-thiol groups, they contribute to the
stabilisation of the three-dimensional structure of proteins. Cysteine bridges of
sunflower albumins have been mapped
29
but those of the globulins have not yet
been directly studied. As most of the dicotyledon globulins have similar
structures,
30
their concentration must be close to those measured for linseed
globulins, that is 61.4 mmol/g of proteins.
31
On the whole protein fraction, their
concentration is probably close to that measured on the soybean protein
isolate, approximately 70 mmol/g.
32
Their reversible reduction
33
frees the
protein chains from native constraints, giving them more mobility (Figure 7.6).
Many molecules have been tested but the cheapest and ecient, is sodium
sulfite.
When denaturation is reached during processing, protein viscosity increases
because of an association/coagulation phenomena. Injection moulding of
d
n
9
r
3
n
g
|
5
d
y
y
f
n
n
3
.
Figure 7.5
Effect of sodium sulfite content on the viscosity of mixtures of SFPI-
glycerol-water-sodium sulfite
¼
100-30-30-x parts in weight. Temperature
of the die
¼
120 1C.
27
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