Biomedical Engineering Reference
In-Depth Information
emulsified meats, egg white replacement in cakes, and substitution of nonfat dry
milk in baked goods, all pointed to a lower cost per unit of performance, while at
the same time, maintaining the required level of consumer acceptance.
In the domestic market, soy protein products gained acceptance as useful and
economical ingredients for the manufacture of conventional foods, and in the design
of new foods. More food package labels show soy protein products in their ingredient
listings. This is a trend that is expected to continue.
C
HEMICALLYAND
E
NZYMATICALLY
M
ODIFIED
P
ROTEINS
Many physical and chemical properties of proteins can be modified to enhance their
surface acivity.
165
Some of the most important characteristics, such as film strength,
viscoelasticity, and colloid stability, can be changed by extrinsic factors (pH, ionic
strength, temperature, etc.) while others such as hydrophobicity, flexibility of the
polymer, net charge, etc. can be modified (chemically or enzymatically) by altering
the intrinsic properties of the protein. Chemical modification of proteins affects both
protein-protein interactions as well as surface activity.
It is beyond the scope of this chapter to elaborate on the different studies
conducted on the various amphiphilic modified proteins. Some of the most common
extrinsic modifications are
166-170
(1) change of pH which will affect the film-forming
properties of BSA (rather than lowering the degree of electrostatic repulsions or
steric stabilization) or will affect the hydrophobicity of BLG (which will result in
TABLE 7.11
Effect of pH on Some Film Properties (Surface Pressure, Surface Yield Stress,
and Film Elasticity) and Foaming Properties (Drainage Half-Life) of Bovine
Serum Albumin (BSA)
Film
Surface Pressure
a
Surface Yield Stress
b
Film Elasticity
Foam Drainage Half-Life
(min)
pH
(mN m
-1
)
(mN m
-1
)
(mN m
-1
)
4.0
2.8
3.0
2.2
5.0
5.0
15.0
3.8
5.0
8.0
5.5
19.0
4.0
5.2
9.6
6.0
14.0
4.3
5.4
8.5
7.0
10.0
3.0
2.3
6.3
8.0
2.0
2.2
1.8
6.0
a
Surface pressure of protein solutions (5
×
10
-3
wt% in 10 mM citrate) was measured after 5 min at
a temperature of 23°C.
b
Surface yield stress was estimated by measuring surface viscosity using a Brookfield viscometer:
protein solutions of 0.1 wt% in 10 mM citrate (pH 3 to 5.5) and phosphate (pH 5.7 to 8) buffers at
3°C after 5 min.
From Kim, S. H. and Kinsella, J. E.,
J. Food Sci.,
50, 1526, 1985. With permission.
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