Thermal instability is manifested by the formation of large coagulums of heat-denatured
protein or peptide molecules, and the light scattering
effect of these coagulums can be observed optically
as turbidity. The turbidity values reflect the varying
rates of protein aggregation combined with the
aggregate size distribution. Thermal stability of the
BPC and BPHs was characterized by determining
turbidity (A400) of the protein solution with varied
NaCl concentrations, after heating at 80o
C for 30 min.
Turbidity observations for the BPC and the BPHs are
shown in Figure 1B. At a fixed NaCl concentration,
the BPHs had dramatically lower turbidities than
the BPC, and the trend was decreasing with DH
(p< 0.05). This indicates that the BPHs are more
tolerant to thermal treatment than the BPC in the
presence of NaCl, regardless of its concentration.
The turbidities of the BPC and the BPHs increased
with NaCl concentrations from 0 to 25 mM. The
NaCl promotes heat induced aggregation of proteins,
such as whey protein (Majhi et al., 2006). The effects
of low ionic strength on the thermal aggregation of
Bambara groundnut protein may explained by effects
on the protein structure that expose hydrophobic
groups. Similarly, Xiong (1992) showed that, at
pH 6.0, addition of up to 20 mM NaCl accelerated
whey protein isolate aggregation when heated
from 60 to 90o
C. In our case, the protein solution
turbidity seemed to decrease beyond 50 mM NaCl
concentration. A high ionic strength might impact
the charge distribution in the side chains, reducing
charge repulsion that prevents thermal aggregation
of the protein. Von Hippel and Schleich, (1969)
suggest that the ability of electrolytes to influence the
conformation and stability of proteins depends on the
concentration and/or ionic strength of the salt.