The addition of calcium (1.23−9.73 mg/g of protein) during
neutralization of isoelectric precipitate
modified the thermal behavior of soybean protein isolates. In the
samples that did not undergo
thermal treatment, the calcium content increase caused an increase in
thermal stability, especially
in the 11S fraction, but no modifications were detected in the enthalpy
values. Samples undergoing
mild thermal treatments (5 min at 80 °C), after neutralization with
different amounts of calcium,
showed enthalpy values lower than those of the unheated samples.
Isolates with no calcium
aggregates or with values below 5 mg/g of protein and intense thermal
treatment (15 min at 90 °C)
showed no endotherms (denaturation enthalpies = 0). Calcium
levels above 5 mg/g of protein
protected the soybean proteins, partially preventing their denaturation
by heating at 90 °C for 15
min. The activation energy of the thermal denaturation process of
soybean protein isolates,
calculated by the Ozawa method (1970), was modified in the presence of
calcium. The E
a of the
11S fraction varied between 51.12 and 102.15 kcal/mol and that of the
7S between 42.69 and 58.05
kcal/mol. A minimal value for glycinin E
a
was observed at a calcium concentration of 6.51 mg/g of
protein. The E
a value of the fraction
enriched in 11S, calculated by the Borchardt and Daniels
method (1957), was higher than that obtained by the Ozawa method (1970)
for a soybean isolate.
This fraction would denature following second-order kinetics
(n = 2).
Keywords: Thermal properties; protein denaturation; interaction calcium;
soybean proteins
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