Mixtures of soy protein (SP) and
gum arabic (GA) formed an electrostatic
complex in a relatively narrow pH range at very low ionic strength.
The conditions under which the complexes could be formed were determined
using turbidimetric measurements first. In salt-free condition and
1:1 SP/GA mixture, critical pH values with the formation of soluble
(pHc = 4.40), insoluble (pHφ1 = 3.55),
and maximum (pHopt = 3.15) complexes were observed. As
SP/GA ratios increased from 1:4 to 8:1, critical pH values shifted
toward higher pH. Charge densities (ZN) for SP and GA were calculated
from electrophoretic mobility using soft particle analysis theory.
Results showed that a 1:1 charge ratio at pHφ1 was
found at any SP/GA ratio, indicating that charge compensation was
fulfilled for SP/GA insoluble complex formation. A SP–GA–water
ternary phase diagram was built at pH 4.0. The influence of the total
biopolymer concentration (0–10% w/w) and SP/GA ratio was represented
in the phase diagram. At a total concentration of 0.10%, results were
consistent with the turbidity measurement; that is, no phase separation
occurred at an SP/GA ratio lower than 1:2 at pH 4.0. Salt effect (NaCl,
0–500 mmol/L) on SP/GA complexes was discussed. Results indicated
that SP/GA complexing, which led to the formation of turbidity peaks
at pH 3.2, was suppressed when NaCl concentrations were ≥50
mmol/L, whereas the remarkable increase in turbidity around pH 5.0
was caused by the aggregation of soy protein molecules on which gum
arabic could be adsorbed.
Proteins in soybean whey were separated by Tricine-SDS-PAGE and identified by MALDI-TOF/TOF-MS. In addition to β-amylase, soybean agglutinin (SBA), and Kunitz trypsin inhibitor (KTI), a 12 kDa band was found to have an amino acid sequence similar to that of Bowman-Birk protease inhibitor (BBI) and showed both trypsin and chymotrypsin inhibitor activities. The complex behavior of soybean whey proteins (SWP) with chitosan (Ch) as a function of pH and protein to polysaccharide ratio (RSWP/Ch) was studied by turbidimetric titration and SDS-PAGE. During pH titration, the ratio of zeta potentials (absolute values) for proteins to chitosan (|ZSWP|/ZCh) at the initial point of phase separation (pHφ1) was equal to the reciprocal of their mass ratio (SWP/Ch), revealing that the electric neutrality conditions were fulfilled. The maximum protein recovery (32%) was obtained at RSWP/Ch = 4:1 and pH 6.3, whereas at RSWP/Ch = 20:1 and pH 5.5, chitosan consumption was the lowest (0.196 g Ch/g recovered proteins). In the protein-chitosan complex, KTI and the 12 kDa protein were higher in content than SBA and β-amylase. However, if soybean whey was precentrifuged to remove aggregated proteins and interacted with chitosan at the conditions of SWP/Ch = 100:1, pH 4.8, and low ionic strength, KTI was found to be selectively complexed. After removal of chitosan at pH 10, a high-purity KTI (90% by SEC-HPLC) could be obtained.
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