THERMAL AND SURFACE-ACTIVE PROPERTIES OF CITRIC ACID-EXTRACTED AND ALKALI-EXTRACTED PROTEINS FROM PHASEOLUS BEANS

Acid-extracted crystalline protein isolates and alkali-extracted amorphous proteins from four Phaseolus beans were investigated for thermal and surface properties. Differential scanning calorimetry (DSC) analysis of crystalline isolates gave denaturation enthalpy (DELTAH) values ranging from 12.4 to 31.0 J/g; for amorphous isolates of the same beans DELTAH ranged from 10.3 to 11.9 J/g, suggesting that the crystalline proteins were less denatured than the corresponding amorphous isolates. Differences in protein content, nitrogen solubility index (NSI), surface hydrophobicity (S0), and foam expansion were observed between the alkali- and acid-extracted isolates. The alkali-extracted isolates showed protein contents ranging from 69.62 to 81.61 %, NSI of 24.23-66.75 %, So of 2128-17 000 Fl % -1, and foam expansions of 30.0-49.0%. The acid-extracted isolates showed higher protein contents (75.84-96.09%) and NSI (52.19-92.37%) but much lower S0 (1966-7479FI%-1) and low foam expansion (3.0-26.0%). Surface tension of crystalline and amorphous proteins solutions ranged from 54.8 to 58.5 and from 54.2 to 56.3 mN/m, respectively. However, the rate of decay of tension in the crystalline isolates was lower than that of the corresponding amorphous isolates. Regression analysis revealed that protein surface activity was dependent on protein content and surface hydrophobicity.