Fractionation of potato proteins: Solubility, thermal coagulation and emulsifying properties

Water-soluble potato proteins were extracted and an acidic and a basic fraction were isolated by ion-exchange chromatography. The acidic fraction contained mainly patatin (67%), the major soluble protein of potato tubers. The basic fraction contained the 20-25-kDa proteins family (64%) and a 16-kDa polypeptide (22%), probably corresponding to the protease inhibitor potato II. Functionality as pH-solubility curves, thermocoagulation sensitivity, kinetics of creaming and resistance to coalescence were determined. The patatin fraction showed a sharp solubility minimum at pH 4 and a marked sensitivity to thermocoagulation of pH 4 or in the presence of NaCl at both pH 4 and 7. The 16-25-kDa fraction was highly soluble whatever the pH and could be readily thermocoagulated only at 80 degrees C at pH 7 or in the presence of NaCl at both pH 4 and 7. Both fractions could not be thermocoagulated at 1 mg/mL at pH values far from pI (pH 7 for the patatin fraction and pH 4 for the 16-25-kDa fraction) and without NaCl. The 16-25-kDa fraction led to stable emulsions towards creaming and coalescence whatever the pH and ionic strength. The patatin fraction exhibited poorer emulsification properties and the stability of the emulsions obtained was more pH- and ionic strength-dependent with the better results obtained near pI in the absence of NaCl. A thermal treatment (80 degrees C, 30 min) of the emulsions had a marked positive effect on the resistance to coalescence of the patatin emulsions whatever the pH and the ionic strength, while it had a deleterious effect on the resistance to coalescence of emulsions from the 16-25-kDa fraction.