Effect of thermal treatment on the proteins of amaranth isolates

The effect of thermal treatment of proteins from Amaranthus hypochondriacus was studied. Two protein isolates were obtained from the defatted flour by water extraction at a pH of 9 (A9 isolate) and 11 (All isolate), followed by isoelectric precipitation at a pH of 5. Effect of thermal treatment (70 and 90 degrees C, during 3, 5, 10, 15 and 30 min) on A9 and A11 dispersions were analyzed by differential scanning calorimetry (DSC), polyacrylamide gel electrophoresis, UV spectrophotometry, superficial hydrophobicity and solubility in water. Thermal treatment induced the aggregate formation of high molecular mass stabilized by disulfide and non-covalent bond. Thermal treatment at 70 degrees C produced a 30% denaturation in both, while at 90 degrees C A9 was more denatured than All (75% and 55% of denaturation, respectively). An increase in thermal stability was also detected by DSC in A9 treated at 90 degrees C. The denaturation process was accompanied at short heating times by an increase in UV absorbance and changes in superficial hydrophobicity. A decrease in water solubility (35-50%, depending on time-temperature conditions) was also observed for the A9 isolates. The results suggest that the A9 isolates, enriched in a globulin protein fraction, are more sensitive to thermal treatment than isolates All enriched in glutelin protein fraction. The changes shown by both isolates, indeed, could affect their functional properties and could definitely limit their use in food products. (c) 2007 Society of Chemical Industry.