Composition and Structural Characterization of Amaranth Protein Isolates. An Electrophoretic and Calorimetric Study

Abstract

Amaranth protein isolates were prepared by (a) extraction at different alkaline pHs and precipitation at pH 5 and (b) extraction at pH 9 and precipitation at different pHs. The isolates were compared with protein fractions. DSC analysis showed that albumin-1 was composed of several species of Td below 80 °C and ΔH below 4.0 J/g. Glutelin had two species of different Td. Both, globulin and albumin-2, had a main component of Td of 94 °C and ΔH of 19.7 ± 3.2 J/g. The thermal behavior and composition of isolates prepared by method a depended on the extraction pH. The isolate extracted at pH 8 was mainly composed of albumin-1 and globulin, whereas at pHs 9, 10, and 11, albumin-2 and glutelin were also present. The increase of the extraction pH led to a decrease in the thermal stability of proteins from pH 8 on and to a decrease in ΔH at pH 11. With method b, different isolates were obtained. At pH 6 and 7, most of the albumin-2 and some of the globulins precipitated, whereas at pHs 4 and 5, all protein fractions precipitated. Acidification at pH 5 and lower lead to denaturation of the proteins, which was only partially reversed by returning to pH 7. Keywords: Protein isolate; amaranth proteins; calorimetry; protein structure