Influence of pH on Structure and Function of Amaranth (Amaranthus hypochondriacus) Protein Isolates

Abstract

Cereal Chem. 87(5):448-453Structural and functional properties of two amaranth protein isolates as a function of pH were studied. Isolates, A9 and A11, were obtained by alkaline extraction at pH 9 and 11, respectively. Gel filtration chromatograms of A9 and A11 showed similar profiles. The A11 isolate contained mainly albumins and globulins, and a small proportion of globulin-P aggregates, suggesting the presence of species with a higher degree of denaturation compared to A9. Differential scanning calorimetry (DSC) showed that A9 was characterized by two thermal transitions (65.8 and 98°C); A11 exhibited only a small endotherm (66.6°C) and a second, less defined one. DSC analysis of A9 at pH 2-4 did not show endotherms, but at pH 5, some protein structures were observed. A11 showed a greater degree of denaturation. FPLC results showed that the proteins in A9 are more folded and their conformation is closer to the native state than those in A11, which are more unfolded due to pH-mediated denaturation, mainly in acid media. The surface hydrophobicity of the isolates in acid media was lower than in alkaline media. The fluorescence emission spectra of the isolates showed differences in acidic pH conditions. As expected, the highest solubility was at alkaline pH. The water-holding capacity was similar for both isolates. The water-imbibing capacity and speed of foaming was higher for A11 than for A9. In summary, intense pH treatment of amaranth isolates generated partial or total protein denaturation and differences in the functional properties.

Proteins of Amaranth (Amaranthus spp.), Buckwheat (Fagopyrum spp.), and Quinoa (Chenopodium spp.): A Food Science and Technology Perspective

“…While 2 distinct denaturation peaks were visible at pH 8.0, no such peaks were observed at pH 2.0. This indicates that low pH induces extensive unfolding of amaranth proteins extracted under alkaline conditions (Abugoch James and others ; Ventureira and others ; Bolontrade and others ). Furthermore, Salcedo‐Chávez and others () noted that 10 different combinations of extraction pH and pH of IEP of amaranth protein isolates (Table ) had no significant effect on their T d readings.…”

“…In addition, differences in foaming may also be explained by differences in protein extraction procedures. Abugoch James and others () showed that whether amaranth proteins are isolated from flour at pH 9.0 or 11.0 influences their conformation and surface hydrophobicity and, as a consequence, their foaming properties. While their foaming capacity was similar, foams prepared from extracts obtained at pH 11.0 were more stable than those from extracts obtained at pH 9.0, especially under acidic conditions (Abugoch James and others ).…”

“…Abugoch James and others () showed that whether amaranth proteins are isolated from flour at pH 9.0 or 11.0 influences their conformation and surface hydrophobicity and, as a consequence, their foaming properties. While their foaming capacity was similar, foams prepared from extracts obtained at pH 11.0 were more stable than those from extracts obtained at pH 9.0, especially under acidic conditions (Abugoch James and others ). High ionic strength favors amaranth protein foam formation at pH 2.0 and pH 8.0 (Bolontrade and others ).…”

Functionality of the storage proteins in gluten-free cereals and pseudocereals in dough systems

“…The changes were suggested to be due to protein conformational changes and partial denaturation of the proteins due to proteolysis by proteases co-extracted with the proteins. When isolated amaranth proteins were exposed to extremes of pH, they also underwent conformational changes and partial or total denaturation (Abugoch et al, 2010). Surface hydrophobicity was found to be lower at acid pH than at alkaline pH.…”

Effect of alkaline extraction on the structure of the protein of quinoa (Chenopodium quinoa Willd.) and its influence on film formation

“…The quenching of fluorescence intensity may be due to the exposure of tryptophan residues to the polar environment from the interior hydrophobic environment (Peterman & Laidler, 1979;Gorinstein et al, 2001). The k max emission of tryptophan is sensitive to the pH of the medium, it varied between 334 and 355 nm for quinoa, amaranth and soy, and it is also closely related to the chromophore groups of proteins, such as tryptophan, exposed to the solvent (Sze et al, 2007;Abugoch et al, 2008Abugoch et al, , 2010. The red shift to higher emission maxima when the extraction pH was increased from 8-10 to 11 and 12 may be associated with a higher degree of protein denaturation (Sripad & Rao, 1987;Thomson & Ananthanarayanan, 2000).…”