Modifying the Cold Gelation Properties of Quinoa Protein Isolate: Influence of Heat-Denaturation pH in the Alkaline Range

Abstract

Heat-denaturation of quinoa protein isolate (QPI) at alkali pH and its influence on the physicochemical and cold gelation properties was investigated. Heating QPI at pH 8.5 led to increased surface hydrophobicity and decreases in free and bound sulfhydryl group contents. Heating at pH 10.5 caused a lesser degree of changes in sulfhydryl groups and surface hydrophobicity, and the resulting solutions showed drastically increased solubility. SDS PAGE revealed the presence of large aggregates only in the sample heated at pH 8.5, suggesting that any aggregates present in the sample heated at pH 10.5 were non-covalently bound and disintegrated in the presence of SDS. Reducing conditions partially dissolved the aggregates in the pH 8.5 heated sample indicating the occurrence of disulphide bonding, but caused no major alterations in the separation pattern of the pH 10.5 heated sample. Denaturation pH influenced the cold gelation properties greatly. Solutions heated at pH 8.5 formed a coarse coagulum with maximum G' of 5 Pa. Heat-denaturation at 10.5 enabled the proteins to form a finer and regularly structured gel with a maximum G' of 1140 Pa. Particle size analysis showed that the pH 10.5 heated sample contained a higher level of very small particles (0.1-2 μm), and these readily aggregated into large particles (30-200 μm) when pH was lowered to 5.5. Differences in the nature of aggregates formed during heating may explain the large variation in gelation properties.

Heat-denaturation and aggregation of quinoa ( Chenopodium quinoa ) globulins as affected by the pH value

“…Each subunit consists of an acidic A-(M r % 28 and 34k) and a basic B-(M r % 17 and 19k; B) chain, that are linked by a disulfide bond (Brinegar & Goundan, 1993;Casey, 1999;Mäkinen, Hager, & Arendt, 2014). These proteins have their isoelectric point between pH 5.0 and 6.5, where they exhibit minimum solubility (Mäkinen et al, 2015). Similar storage proteins are found in e.g.…”

“…Heating quinoa protein isolate at alkaline pH (10.5) improved its solubility and emulsifying activity, and it also formed a gel with a regular structure and high storage modulus upon acidification of the solution. Heating at mildly alkaline pH (8.5-9.5) led to the formation of a weak coagulum, while protein heated at a neutral pH formed no structures (Mäkinen et al, 2015). The observed differences were linked to the higher solubility of the polypeptides released from the chenopodin subunits during heating at highly alkaline pH.…”

“…The protein storage vacuoles are primarily located in the embryonic tissue that surrounds the seed (Prego, Maldonado, & Otegui, 1998). This tissue can be readily separated by milling into a fraction that contains 23% protein (Ando et al, 2002;Mäkinen, Zannini, & Arendt, 2015), leaving the starchy perisperm available for other applications (Elgeti et al, 2014;Rayner, Timgren, Sjöö, & Dejmek, 2012).…”

“…It can be isolated by alkali extraction and isoelectric precipitation (Brinegar & Goundan, 1993;Mäkinen et al, 2015). Chenopodin consists of $49 and 57 kDa subunits (AB-11S) that are associated into a hexamer by non-covalent interactions.…”

“…during heating, and formed a coarse coagulum with no gel-like properties when acidified (Hermansson, 1979;Mäkinen et al, 2015). The structural changes leading to these differences in acid gelation as a result of heating are however not known.…”