The aim of this study was to determine the influence of heat processing on denaturation and digestibility properties of protein isolates obtained from sweet quinoa (Chenopodium quinoa Willd) at various extraction pH values (8, 9, 10 and 11). Pretreatment of suspensions of protein isolates at 60, 90 and 120 °C for 30 min led to protein denaturation and aggregation, which was enhanced at higher treatment temperatures. The in vitro gastric digestibility measured during 6 h was lower for protein extracts pre-treated at 90 and 120 °C compared to 60 °C. The digestibility decreased with increasing extraction pH, which could be ascribed to protein aggregation. Protein digestibility of the quinoa protein isolates was higher compared to wholemeal quinoa flour. We conclude that an interactive effect of processing temperature and extraction pH on in vitro gastric digestibility of quinoa protein isolates obtained at various extraction pH is observed. This gives a first indication of how the nutritional value of quinoa protein could be influenced by heat processing, protein extraction conditions and other grain components.

The objective of this study was to analyse the impact of the gel structure obtained by different heat-induced temperatures on the in vitro gastric digestibility at pH 2. To achieve this, gels were prepared from soy protein, pea protein, albumin from chicken egg white and whey protein isolate at varying temperatures (90, 120 and 140 °C) for 30 min. Gels were characterised prior to digestion via microstructure and SDS-PAGE analysis. Subsequently, the gastric digestion process was followed via the protein hydrolysis and HPSEC analysis up to 180 min. Peptides of different sizes (<5 kDa) were gradually formed during the digestion. Our results showed that gels induced at 140 °C were digested faster. The protein source and gelation temperature had great influence on the in vitro gastric protein digestibility.Electronic supplementary materialThe online version of this article (10.1007/s11483-018-9518-7) contains supplementary material, which is available to authorized users.

The in vitro gastric digestibility of the quinoa variety Riobamba was investigated, especially the influence of the quinoa matrix. Dry-fractionated quinoa protein concentrate, which is just milled and sieved, was much better digestible than the same concentrate that was reconstituted from wet fractionated quinoa protein isolate, quinoa starch isolate, and quinoa fibre isolate. In the reconstituted concentrate, the presence of quinoa starch and fibre next to quinoa protein reduces its in vitro gastric digestibility significantly. However, the effect of starch is partially counteracted if the fibre is also present. While the effects of starch and fibre separately can be understood from the decrease of the accessibility for pepsin to hydrolyse proteins, due to the hydrated starch and fibre, we suspect that the synergistic effect of starch and fibre may be due to a relative reduction of the hydration of starch due to the presence of the also strongly hydrating fibre. We concluded that the presence of starch and fibre decreases the protein in vitro gastric digestibility. Therefore, the presence of fibre partially countered the decreased of the protein digestibility of starch. Heating of the matrices to 120°C generally resulted in much lower digestion rates, due to extensive aggregation of the protein.

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