Exploring novel food proteins and processing technologies : a case study on quinoa protein and high pressure –high temperature processing

Foods rich in protein are nowadays high in demand worldwide. To ensure a sustainable supply and a high quality of protein foods, novel food proteins and processing technologies need to be explored to understand whether they can be used for the development of high-quality protein foods. Therefore, the aim of this thesis was to explore the properties of a novel food protein and a novel processing technology for the development of high-quality protein foods. For this, quinoa was chosen as an alternative protein source and high pressure – high temperature (HPHT) processing was chosen as a novel processing technology. Quinoa protein has been found to have a balanced amino acid profile and to be allergen-free. As this combination is not common among plant proteins, it is worth studying physicochemical and functional protein properties of quinoa further (Chapter 1). Extraction and processing conditions can influence protein properties and thus functionality. Therefore, quinoa protein properties were examined at different extraction and processing conditions (Chapter 2 and 3). For this, the protein was isolated from the seed using alkaline extraction and subsequent acid precipitation. The quinoa protein isolates (QPIs) obtained were examined in terms of protein purity, yield, solubility, denaturation, aggregation and gelation behaviour, and digestibility. It was found that when extraction pH increased, protein yield and denaturation increased, which was explained by a higher protein charge, leading to increased unfolding and solubilisation (Chapter 2). Protein purity decreased with increasing extraction pH, which was associated with a possible co-extraction of other seed components. QPIs obtained at extraction pH 8 (E8) and 9 (E9) had a higher solubility in the pH range of 3-4.5 (E9 solubility was highest at pH 7) compared to the isolates obtained at extraction pH 10 (E10) and 11 (E11). It was hypothesised that at a higher extraction pH, the larger extent of protein denaturation led to the exposure of hydrophobic groups, thus decreasing surface polarity and solubility. When suspensions of E8 and E9 were heated, protein aggregation increased and semi-solid gels with a dense microstructure were formed. In contrast, suspensions of E10 and E11 aggregated to a lower degree and did not form self-supporting gels upon heating. The gels obtained with E10 and E11 had furthermore a microstructure showing loose particles. Increased protein aggregation and improved gel formation at lower extraction pH were hypothesised to be due to a higher degree of hydration and swelling of protein particles during heating, leading to increased protein-protein interactions. These findings show that QPI obtained at an extraction pH below 9 might be used to prepare semi-solid gelled foods, while QPI obtained at pH values higher than 10 might be more suitable to be applied in liquid foods. Heat treatments of QPI suspensions lead to an increased protein denaturation and aggregation but to a decreased in vitro gastric protein digestibility, especially at a high temperature (120°C) and extraction pH (11) (Chapter 3). It was hypothesised that QPIs obtained at a higher extraction pH and treated at higher temperature were denatured to a greater extent and contained stronger protein crosslinks. Therefore, enzyme action was impaired to a higher degree compared to lower temperatures and extraction pH values. This means that by controlling extraction pH and treatment temperature the digestibility of quinoa protein can be optimised. The disadvantage of the conventional fractionation method used in Chapter 2 and 3 is that it requires high amounts of energy and water and the solvents used can denature the protein, possibly leading to a loss in functionality. Therefore, recently, a new method has been developed, hybrid dry and aqueous fractionation, which uses less energy and water and has proved successful for obtaining protein-rich fractions from pea. It was not known whether hybrid dry and aqueous fractionation can be used to obtain protein-rich fractions of quinoa (Chapter 4). Quinoa seeds were carefully milled to disentangle the protein-rich embryo from the starch-rich perisperm. Using subsequent air-classification, the embryo and perisperm were separated based on size into a protein-rich fraction and a starch-rich fraction, respectively (dry fractionation). The protein-rich fraction was further milled to a smaller particle size and suspended in water. This step was to solubilise the protein (aqueous fractionation), whereby a smaller particle size and adding NaCl optimised the solubilisation efficiency. The addition of salt helped to extract more salt-soluble proteins from quinoa, next to the water-soluble proteins. After centrifugation, the protein-enriched top aqueous phase was decanted and ultrafiltered for further protein concentration. The process generated a quinoa protein-rich fraction with a protein purity of 59.4 w/dw% and a protein yield of 62.0%. Having used 98% less water compared to conventional protein extraction, this new method is promising for industry to obtain quinoa protein concentrates in a more economic, sustainable and milder way. Next to exploring novel food proteins for the development of high-quality protein foods, novel processing technologies are also important to study. This is because traditional thermal processing can deteriorate the quality of protein-rich foods and beverages by causing undesired browning or too high viscosities. Therefore, for sterilisation purposes, HPHT processing was investigated for the treatment of protein foods (Chapter 5). Model systems, whey protein isolate – sugar solutions, were used to study the effect of pressure at high temperature on Maillard reactions, browning, pH, protein aggregation and viscosity at different pH. It was found that pressure retarded early and advanced Maillard reactions and browning at pH 6, 7 and 9, while it inhibited protein aggregation and, thereby, a high viscosity at pH 7. The mechanism behind this might be that pressure induces a pH drop, possibly via dissociation of ionisable compounds, and thus slows down Maillard reactions. Differences in protein conformation, protein-protein interactions and sensitivity of whey proteins, depending on pH, pressure and heat, might be at the base of the reduced protein aggregation and viscosity observed at pH 7. The results show that HPHT processing can potentially improve the quality of protein-sugar containing foods, for which browning and high viscosities are undesired, such as high-protein beverages. Finally, the properties of quinoa protein and HPHT processing were discussed in a broader context (Chapter 6). It was concluded that QPI obtained at pH 9 is a promising alternative to pea and soy protein isolate from a technical perspective and that QPI protein yields can be optimised. Also, quinoa protein-rich fractions obtained with the hybrid dry and aqueous fractionation method were predicted to have comparable properties to QPI, soy and pea protein isolates. However, from a marketing perspective, the protein-rich fraction was considered more advantageous to be up-scaled compared to QPI. High pressure at ambient or high temperature was found to have an added value compared to heat, which can be used for the development of high-quality protein food. Lastly, quinoa protein and HPHT processing might become more attractive for industry in the light of current trends, if present predictions can be confirmed and remaining issues can be resolved.