Pre-treatment and digestion of plant proteins - The quinoa case

The growing global population will place increased pressure on the world’s resources to provide more proteins. It is expected that we need to switch at least partly from animals as sources of protein, to plant-based proteins, to ensure sufficient and sustainable production of proteins for everyone. Animal protein is nutritionally of very high quality, since it contains all essential amino acids, whereas vegetable sources generally lack one or more of the essential amino acids. However, this thesis shows that the image that plant proteins have an inferior amino acid profile and poor digestibility is not accurate. For instance, quinoa contains protein with an almost ideal amino acid profile. Especially the essential amino acids profile is considered to be well-balanced for human nutrition. Besides, quinoa contains almost twice as much dietary fibre as most other grains and is high in phosphorus, magnesium and iron. In addition, the quinoa is a good source of calcium, which is useful for vegans and lactose intolerant people. The gluten-free nature of quinoa, being a non-cereal, is considered safe for celiac patients. Next to the amino acid profile including the essential amino acids, the digestibility is another important factor in determining the quality of a protein source. Generally, the potential use of plant proteins and thus also quinoa protein as a food ingredient is limited by their relatively lower digestibility as compared with animal proteins. However, this thesis shows that the reformulate digestibility can be improved by choosing a proper pre-treatment.

This thesis starts with a study on the effect of pre-treatment on in vitro gastric digestion of quinoa obtained via wet and dry fractionation (Chapter 2).  Quinoa protein was isolated (QPI) from quinoa seeds using a wet fractionation method with a purity of 87% (w/dw) and concentrated (QPC) via a dry fractionation method with a purity of 28% (w/dw). The dry fractionation process only involved milling and sieving and kept the protein in its natural, native state. The wet fractionation method affected the protein digestibility negatively in comparison to the dry fractionation method. In turn, heating decreased the protein digestibility of both types of quinoa. However, the effect of the temperature was lower in the QPC than in the QPI. The better digestibility of the QPC was attributed to the prevention of the formation of large aggregates during the heating of the protein.

The influence of heating on the denaturation and the digestibility properties of QPI obtained from a sweet quinoa variety at various extraction pH values was analysed in Chapter 3. Heating the quinoa protein suspensions led to protein denaturation and aggregation, which was stronger at higher treatment temperatures. The protein digestibility was also lower when the protein dispersions had been heated at 90 and 120 °C instead of 60 °C, while the digestibility decreased with increasing extraction pH. Both the effects of high temperature and of the extraction pH on the protein digestibility were ascribed to protein aggregation.

Chapter 4 extended the study from protein dispersions, towards protein gels. The type protein source (soy, pea, whey and albumin) and the temperature at which the protein is gelled into a semi-solid product, has great influence on the in vitro gastric protein digestibility. Gels formed at 140 °C digest faster as compared to gels induced at 90 and 120 °C. It is thus clear that by adapting the gel morphology, one can also adapt the gastric digestibility of food products, which is not just a function of the source of the protein, but also very much on the structure of the food products, and on its processing history.

Dry milling and subsequent sieving of quinoa sweet varieties (Riobamba and Atlas) produced fractions that were enriched in protein and in starch (Chapter 5). This new dry fractionation method is a more resource efficient alternative to the conventional wet extraction of quinoa proteins and starch. The quinoa protein could be concentrated to a purity of around 32% (32 g/100 g dry solids) for both quinoa varieties analysed, while starch could be isolated to a fraction with a purity of 86-89 % (86-89 g/100 g dry solids). The protein yield and protein separation efficiency were higher for the Riobamba variety. The protein-enriched fraction is rich in oil and fibre as well. The proteins concentrated via the dry fractionation method proposed retained their native properties and showed a high water retention capacity and solubility when unheated. The gelatinization temperature of the starch-rich fraction was influenced by the residual presence of proteins. The starch isolation method had a pronounced effect on the pasting and textural properties. The protein-enriched fractions can be of relevance as functional food ingredients, with a high potential for application in gluten-free products.

The influence of starch and fibre on the in vitro gastric digestion of unheated and heated quinoa protein suspensions was studied in Chapter 6. The presence of either starch or fibre reduced the protein digestibility, which is explained by the lower accessibility of pepsin to hydrolyse the proteins, due to the swelling of these components. However, it was found that when fibre was added to a protein-starch system, the presence of fibre partially counteracted the reducing effect of starch on the protein digestibility. Therefore, there is a synergistic effect between the two that merits further study. The quinoa protein systems that had been heated at 120 °C showed reduced protein digestibility, which is due to the formation of large aggregates during pre-heating of the suspensions, as was also found in Chapter 2.

In Chapter 7, it was found that heating does not affect the in vitro protein digestibility of SPI dispersions, while heating of PPC dispersions at 120 °C increased its protein digestibility, even though both protein types became partially insoluble. The soy protein isolate (SPI) and pea protein concentrate (PPC) dispersions were then separated into a soluble and an insoluble fraction to study the effect of heat-induced aggregation on protein digestibility. The insoluble fractions contained heat-induced aggregates and were less digestible than their soluble counterparts, which became more digestible with heating. This compensated for the relatively low digestibility of the insoluble fractions. Thus, the solubility of proteins is not always a prerequisite for protein digestion.

Finally, the main findings of this thesis were discussed and an outlook for further research was given around the major themes of this thesis (Chapter 8). The developments of a new separation process of plant proteins was discussed as well as the implications of the separation process on the protein functionality. A dry fractionation process for protein concentration of amaranth was presented. The possible effects of the protein isolation or concentration process were mentioned. Overall, three main conclusions could be drawn from this thesis.

1. Quinoa protein can be well isolated using conventional wet processes, yielding a QPI with good purity, but can also be concentrated using the new dry fractionation process. Both types of quinoa protein are well digestible according to the in vitro gastric assay that was used.

2. The conditions during processing of the raw materials into protein isolate or concentrate strongly influence the gastric digestibility: the thermal load, but also the pH applied during the isolation change the gastric digestion perhaps even stronger than the original differences between different protein sources.

3. The digestion of dissolved protein is relatively fast, while that of gelled protein is significantly slower; the presence of other components such as starch or fibre slows the gastric digestion significantly down. This may be due to the lower amount of gastric fluid that is available for the protein, the lower swelling of the protein, and the subsequent slower of pepsin into the protein matrix.