The impact of sour cherry phenolic compounds on the functional properties and in vitro digestibility of pea protein

Abstract

In the last years, there has been a considerable and continuous increase in the number of people who follow plant-based diets. Especially in the Western world, diet trends are focusing on either cutting out meat and animal products completely, or at least consuming them less often. Additionally, allergenicity issues are also gaining more attention as the number of people who suffer from allergic reactions are increasing. Finally, the general population is becoming more conscious and environmentally aware, with both companies and individuals focusing on and actively working towards lowering their carbon footprint. Plant-based ingredients are important in this regard as animal based products require 75% times more resources like land and water than those that are plant based. All of these factors ultimately result in the need to provide plant-based, allergen-free, and sustainable food products and ingredients to be utilized in the food industry. There is a demand towards animal based ingredient alternatives which at the same time provide similar taste and functionality properties. Pea protein has been of interest both from a scientific and industrial point of view as it has excellent nutritional properties, functionality, low cost and low allergenicity. In this study, the impact of sour cherry phenolic compounds on the functional properties and in vitro digestibility of pea protein were investigated. Sour cherry phenolics at concentrations of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 1 mg/mL were added to 5 mg/mL of pea protein. The protein-phenolic solutions were then used to form oil-in-water emulsions using medium-chain triglycerides oil at two different weight oil to water ratios of 50:50 and 25:75. MCT oil was preferred over oils such as sunflower oil and palm oil as these oils inherently contain phenolic compounds, and the focus of this study was to observe the changes resulting from sour cherry phenolics only. The foaming capacity and foaming stability of pea protein alone were found to be 36.67% and 26.79%, respectively. Addition of sour cherry phenolics overall decreased the FC and FS values of PP. However, the increasing concentrations did not result in a constant decrease of the values. Addition of SCP at the lowest concentration initially decreased FC and FS up to 0.2 mg/mL. Addition of 0.2 mg/mL of SCP yielded FC and FS values of 33.33%, and 12.63%, respectively. Concentration increases upon this point again lead to a decrease in the values. Emulsifying capacity and emulsifying stability values were measured for both oil ratios for all SCP concentrations. EC and ES of pea protein was 44.29% and 43.79% for 50:50 oil to water ratio and 26.60% and 23.88% for 25:75 oil to water ratio. The addition of SCP increased EC and ES from the concentration of 0.05mg/mL to 0.2 mg/mL, with the highest results obtained for EC and ES being 47.75% and 45.94%, respectively for 50:50 oil ratio. Concentration increase after this showed a decline in EC and ES with no statistical difference between the values between 0.3 mg/mL to 1 mg/mL. For all concentrations, the ES and EC values of the 50:50 protein-phenolic and oil to water ratio was considerably higher than that of the 75:25 ratio. Compared to PP alone, although the EC and ES of the emulsions showed a slight decrease when phenolics were initially added, the values of 0.2 mg/mL SCP+PP surpassed PP. Further increase of phenolics did lower EC and ES values, however there were no drastic differences between the concentrations. The absolute zeta potential of the mixtures marginally increased until concentrations of 0.2 and 0.3 mg/mL, of which similar values were measured. Further increase resulted in a decline in the values, however the differences between these values are negligible. A consistent observation was that the absolute zeta values of the emulsions with 50:50 oil ratio were higher than those of the 25:75. Higher absolute zeta potential indicates better stability in emulsions, in line with the abovementioned results, higher oil ratio provided improved stability. Protein solubility was found to increase with increasing concentrations of SCP. Generally in literature contradictory results have been presented. However, the increase observed could potentially be a result of the high amounts of (70-80%) of globulins present in pea proteins, whose solubilities have reported to increase over 80% with the addition of phenolics. In vitro digestion showed the protein solubility of the intestinal phase to be significantly higher than that of the gastric phase, with the emulsions at the gastric phase exhibiting the highest results. Total phenolics content was found to increase with increasing concentrations of phenolics as expected for all 3 groups: phenolics only solutions, phenolic-protein solutions and for their corresponding emulsions. The TPC values for all samples showed an increase throughout in vitro digestion, with the highest results being obtained in the intestinal phase. Increase of TPC during simulated digestion is correlated in literature. Antioxidant capacity was evaluated via the CUPRAC assay. The antioxidant capacities were found to increase throughout in vitro digestion, with the highest results observed in the intestinal phase, in correlation with the results obtained for TPC. Antioxidant capacities of each of the groups showed an increase with increasing phenolics concentrations within themselves. The results were significantly high for all 3 samples of the 1 mg/mL SCP containing groups, showing that the concentration of the phenolics had the most dominant effect on antioxidant capacity. The emulsions had higher values in the intestinal phase than their protein-phenolic counterparts. Overall it can be said that the mechanisms of protein-phenolic interactions can have both positive and negative effects on the functional properties of proteins and in vitro gastrointestinal digestibility. These can differ according to the type of protein and phenolics and their respective amounts, along with experiment conditions such as pH and temperature.