Soluble and insoluble complexes formed between protein, phytic acid and calcium in pea concentrate suspensions around neutral pH

Abstract: Recently, the consumption of pulses was promoted for sustainability and nutritional purposes. They are rich in proteins and good source of minerals, such as calcium. However, these benefits can be impaired by their phytic acid content. This polyphosphate is a major antinutrient which binds to proteins and minerals, thereby reducing their digestibility and bioavailability, respectively. Previous research focused on model proteins, such as BSA and soybean. The need to extend the understanding to pulses rich in phytate becomes a major interest.

The aim of this study was to investigate associations between phytic acid, calcium and pea proteins in suspensions made from an air-classified concentrate.

The solubility of the three compounds was quantified at pHs 6.5, 7.0 and 7.5. Further complex formation was investigated by chelating calcium with EDTA addition or removing proteins via ultrafiltration. Size exclusion chromatography (SEC-HPLC) was used to compare protein profiles of samples.

As expected, protein solubility increased with raising pH. Phytic acid solubility was minimum at pH 7.0 while simultaneously calcium solubility reached a maximum (Figure 1A). Protein solubility increased after EDTA addition at pH 6.5 and 7.0, supporting the formation of insoluble binary complexes between calcium and proteins. The concomitant rise in phytic acid solubility highlighted the formation of either insoluble binary complexes between phytic acid and calcium, or insoluble ternary complexes. Further measurements showed about 40% of soluble protein complexes formed with phytic acid and, possibly, calcium. Small amounts of soluble ternary complexes were observed at pH 7.5. Alongside, SEC-HPLC study showed higher molecular weight protein fractions at pH 7.5 (Figure 1B). Also, chromatogram’s shape suggested no soluble polymer, but rather protein units associated with small molecules like calcium and phytic acid.

Further studies on isolated protein fractions are required to determine affinity with calcium and phytic acid and detailed understanding of interaction mechanisms.