The enzymatic modification of a chickpea protein isolate for improved nutritional and functional properties

Abstract

In this study the effectiveness of limited enzymatic hydrolysis on improving the functional and nutritional properties of a chickpea protein isolate (CPI) was examined. Three different proteases (trypsin, pepsin, and papain) were used under controlled pH (7.0, 2.6, and 6.2) to hydrolyze the protein to between 5 and 15 percent degrees of hydrolysis (% DH). Following hydrolysis net negative charge increased when measured at pH 7.0, increasing further with higher levels of trypsin and papain treatment from -17.1 mV in the untreated isolate up to -21.4 mV. Surface hydrophobicity increased from 23.9 AU in the untreated isolate to 37.6 ~53.9 AU following enzyme treatments. Trypsin and papain hydrolysis were noted to improve CPI solubility at pH 4.0, up to 19.0% from an initial 6.5% in the untreated isolate. Hydrolysis slightly decreased CPI solubility at pH 7.0 (from 54.6% to 50.7%), while at pH 10.0, 10% DH and 15% DH trypsin and papain hydrolysis increased CPI solubility (from 57.4% to between 66.0% and 67.7%). However, 15% DH pepsin hydrolysis decreased solubility (from 63.7% to 52.1%). Increasing levels of pepsin and papain hydrolysis were found to improve water holding capacity (WHC) (from 3.1 g/g to between 3.6 and 5.1 g/g). In contrast, all hydrolyzed samples showed minor decreases in oil holding capacity (OHC), when compared to controls (4.0 g/g to between 2.9 and 3.7 g/g). Emulsifying activity and stability (EAI and ESI) were both found to be strongly linked with pH, with these parameters improving under more alkaline conditions. Hydrolysis of the CPI was found to lead to improvements in EAI for emulsions prepared at pH 4.0 (from 33.2 m2•g-1 to between 39.7 and 61.2 m2•g-1), as well as at pH 10.0(from 99.2 m2•g-1 to between 94.4 and 293.5 m2•g-1), with papain treatment generating the largest increases. Changes to EAI at pH 7.0 were not noted for any of the samples. In terms of ESI, trypsin hydrolysis led to an increase in stability of emulsions prepared at pH 4.0, while no changes were noted at either pH 7.0 or 10.0. Emulsions prepared at pH 10.0 were stable, displaying little separation. Foaming capacity and stability (FC and FS) were also found to be linked with pH, improving under alkaline conditions. When foams were generated at pH 4.0, heat treatment led to decreases in foaming capacity (from 81.1% to 51.7%) while enzymatic treatment led to significant increases thereafter (from 51.7% to 84.4%). Enzyme treatments moderately increased foaming capacity at pH 7.0 (from 128.9% to between 124.2% and 174.4%). All of the foams prepared at pH 10.0 using the hydrolysates were superior to controls, with optimal conditions observed at 5% DH for pepsin and papain and 15% DH for trypsin. Foaming stability varied between pH 4.0 and the higher pH conditions (65.8% to 96.8% volume lost, compared to between 6.8% and 37.2% volume lost). Foams prepared at pH 10.0 showed decreased stability following higher degrees of hydrolysis (DH >10%), with trypsin treatment resulting in the most unstable foams (from 19.3% to 37.2% volume lost, compared to between 12.4% and 20.5% volume lost). Phenolic contents decreased from 1.6 gallic acid equivalents (GA) per gram in the untreated isolate to between 0.6 and 1.3 mg GA/g in the hydrolysates. Tannin levels were not detectable in any of the chickpea protein isolates, despite considerable levels present in the original flour (176.7 mg/ml extract). Trypsin hydrolysis was found to be less effective at reducing trypsin inhibitory capacity (from 7.2 to 5.6 TIU) when compared to pepsin and papain (from 4.2 to 1.7 TIU). Chymotrypsin inhibitor levels decreased following enzyme treatment (from 5.5 CIU to between 1.7 and 4.2 CIU). Tryptophan was found to be the most limiting amino acid in the untreated isolate (83% the recommended value). Cysteine and methionine became limiting following every hydrolysis, potentially due to the release of small peptides which could not be reclaimed during sample preparation. In vitro protein digestibility corrected amino acid scores (IV-PDCAAS) decreased substantially following hydrolysis compared to the untreated CPI (from 64.80% to 55.20%), compared to 68.24% in the untreated isolate. In summary, enzymatic hydrolysis was observed to have powerful effects on the various chemical and functional properties. Notably, most hydrolysates possessed improved emulsifying capacity/stability and foaming properties. Hydrolysis had mixed effects on the nutritional properties, with decreases noted for all analyzed bioactive compounds, alongside a decline in the overall quality of the protein content, particularly following high levels of hydrolysis. Enzymatic hydrolysis therefore has been identified as a feasible means of improving the functionality of chickpea protein isolates. However, in order to minimize negative effects to the nutritional profile, 5% to 10% hydrolysis may prove most desirable.