Effect of enzymatic hydrolysis on solubility and surface properties of pea, rice, hemp, and oat proteins: Implication on high protein concentrations

Abstract

Pea, rice, hemp, and oat proteins at concentrations of 5% and 10% were modified using controlled hydrolysis by Alcalase® (4%, g enzyme/g protein) during 30, 60, 120, and 300 min, and their solubility and surface properties were measured. The degree of hydrolysis (DH%) gradually increased with hydrolysis time (R2 ≥ 85%) and (R2 ≥ 96%) at 5% and 10% protein concentrations, respectively. The protein profile of hydrolysates suggests that Alcalase® cleaved the pea, rice, and oat proteins to smaller sizes up to 15 kDa and the hemp protein to larger polypeptides with exposure of basic subunits (20 kDa). All plant proteins at 5% and 10% protein concentrations were greatly solubilised after 30 min of hydrolysis (at pH 8.0 and temperature 50 °C) compared with their unhydrolysed counterparts. Increasing the hydrolysis time to four, five, and ten times more than 30 min further increased protein solubility. The relative fluorescent intensity from the spectrofluorometer suggests that hydrophobic groups remained unchanged after hydrolysis in rice protein. This agrees with the low DH% of rice protein. Unhydrolysed pea had the highest hydrophobicity with values of 59.69 ± 1.94 and 86.28 ± 2.88 at 5% and 10% protein, respectively, which decreased through hydrolysis. On the other hand, hemp hydrolysates had the highest hydrophobicity compared with their unhydrolysed counterparts at both protein concentrations. Hydrolysis increased surface charge only in oat protein. Higher protein concentration led to lower protein solubility in oat and hemp proteins. These results showed that prolonged hydrolysis and higher protein concentration can affect the dynamic association and dissociation of proteins leading the functional groups previously exposed by hydrolysis to hide in the interior of the protein structure.