Impact of heat and enzymatic treatment on ovalbumin amyloid-like fibril formation and enzyme-induced gelation

Heating aqueous solutions of ovalbumin (OVA) may cause gel formation. When heated at pH conditions close to the protein's isoelectric point (towards neutral pH), turbid particulate gels are formed, whereas at acidic pH fine-stranded, transparent gels are formed already at lower concentrations. Here, transparent gels were formed when subjecting 2.0% OVA to a combined heat (78 °C for 22 h at pH 7) and trypsin (37 °C for 48 h) treatment. Transmission electron microscopy clearly revealed the presence of long, straight OVA fibrils which contributed to the gel formation. Quartz crystal microbalance with dissipation (adsorption of small structures), size exclusion – HPLC (presence of both structures larger and smaller than native OVA) and atomic force microscopy (presence of long fibrils with a higher thickness, whereas heated OVA mainly showed amorphous aggregates) analyses confirmed that the additional enzymatic treatment was able to break down the amorphous aggregates formed by heating OVA into peptides, which then partly re-assembled into longer OVA fibrils. The above mentioned heat and enzymatic treatment conditions brought about gelation after 17 h with a gel strength of 68 Pa which broke at a stress of 38 Pa. By varying the temperature during heat (58–88 °C) and enzymatic (27–67 °C) treatments, gels were formed the fastest when heated at 78 °C followed by enzymatic treatment at 57 °C. A design of experiments for evaluating the impact of OVA and trypsin concentration revealed that the fastest gelation occurred at the higher considered OVA and trypsin concentrations. Additionally, the gel strength was also higher under the latter conditions. It is clear that different gel characteristics can be reached when varying the different process conditions, creating the opportunity for reconsidering the formulation of various foods such as jellies, marmalades and desserts.