In-situ disintegration of egg white gels by pepsin and kinetics of nutrient release followed by time-lapse confocal microscopy

Abstract Digestion behaviour of food protein-based hydrogels is greatly influenced by the gel characteristics and in particular the microstructure. Using egg whitegels (EWGs) as a model food, this study aimed to explore the real time disintegration by pepsinof different microstructuresand subsequent nutrient release kinetics. Using thermal treatment at 80 °C, EWGs with two different microstructuresbut similar protein concentration (10%) were produced by varying the pH conditions (pH 5 and pH 9). The in situ spatiotemporal disintegration of the microstructureduring static in vitro gastric digestion was followed using a high resolution confocal microscopic technique. Tetramethylrhodamine isothiocyanate (TRITC)- dextran(4400 Da) was incorporated into the gels as a model fluorescent molecule of peptide-like size, to trace its release due to the pepsinaction. The looser microstructureof pH5-EWG caused the gel to disintegrate more quickly and to a greater extent, leading to a higher rate of TRITC- dextranrelease. In contrast, the compact-dense microstructureof the pH9-EWG showed slower kinetics of disintegration and (TRITC)- dextranrelease, likely due to a reduced accessibility of pepsinto its substrates. Pepsinactivity being highly pH-dependent, high local pH and high buffering capacity of pH9-EWG may also play a pivotal role in the slower disintegration observed for this gel. In both EWGs, spatial degradation was mainly observed at the gel surface while the interior area fractions remained unchanged. Thus, surface erosion is possibly the underlying mechanism of EWG disintegration by pepsinin these experimental conditions.