Exploring the gastric breakdown of milk protein gels by time-lapse synchrotron deep-UV microscopy

Gastric digestion of food is the result of physical disintegration and enzymatic reactions leading to the release of nutrients. Depending on the gelation conditions, food protein networks with specific structural and rheological features can be formed. Several in vivo and in vitro studies have shown an influence of the macro- and microstructure on the kinetics of milk protein hydrolysis. Despite growing interest to modulate food structure to potentially control digestion rates, little is known concerning the fundamental mechanisms by which the structure of dairy gels affects the digestion kinetics.An original time-lapse synchrotron deep-UV microscopy method was developed using the natural tryptophan fluorescence of proteins. It allowed monitoring in situ the microstructural evolution of protein gels during simulated gastric digestion. Two dairy gels of identical composition but different coagulation modes (acid and rennet) were submitted to static in vitro digestion with the aim of decoupling the acidification process from the enzymatic action of pepsin. Automated analysis procedures of the collected images allowed to model the kinetics of disintegration of sub-millimetric gel pieces. Results confirmed the propensity of rennet gels, but not acid gels, to undergo large microstructural modifications and form compact protein aggregates (syneresis) under acidic conditions. Consequently, the kinetics of proteolysis was much slower for the rennet gel, reiterating the hypothesis of a pepsin accessibility reduced to the surface of solid casein substrates (Fig 1). Finally, the morphological shapes of particles were almost preserved during the full digestion process and the kinetics of disintegration could be fitted by a simple exponential model. Therefore, we concluded that the erosion mode is probably the predominant mechanism during the gastric digestion of dairy gels.