Exploring the breakdown of dairy protein gels during in vitro gastric digestion using time-lapse synchrotron deep-UV fluorescence microscopy

•In vitro gastric digestions of dairy gels were monitored by synchrotron deep-UV microscopy.•Variations of tryptophan fluorescence allowed exploring pH gradients inside particles.•Syneresis of rennet gel led to compact protein aggregates under gastric acid conditions.•Consequently kinetics of pepsin hydrolysis were slower for rennet gel than acid gel.•Erosion is probably the predominant mechanism of dairy gel disintegration.

A novel time-lapse synchrotron deep-UV microscopy methodology was developed that made use of the natural tryptophan fluorescence of proteins. It enabled the monitoring in situ of the microstructural changes of protein gels during simulated gastric digestion. Two dairy gels with an identical composition, but differing by the coagulation mode, were submitted to static in vitro gastric digestion. The kinetics of gel particle breakdown were quantified by image analysis and physico-chemical analyses of digesta. The results confirm the tendency of rennet gels, but not acid gels, to form compact protein aggregates under acidic conditions of the stomach. Consequently, the kinetics of proteolysis were much slower for the rennet gel, confirming the hypothesis of a reduced pepsin accessibility to its substrate. The particle shapes remained unchanged and the disintegration kinetics followed an exponential trend, suggesting that erosion was the predominant mechanism of the enzymatic breakdown of dairy gels in these experimental conditions.