Gastric digestion of milk protein gels as assessed by time-lapse Synchrotron UV-microscopy

Gastric digestion is the result of physical disintegration, acidic hydrolysis and enzymatic reactions leading to therelease of nutrients which are absorbed in the upper intestinal tract. Protein is one of the essentialmacro-nutrient and can be eaten in a great variety of forms (solubilized, cross-linked, in their native ordenatured states). Controlling food protein gelation conditions result in the formation of particles with specificstructural features. Several in vivo and in vitro studies have shown an influence of the macro- andmicrostructure on the kinetics of milk protein hydrolysis. Nevertheless, the mechanisms by which the structureof dairy gels can affect the digestion kinetics remain largely unknown.The aim of the study was to assess the part play by HCl and gastric enzyme (i.e. pepsin) during gastric digestionusing a dynamic and label-free imaging technique on the DISCO beamline of Synchrotron SOLEIL to visualize insitu the milk protein gels breakdown kinetics. The DISCO beamline uses the deep ultraviolet range to probe theintrinsic UV tryptophan fluorescence without the need of specific external probes. Two milk gels with the sameprotein concentration but different microstructures were prepared either by rennet or acid coagulation ofnon-fat milk. The disintegration of the different networks was monitored under digestion at body temperature insimulated gastric fluids and the effect of the acidic environment uncoupled from the enzyme effect. Theevolution of particle area and mean fluorescence intensity has been determined, and used to estimate thekinetics of food particles breakdown.The kinetics of acid gel in vitro digestion was significantly reduced compared to rennet gel. Our data indicatethat rennet gel has a two-step behavior during the acidification phase with a swelling followed by a contractionof the particle, not observed for acid gel. In addition, these microstructural modifications of rennet gel affectnegatively the enzymatic breakdown kinetics of particles compared to acid gel.This study leads to original methodological developments both from the point of view of the acquisition of dataand their joint analysis. Getting in situ information about digestion kinetics, microstructural transformation andenzymatic reaction, allow further analysis of the digestion process.