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 the release of nutrients which are absorbed in the upper intestinal tract. Protein is one of the essential macro‐nutrient and can be eaten in a great variety of forms (solubilized, cross‐linked, in their native or denatured states). Controlling food protein gelation conditions result in the formation of particles with specific structural features. Several in vivo and in vitro studies have shown an influence of the macro‐ and microstructure on the kinetics of milk protein hydrolysis. Nevertheless, the mechanisms by which the structure of 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 digestion using a dynamic and label‐free imaging technique on the DISCO beamline of Synchrotron SOLEIL to visualize in situ the milk protein gels breakdown kinetics. The DISCO beamline uses the deep ultraviolet range to probe the intrinsic UV tryptophan fluorescence without the need of specific external probes. Two milk gels with the same protein concentration but different microstructures were prepared either by rennet or acid coagulation of non‐fat milk. The disintegration of the different networks was monitored under digestion at body temperature in simulated gastric fluids and the effect of the acidic environment uncoupled from the enzyme effect. The evolution of particle area and mean fluorescence intensity has been determined, and used to estimate the kinetics of food particles breakdown. The kinetics of acid gel in vitro digestion was significantly reduced compared to rennet gel. Our data indicate that rennet gel has a two‐step behavior during the acidification phase with a swelling followed by a contraction of the particle, not observed for acid gel. In addition, these microstructural modifications of rennet gel affect negatively 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 data and their joint analysis. Getting in situ information about digestion kinetics, microstructural transformation and enzymatic reaction, allow further analysis of the digestion process.