The tools developed in soft matter physics are of particular interest to characterize food micro- and nanostructures and monitor how they evolve under various conditions. For instance, X-ray scattering from dairy products has provided decisive knowledge on the internal structure of casein micelles, the main dairy protein, and on their response to industrial processing (de Kruif ACIS 2012). Food structure can modify the release of nutrients during digestion, and should therefore be considered when evaluating nutritional properties or related health effects. In vivo experiments on mini-pigs showed that enzymatic dairy gels (curd-type, made with rennet) led to a remarkable slowdown of the kinetics of amino-acid absorption, compared to milks and acid dairy gels (yogurt type) of identical composition (Barbe Food Res. Int. 2014). This is attributed to delayed gastric retention arising from acid-induced rearrangements of the protein network of enzymatic gels. Results obtained by SAXS at synchrotron SOLEIL on SWING beamline show, at a nanometer scale, the structural evolution of the gels under in vitro gastric digestion. The evolution of the SAXS spectra during digestion is very informative. -calcium-phosphate nanoclusters, which provide a well-defined signature in the 0.03-0.15 Å-1 range (size ~ 2.5 nm), are immediately dissolved upon HCl addition. This is the main reason why enzymatic gels, but not acid gels (for which nanoclusters are already dissolved), undergo important microstructural rearrangements upon acidification. - one can follow quantitatively how the large signal from casein particles at low q progressively disappears during protein hydrolysis. - a signature appears suggesting that molecular edifices of few nanometers, which could result from peptides self-assemblies, are created during digestion. Finally, the evolution at large distances (scattering at low q), is slower for rennet gels than for acid gels, showing a structure effect on digestion kinetics.