MRI assessment of pH and coagulation during semi-dynamic in vitro gastric digestion of milk proteins

Gastric digestion is essential for protein breakdown, which is commonly studied in vitro. However, validating in vitro models with in vivo data is necessary. ¹H MRI techniques, including Chemical Exchange Saturation Transfer (CEST) and Magnetization Transfer (MT), are promising for non-invasive assessment of in vitro and in vivo digestion. We previously demonstrated the ability of these techniques for monitoring in vitro milk protein digestion in static conditions. Here, we investigated the performance of CEST and MT for detecting pH and milk protein coagulation in semi-dynamic in vitro conditions using whey protein isolate solution (WPIS), low- or high-pasteurized skim milk (LPSM and HPSM, respectively). The asymmetric MT ratio (MTR_asym), reflecting the soluble proteins/peptides, decreased with the pH and protein concentration during digestion, with distinct trends observed for the different products. The dual-power CEST method allowed accurate and precise estimation of the pH for WPIS within a pH range of 5–7 on a 7 T NMR spectrometer. In situ pH mapping was achieved using a 3 T clinical MRI scanner. The T₂-weighted images, combined with MT ratio (MTR) maps, reflecting semi-solid proteins, enabled assessing the coagulum volume and structure during digestion. The MTR allowed differentiating the coagulation behaviour of LPSM and HPSM induced by the heat treatment. In conclusion, CEST and MT can be used to monitor milk protein digestion, and the effect of milk heat treatment in semi-dynamic gastric conditions. Future work will investigate the feasibility of these techniques for in vivo gastric milk protein digestion studies.