Exploring in vitro gastric digestion of whey protein by time-domain nuclear magnetic resonance and magnetic resonance imaging

Gastric digestion is crucial for protein breakdown. Although it has been widely studied with in vitro models, verification in vivo remains a big challenge. Magnetic resonance imaging (MRI) has the potential to bridge this gap. Our objective was to use the transverse relaxation time (T-2) and rate (R-2 = T-2(-1)) to monitor hydrolysis of protein-rich food during in vitro gastric digestion. Whey protein solution and heat-induced hydrogels were digested by means of simulated gastric fluid (SGF). Free amino groups (-NH2 groups) and protein concentration in the supernatant were measured. T-2 and R-2 of the digestion mixture were determined by time-domain nuclear magnetic resonance (TD-NMR) and MRI. Subsequently, relative amplitudes (TD-NMR) for different T-2 values and T-2 distribution (MRI) were determined. For the solution, protein concentration and T-2 did not change during digestion. For the gels, water in supernatant and gel phase could be discriminated on the basis of their T-2 values. During digestion, R-2 of supernatant correlated positively with protein (-NH2 groups) concentration in SGF. Also, the decrease in relative amplitude of gel fraction correlated linearly with the increase of supernatant protein concentration. MRI T-2-mapping showed similar associations between R-2 of supernatant and protein (-NH2 groups) concentration. In conclusion, T-2-measurements by TD-NMR and MRI can be used to monitor in vitro gastric digestion of whey protein gels; TD-NMR measurements contributed to interpreting the MRI data. Thus, MRI has high potential for monitoring in vivo gastric digestion and this should be further pursued.