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 (T2) and rate (R2 = T2−1) 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. T2 and R2 of the digestion mixture were determined by time-domain nuclear magnetic resonance (TD-NMR) and MRI. Subsequently, relative amplitudes (TD-NMR) for different T2 values and T2 distribution (MRI) were determined. For the solution, protein concentration and T2 did not change during digestion. For the gels, water in supernatant and gel phase could be discriminated on the basis of their T2 values. During digestion, R2 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 T2-mapping showed similar associations between R2 of supernatant and protein (–NH2 groups) concentration. In conclusion, T2-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.