Real-time spatial quantification of gastric acid diffusion in whey protein gels with different NaCl concentrations by wide-field fluorescence microscopy

Gastric acid diffusion and penetration constitute an essential process in the structural breakdown and enzymatic hydrolysis of solid food during digestion. This study aimed to quantify the real-time diffusion and spatial distribution of gastric acids in whey protein isolate (WPI) gels in the presence of 0–0.05 M NaCl during simulated digestion using a wide-field fluorescence microscope. For all the gels regardless of NaCl concentration, the outer surface rapidly developed a near-saturated layer, resulting in a higher normalized gastric acid concentration in the outer layer than in the inner layer. The pH decrease was more significant for the gels at a higher NaCl concentration (i.e., 0.05 M) due to the formation of a more discontinuous and looser network structure that would facilitate acid diffusion into the gel matrix and decrease the gel buffering capacity. Consistently, the effective diffusion coefficient (DA) estimated via the Fick diffusion model was 6.19 × 10−10 m2/s for 0.05 M WPI-RITC gels, significantly higher than 0.015 M (4.46 × 10−10 m2/s) and 0 M (3.54 × 10−10 m2/s) gels. The present study has provided a quantitative understanding of the diffusion process and spatial distribution of gastric acids within the WPI gel matrix in real-time during in vitro gastric digestion as influenced by NaCl concentration.