New mathematical model for interpreting pH-stat digestion profiles: impact of lipid droplet characteristics on in vitro digestibility.

The pH-stat method is commonly used to characterize the in vitro digestibility of lipids under simulated small intestine conditions. This method measures the fraction of free fatty acids (FFA) released from triacylglycerols over time. A new mathematical model has been developed to characterize the FFA versus time profiles generated by the pH-stat method, which can be used to quantify the influence of physicochemical parameters on the rate (k) and extent (phi(max)) of lipid digestion. In this model, k is the amount of FFA produced per unit time per unit surface area, whereas phi(max) is the maximum fraction of digestible FFAs released. This model is used to quantify the influence of lipid droplet characteristics (size, concentration, composition, and emulsifier type) on the digestion of emulsified lipids. The rate (k) of lipid digestion increased with decreasing lipid content (from 2.5 to 0.5 wt %), increasing droplet diameter (from d = 200-15000 nm), and decreasing fatty acid molecular weight (MCT versus corn oil). The extent (phi(max)) of lipid digestion was also considerably less for corn oil than for MCT. The rate and extent of lipid digestion did not depend strongly on initial emulsifier type: beta-lactoglobulin, Tween 20, lecithin, or lyso-lecithin. These results are interpreted in terms of differences in the concentrations of reactants, products, catalysts and cofactors at the lipid droplet surfaces during digestion, for example, triacylglycerols, emulsifiers, FFA, lipase, and bile salts. This model provides a useful means of quantifying the influence of specific parameters on lipid digestion using the pH-stat method.