Dynamic modeling highlights the major impact of droplet coalescence on the in vitro digestion kinetics of a whey protein stabilized submicron emulsion

Submicron oil-in-water emulsions stabilized with whey proteins have been reported to be relatively resistant and highly sensitive to coalescence during the gastric and intestinal phases of in vitro digestion experiments, respectively. The aim of this study was to assess the impact of droplet coalescence on the intestinal lipolysis kinetics of such emulsion, and to develop a mathematical model able to predict the experimental observations. A submicron whey protein stabilized emulsion made of a mixture of medium-chain (MCT) and long-chain triacylglycerols (LCT) was prepared and submitted to gastrointestinal in vitro digestion. Triacylglycerol concentrations and droplet size distributions were measured before and after the gastric phase and during the intestinal phase using HPLC and laser granulometry, respectively. MCT were fully digested within 15 min of intestinal digestion, whereas LCT were still detected after 5 h. Moreover, the intestinal lipolysis of LCT showed a two-stage behavior with an initial fast rate that markedly slowed down after about 30 min, a time at which a sudden rise in the droplet sizes, attributed to coalescence, was also observed. A mathematical model based on the experimentally measured droplet sizes and assuming a rate of lipolysis proportional to the interfacial area was developed and successfully used to reproduce the observed kinetics. Our results support the idea that droplet coalescence during the intestinal phase was the main reason for the marked slowdown of the kinetics of lipid digestion, hence suggesting that other possible mechanisms, such as an inhibition of the lipolysis reaction, could only explain secondary order effects.