Application of static in vitro digestion models for assessing the bioaccessibility of hydrophobic bioactives: A review

Some hydrophobic substances in foods may improve human health and wellbeing, including oil-soluble vitamins and nutraceuticals. The fortification of foods and beverages with these substances has therefore become an important focus of modern food research. However, many hydrophobic bioactives have low bioavailability because of their poor water-solubility and chemical instability, which limits their application as health-promoting food ingredients. This review article provides an overview of static in vitro digestion models available to provide insights into the factors impacting the bioavailability of hydrophobic bioactives, as well as presenting many examples of the application of these models. In vitro digestion models are widely used because they are faster, cheaper, and more reproducible than in vivo methods, and their results often correlate well with in vivo ones. The INFOGEST in vitro digestion model is commonly used to simulate the behavior of foods within the human gastrointestinal tract. Insights gained from in vitro digestion methods, suggest that food matrix effects significantly influence the bioaccessibility of hydrophobic bioactives in emulsified foods, i.e., the fraction of the ingested substance that is available for absorption, which is often the rate limiting factor determining their overall bioavailability. Bioaccessibility can be enhanced by optimizing the composition and structure of the food matrix. After digestion, the lipid phase in foods should generate enough fatty acids and monoacylglycerols to assemble into mixed micelles with large enough hydrophobic domains to solubilize all the bioactives released from the food matrix. The oil droplets in emulsions should be small enough to ensure complete lipid digestion, so all the hydrophobic bioactives are released and solubilized. The emulsifier used should produce gastric-resistant oil droplets that do not inhibit lipase adsorption and activity. The potential for food additives to inhibit lipid digestion and/or precipitate bioactive-loaded mixed micelles should also be considered. In vitro digestion models are extremely powerful tools for optimizing food matrix properties to enhance the bioavailability of bioactive substances, which may lead to healthier food products.