Influence of product and process factors on the bioaccessibility and bioavailability of carotenoids in plant-based food systems

Fruits and vegetables are part of a healthy diet as they are important in maintaining good health as well as to reduce coronary heart diseases, strokes and certain types of cancer. Carotenoids are lipophilic micronutrients present in all fruits and vegetables. They are located in plastids of the plant cell, where they can be associated with other components. Due to their high level of bio-encapsulation and low solubility in an aqueous phase, absorption by the human body is rather low. In previous research, carotenoid bioaccessibility appeared to be the highest in food systems where natural barriers were destroyed through processing, to which lipids were added and whereby the carotenoids were transferred to the lipid fraction added prior to digestion. In this context, emulsification of a carotenoid-enriched oil phase could be explored as an adequate delivery system for carotenoids. Due to their lipophilic nature, the absorption pathway of carotenoids is strongly linked to the one of lipid digestion. However, it can be questioned to what extent emulsion characteristics play a role in lipid digestibility and how this can influence carotenoid bioaccessibility.

Therefore, this doctoral project focused on exploring the effect of specific emulsion characteristics on lipid digestibility, micelle formation and carotenoid bioaccessibility. For this, an in vitro digestion approach was proposed that enables the study of digestion time (kinetic approach). From an analytical perspective, multiple lipid digestion species were evaluated for the first time, to obtain detailed insight in the lipid digestion process. Furthermore, this study aimed to link the evolution of lipid digestion and carotenoid bioaccessibility to (micro)structural changes occurring along the digestive tract.

In this context, the effect of (i) the unsaturation degree of the emulsified oil phase, (ii) the gastrointestinal stability of emulsions stabilized by small molecule surfactants and (iii) the presence, location and structure of fiber on lipid digestion, mixed micelle formation and carotenoid bioaccessibility was investigated. Hereto, oil-in-water emulsions were formulated with 5% (w/v) carotenoid-enriched oil and stabilized by one or two emulsifiers (0-1.5% w/v), such as Tween 80, sucrose esters, citrus pectin or phosphatidylcholine. All emulsions studied, presented small initial oil droplet sizes (smaller than 2 µm). The emulsions were submitted to in vitro digestion during which the small intestinal phase was simulated kinetically. In this way, the time dependency of lipolysis, micelle formation and carotenoid bioaccessibility could be evaluated, allowing to gain more detailed insight in the mechanism behind these processes.

Different emulsion characteristics led to different reaction rate constants as well as different final extents of triacylglycerol hydrolysis and carotenoid micellarization. For example, oils rich in polyunsaturated fatty acids (soybean or linseed oil) were digested slower and sometimes to a lesser extent in comparison to olive oil rich in monounsaturated fatty acids. This can be partially attributed to the more bended structure of polyunsaturated fatty acids, which might retard and/or hinder lipase hydrolysis. This doctoral project showed that not only the oil droplet size of the initial emulsion is of importance, but also the gastrointestinal stability of these oil droplets. More specifically, emulsions unstable in the gastric phase presented a larger particle size at the beginning of the small intestine, resulting in a slower and eventually incomplete lipolysis. The work regarding pectin was divided into two parts. First, the emulsifying properties of citrus pectin with different degree of methylesterification were evaluated, after which the effect of the presence of pectin in emulsions was studied on the gastrointestinal stability of oil droplets as well as on the kinetics of lipolysis and carotenoid bioaccessibility. It was quantitatively shown that citrus pectin is capable of reducing the interfacial tension of an oil droplet regardless its structure, evidencing its adsorption at the oil-water interfaces and surface-active properties. The results of the digestion study showed that the pectin structure influenced the lipolysis rate, while the pectin location in the emulsion had a large impact on lipolysis extent. In this sense, the presence of pectin at the oil-water interface led to a fast lipolysis, but low extent. This suggests that pectin does not hinder lipase adsorption, yet it may interact with other components within the intestinal juices, such as bile salts, inhibiting a complete lipid digestion. In all digestion studies performed, a strong interrelation was observed between triacylglycerol hydrolysis, micelle formation and carotenoid bioaccessibility.

The data obtained in the digestion studies discussed above were first modelled using a single response empirical model (fractional conversion model). In this single response approach, all lipid digestion species evaluated were modelled independently. However, they are part of a common reaction scheme and are interrelated. Therefore, in the last part of this doctoral project, it was aimed to build a mechanism-based multiresponse model. Using the iterative process of multiresponse kinetic modelling and the data obtained previously in this work, it was possible to propose a generally applicable reaction scheme of lipid digestion, which could be translated to a set of corresponding differential equations describing the evolution of all evaluated lipolysis species together. This multiresponse model is expected to have a higher reliability and applicability compared to the selected single response empirical model applied before. The mechanistic multiresponse kinetic model proposed, was successful to describe all datasets obtained in this work.

From this work, it can be concluded that emulsion design can be of great interest to tailor lipid digestion and carotenoid bioaccessibility. Moreover, if one wants to manipulate carotenoid bioaccessibility, one can focus on influencing lipolysis in case carotenoids are incorporated in an emulsified lipid fraction prior to digestion.