Structural barriers for carotenoid in vitro bioaccessibility: A case study on carrots and tomatoes

Carotenoids are lipophilic molecules that occur naturally in fruits and vegetables, accounting for their characteristic colors. These compounds possess important biological activities that are related to health promoting effects such as a reduced risk to develop cancer and cardiovascular diseases. To exert the positive effects, carotenoids should be readily absorbed to reach their site of action. However, before being absorbable during digestion, carotenoids have to overcome the natural physical structural barriers within the food matrix that hinder their release. As carotenoids are located within the chromoplast organelles present in plant cells, the organization and localization within the chromoplast substructure and the cell wall constitute the two main natural structural barriers governing carotenoid release. In light of these considerations, the aim of the present thesis consists in gaining a more detailed insight in the effect of the natural structural barriers on carotenoid bioaccessibility. In order to evaluate the individual contribution of the natural structural barriers towards carotenoid bioaccessibility, a new experimental approach was developed: fractions containing carotenoids with different levels of bio-encapsulation were isolated from plant matrices (i.e. tomatoes and carrots) and further evaluated for carotenoid bioaccessibility using an in vitro digestion model. The fractions with decreasing level of structural barriers consisted of large cell clusters, small cell clusters, chromoplasts and carotenoid enriched oil. In this way, respectively (multiple) cell walls and the chromoplast substructure were considered as the main natural structural barriers governing the carotenoid bioaccessibility in the fractions. Results confirmed that the natural structural barriers constitute important limiting factors for carotenoid release from the matrix during digestion. However, depending on the type of matrix considered, different natural structural barriers proved to be significant in limiting carotenoid bioaccessibility. The cell wall and chromoplast substructure are important barriers for carotenoid bioaccessibility in carrots. However, in the case of tomato, the chromoplast substructure constitutes the most important natural structural barrier limiting lycopene bioaccessibility. The second objective of the present thesis consists in evaluating the effect of different types of processes (i.e. thermal, mechanical) as potential tools to favor the disruption of the natural structural barriers with a subsequently carotenoid bioaccessibility enhancement. The same experimental approach as mentioned above was used to evaluate the effect of processing on the natural structural barriers and the consecutive impact on the carotenoid bioaccessibility in carrot and tomato matrices. It is important to remark, however, that, besides disruption of barriers, processing may induce the formation of new barriers that hinder carotenoid release. Previous studies reported a negative effect of high pressure homogenization on lycopene bioaccessibility in tomato. In this context, enzyme treatments were also applied to a high pressure homogenized tomato puree in order to disrupt the potentially new formed barriers that hinder lycopene release and micellarization. Overall results showed that the carotenoid type, independently of the type of matrix wherein they are incorporated, significantly determines the effect of processing on their bioaccessibility. ß-Carotene bioaccessibility is significantly enhanced upon thermal and high pressure homogenization processes. However, lycopene bioaccessibility is negatively affected upon the thermal treatments applied. This indicates that thermal processing promotes the formation of new barriers that hinder the lycopene bioaccessibility. Further experimental work showed that high pressure homogenization and enzyme treatments did not considerably improve the lycopene bioaccessibility in tomato derived products. It is suggested that lycopene might be strongly associated with other components and/or that lycopene crystals are unmodified through processing thus accounting for a limited enhancement of its bioaccessibility. Moreover, it was shown that the molecular structure of the carotenoid plays an important role in the transfer efficiency from the oil to the micellar phase during digestion. According to the results, ß-carotene transfer efficiency to the micelles is higher than that of lycopene. Nonetheless, for both types of carotenoids (i.e. ß-carotene and lycopene), the transfer from oil to micelles represents the most crucial step during digestion. It is generally concluded that the natural structural barriers constitute important physical limitations for carotenoid release from the matrix during digestion. Processing might be used as a tool to improve the nutritional characteristics of tomato and carrot based-products by disrupting those barriers. Nonetheless, it should be considered that the carotenoid bioaccessibility upon processing is affected differently depending on the type of carotenoid considered. Lycopene has a low bioaccessibility by nature which is only slightly affected upon processing. However, other types of carotenoids present within the same matrix, such as ß-carotene, have higher bioaccessibility values which can be further improved upon thermal or high pressure homogenization processes.status: publishe