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Processing and Bioavailability (WG2) page 54
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Digestion
The carotenoids are insoluble within the aqueous environment of the GI tract and therefore
to be absorbed by the body they must be released from the food matrix and solubilised within
a lipophilic pool. Within the gastric environment the principal pool is the lipid ingested within
the meal. The lipid forms droplets (an emulsion) within the chyme and thus provides a large
surface area of dissolution of the carotenoids. The droplets are emptied into the duodenum
where the pancreatic enzymes, principally lipase, hydrolyse the lipid into free fatty acids and
monoglycerides. These lipolysis products form a complex phase at the droplet surface that
dissociates to form a micellar (molecular aggregate) phase with bile constituents such as the
phospholipids and bile salts. It is the micellar phase that is the diffusion vehicle of the
carotenoids to the point of absorption at the mucosal surface.
The carotenoids released from the food matrix can be solubilised within the emulsion
droplets in the stomach or directly into the micellar phase within the duodenum. The ease of
incorporation of lycopene within the micellar phase is determined not only by its ease of
release from the food matrix itself but also by the ease of inclusion into the micellar phase
either directly or via the emulsion droplets. Release from the food matrix is enhanced by
efficient particle breakup of the food material (Rich et al., 1998) either by separation of the
plant cells or by their complete disruption. Heating of foods prior to ingestion can also
improve carotenoid availability as a result of the dissociation of the protein-carotenoid
complexes or dispersion of the crystalline aggregates. (Grtner et al., 1997, Porrini et al.,
1998).
Similarly, if the solubilisation of the carotenoid within a lipid phase is allowed to occur
during processing, for example by heating tomato juice with supplemental lipid then the
measured absorption of lycopene is enhanced (Stahl et al., 1992b). It should be noted that the
solubility of the carotenoids is still very limited within bulk lipid (0.112-0.141% for
hydrocarbon carotenoids and 0.022-0.088% for hydroxyl carotenoids). Transfer of the
carotenoid from the emulsion to the micellar phase may also be influenced by the
hydrophobicity of the carotenoid. The hydroxy-carotenoids (eg. lutein) are located
preferentially at the surface of the lipid droplets hence exchange with the micellar phase will
occur. For the hydrocarbon carotenoids, (eg. lycopene) which are located within the random
core of the lipid droplets, exchange with the micellar phase does not occur and lipolysis is