194 Produce Degradation: Reaction Pathways and their Prevention
7.2.2 OCCURRENCE AND BIOSYNTHESIS
Detailed descriptions of the content of carotenoids in various species of fruits and
vegetables may be found in several monographs (Karrer and Tucker, 1950; Goodwin,
1976; Gross, 1991). Goodwin and Goad (1970) proposed the following seven pat-
terns of carotenoid distribution, which frequently merge into one another: (1) insig-
nificant amounts of carotenoids; (2) small amounts found in chloroplasts, mainly
beta-carotene, lutein, violaxanthin, and neoxanthin; (3) comparatively large amounts
of acyclic lycopene and partly saturated related compounds; (4) relatively large
amounts of beta-carotene and its derivatives, cryptoxanthin and zeaxanthin;
(5) unusually large amounts of epoxides; (6) unique carotenoids (e.g., capsanthin);
and (7) poly-cis-carotenoids.
From the point of view of aesthetic quality of the produce, carotenoid synthesis
is especially important during ripening. At this stage the formation of carotenoids
accelerates by chloroplasts changing to chromoplasts with concurrent degradation
of chlorophyll. The synthesis of carotenoids may be summarized in the following
manner. Initially, mevalonic acid is formed. The next steps involve formation of
geranylgeranyl and phytoene and desaturation of phytoene to lycopene. Cyclization
of the end groups of lycopene leads to the formation of cyclic carotenes. Further
steps may include incorporation of oxygen with generation of xanthophylls (Gross,
1991). The final stages of biosynthesis leading to structural modifications involve
complex enzymatic transformations that are not fully understood. The end result of
these reactions is the high carotenoid content of the ripened produce and optimal color,
which varies from commodity to commodity. In many fruits and vegetables the color
at the ripened stage is critical for consumer acceptability and perception of quality.
Red pepper is an important example of this phenomenon. Several recent studies
concentrated on the changes in carotenoid composition of different varieties of red
pepper during ripening (Minguez-Mosquera and Hornero-Mendez,1994a,b; Hor-
nero-Mendez and Minguez-Mosquera, 2000a,b). Chloroplast carotenoid pigments,
including lutein and neoxanthin, vanish, together with chlorophylls. At the same
time, new carotenoids characteristic of chromoplasts are synthesized. Marin et al.
(2004), as part of their broader study of antioxidant constituents of sweet pepper,
identified and quantified carotenoids at four maturity stages (immature green, green,
immature red, and red). Neoxanthin, lutein (predominating at this stage), cis-lutein,
violaxanthin, beta-cryptoxanthin, and beta-carotene were found in immature green
peppers. In the green stage the same compounds were present, with beta-carotene
predominating. In the immature red stage lutein and cis-lutein were not detected but
new compounds occurred, including capsanthin-5-6,-epoxide capsanthin, antherax-
anthin, cucurbitaxanthin A, and zeaxanthin. In the red stage, capsorubin, cis-cap-
santin, and cis-zeaxanthin were also detected. The content of capsanthin increased
19 times from the immature stage. Also, the total amount of pigments increased
approximately five times. This is an example of a biosynthesis pattern in a produce
wherein unique carotenoids are synthesized at the maturity stage.
The qualitative and quantitative carotenoid composition and color of carrots also
has an impact on consumer preferences. The content of carotenes in carrots is