Carotenoids
Carotenoids are lipid-soluble plant pigments com- mon in photosynthetic plants. The term carotenoid summarizes a class of structurally related pigments, mainly found in plants. At present, more than 600 different carotenoids have been identified, although only about two dozens are regularly consumed by humans. The most prominent member of this group is-carotene. Most carotenoids are structurally ar- ranged as two substituted or unsubstituted ionone rings separated by four isoprene units containing nine conjugated double bonds, such as␣- and -carotene, lutein, and zeaxanthin, and␣- and -cryptoxanthin (Goodwin and Merce, 1983; Van den Berg et al., 2000). These carotenoids, along with lycopene, an acylic biosynthetic precursor of-carotene, are most commonly consumed and are most prevalent in hu- man plasma (Castenmiller and West, 1998).
38 Part I: Processing Technology I II 2 3 4 5 20' 19' 16 17 19 10 11 12 14 15' 13' 9' 7' 1 7 18 16' 17' 6 8 9 13 15 14' 12' 11' 10' 8' 6' 20 18' 5' 4' 1' 3' 2'
Figure 2.1. Structure and numbering of the carotenoid carbon skeleton. (Source: Shahidi et al., 1998.)
All carotenoids can be derived from an acyclic C40H56 unit by hydrogenation, dehydrogenation, cyclization and/or oxidation reactions (Fig. 2.1). All specific names are based on the stem name carotene, which corresponds to the structure and numbering in Figure 2.1 (Shahidi et al., 1998).
The system of conjugated double bonds influences their physical, biochemical, and chemical properties. Based on their composition, carotenoids are subdi- vided into two groups. Those contain only carbon and hydrogen atoms, which are collectively assigned as carotenes, e.g., -carotene, ␣-carotene, and ly- copene. The majority of natural carotenoids contain at least one oxygen function, such as keto, hydroxy, or epoxy groups, and are referred to as xanthophylls or oxocarotenoids. In their natural sources, carotenoids mainly occur in the all-trans configuration (Goodwin and Merce, 1983; Van den Berg et al., 2000).
Carotenoid pigments are of physiological interest in human nutrition, since some of them are vita- min A precursors, especially-carotene. ␣-Carotene, and ␣- and -cryptoxanthin possess provitamin A activity, but to a lesser extent than-carotene. On the basis of epidemiological studies, diet rich in fruits and vegetables containing carotenoids is suggested to protect against degenerative diseases such as cancer, cardiovascular diseases, and macular degeneration. Recent clinical trials on supplemental -carotene have reported a lack of protection against degener- ative diseases. Much of the evidence has supported the hypothesis that lipid oxidation or oxidative stress is the underlying mechanism in such diseases. To date carotenoids are known to act as antioxidants in vitro. In addition to quenching of singlet oxygen, carotenoids may react with radical species either by addition reactions or through electron transfer reac- tions, which results in the formation of the carotenoid
radical cation (Canfield et al., 1992; Sies and Krinsky, 1995; Van den Berg et al., 2000; S´anchez-Moreno et al., 2003c).
Carotenoid intake assessment has been shown to be complicated mainly because of the inconsisten- cies in food composition tables and databases. Thus, there is a need for more information about indi- vidual carotenoids. The estimated dietary intake of carotenoids in Western countries is in the range of 9.5–16.1 mg/day. To ensure the intake of a sufficient quantity of antioxidants, the human diet, which real- istically contains 100–500 g/day of fruit and vegeta- bles, should contain a high proportion of carotenoid- rich products. No formal diet recommendation for carotenoids has yet been established, but some ex- perts suggest intake of 5–6 mg/day, which is about twice the average daily U.S. intake. In the case of vi- tamin A, for adult human males, the RDA is 1000g retinyl Eq/day, and for adult females, 800g retinyl Eq/day (O’Neill et al., 2001; Trumbo et al., 2003).
Citrus fruits are the major source of - cryptoxanthin in the Western diet. The major fruit contributors to the carotenoid intake in Western diets are orange (-cryptoxanthin and zeaxanthin), tanger- ine (-cryptoxanthin), peach (-cryptoxanthin and zeaxanthin), watermelon (lycopene), and banana (␣- carotene). Other relatively minor contributors are kiwi fruit, lemon, apple, pear, apricot, cherry, melon, strawberry, and grape (Granado et al., 1996; O’Neill et al., 2001).
Flavonoids
Flavonoids are the most common and widely dis- tributed group of plant phenolics. Over 5000 different flavonoids have been described to date and they are classified into at least 10 chemical groups. Among
A C B HO HO OH OH O O R1 OH R1 OH OH HO OH OH OH HO OH O R1 R2 HO OH OH OH R1 R1 O R2 HO OH Flavones Flavanones Anthocyanidins Isoflavones Flavonols Flavanols R1 Apigenin H Luteolin OH R1 R2 Kaempferol H H Quercetin OH H Myricetin OH OH R1 R2 Catechin H OH Epicatechin OH H R1 R2 Naringenin H OH Hesperetin OH OCH3 R1 R2 Cyanidin OH H Pelargonidin H H Malvidin OCH3 OCH3
R1 Daidzein H Genistein OH R1 R2 R2 O O O O O O
Figure 2.2. Structures of the main flavonoids in fruits. (Source: Harborne, 1993.)
them, flavones, flavonols, flavanols, flavanones, an- thocyanins, and isoflavones are particularly common in fruits (Fig. 2.2). The most-studied members of these groups are included in Table 2.5, along with some of their fruit sources (Bravo, 1998).
Numerous epidemiological studies support the concept that regular consumption of foods and bever- ages rich in antioxidant flavonoids is associated with a decreased risk of cardiovascular disease mortality. There is also scientific evidence that flavonoids may
40 Part I: Processing Technology
Table 2.5. Classification of Flavonoids and Their Presence in Fruits
Subclasses Flavonoids Fruits
Flavones Apigenin, luteolin Apples, blueberries, grapefruit, grapes, oranges
Flavonols Quercetin, kaempferol, myricetin Apples, berries, plums Flavanols Catechin, epicatechin,
epigallocatechin gallate
Apples, berries, grapes, plums Flavanones Hesperetin, naringenin Citrus fruits
Anthocyanins Cyanidin, pelargonidin, malvidin Berries, grapes Isoflavones Genistein, daidzein Currants, passion fruit Source: De Pascual-Teresa et al. (2000) and Franke et al. (2004).
protect against some cancers. It has been shown in the past that flavonoid content and structure may change with technological processes increasing or decreas- ing their contents and biological activity (Garc´ıa- Alonso et al., 2004).
Most of the existing flavonoids in fruits have shown antioxidant activity in in vitro studies, and almost all the fruits that have been screened for their antioxidant activity have shown to a lower or higher extent some antioxidant and radical scavenger activity.
Other biological activities of flavonoids seem to be independent of their antioxidant activity. This is the case of the oestrogen-like activity showed by isoflavones. Isoflavones have also shown an effect on total and HDL cholesterol levels in blood.
Anthocyanins have shown to be effective in de- creasing capillary permeability and fragility and also have anti-inflammatory and anti-oedema activities.
Flavonols inhibit COX-2 activity and thus may play a role in the prevention of inflammatory diseases and cancer (De Pascual-Teresa et al., 2004).
Factors like modification on the flavonoid struc- ture or substitution by different sugars or acids may deeply affect the biological activity of flavonoids and in this sense different processing of the fruits may also influence their beneficial properties for human health.
Phytosterols
Plant-based foods contain a large number of plant sterols, also called phytosterols, as minor lipid com- ponents. Plant sterols have been reported to include over 250 different sterols and related compounds. The most common sterols in fruits are-sitosterol, and its 22-dehydro analogue stigmasterol, campesterol and avenasterol (4-desmethylsterols). Chemical struc- tures of these sterols are similar to cholesterol dif- fering in the side chain (Fig. 2.3).-Sitosterol and stigmasterol have ethyl groups at C-24, and campes- terol has a methyl group at the same position. Plant sterols can exist as free plant sterols, and as bound conjugates: esterified plant sterols (C-16 and C-18
21 HO A B C D 4 6 7 2 1 19 11 18 20 23 25 27 12 16 15 22 24 26 R 22 24 R sitosterol campesterol stigmasterol 5α-cholestan-3β-οl Δ5-avenasterol 24 R 24 R
Figure 2.3. Structures of cholesterol (5␣-cholestan-3-ol), sitosterol, campesterol, stigmasterol, and 5-avenasterol. (Source: Piironen et al., 2003.)
fatty acid esters, and phenolic esters), plant steryl glycosides (-D-glucose), and acylated plant steryl glycosides (esterified at the 6-hydroxy group of the sugar moiety). All of these forms are integrated into plant cell membranes (Piironen et al., 2000, 2003).
Plant sterols are not endogenously synthesized in humans, therefore, are derived from the diet entering the body only via intestinal absorption. Since plant sterols competitively inhibit cholesterol intestinal up- take, a major metabolic effect of dietary plant sterols is the inhibition of absorption and subsequent com- pensatory stimulation of the synthesis of cholesterol. The ultimate effect is the lowering of serum choles- terol owing to the enhanced elimination of cholesterol in stools. Consequently, the higher the dietary intake of plant sterols from the diet, the lower is the choles- terol absorption and the lower is the serum cholesterol level (Ling and Jones, 1995; De Jong et al., 2003; Trautwein et al., 2003).
The usual human diet contains currently around 145–405 mg/day of plant sterols. Dietary intake val- ues depend on type of food intake. Intakes, especially that of-sitosterol, are increased two- to threefold in vegetarians. For healthy humans, the absorption rate of plant sterols is usually less than 5% of dietary levels. Serum sterol levels of around 350–270g/dl in non-vegetarians have been observed (Ling and Jones, 1995; Piironen et al., 2000).
Vegetables and fruits are generally not regarded to be as good a source of sterols as cereals or vegetable oils. The plant sterol content in a food may vary depending on many factors, such as genetic background, growing conditions, tissue maturity, and postharvest changes (Piironen et al., 2000). There are scarce data available on the content of plant sterols in the edible portion of fruits (Wiehrauch and Gardner, 1978; Morton et al., 1995). Recently, the fruits more commonly consumed in Finland have been analyzed. Total sterols ranged from 6 mg/100 g (red currant) to 22 mg/100 g (lingonberry) of fresh weight, in all fruits, except avocado, which contained significantly more sterols, 75 mg/100 g. The content on dry weight basis was above 100 mg/100 g in most products. Peels and seeds were shown to contain more sterols than edible parts (Piironen et al., 2003). In Sweden, the range of plant sterol for 14 fruits is 1.3–44 mg/100 g (fresh weight), only passion fruit contains more than 30 mg/100 g (Normen et al., 1999). Among the fruits found in both reports, orange shows the high- est plant sterol content, and banana the lowest. In all the items analyzed,-sitosterol occurred at the high- est concentrations, followed by campesterol or stig- masterol. Detectable amounts of five-saturated plant
stanols, sitostanol, and campestanol, were found in specific fruits such as pineapple.
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