Phytochemicals
Phytochemicals can be defined as any compound found in plants (the ancient Greek word phyton means plant). However, the term phytochemical is often used to describe a diverse range of biologically active compounds found in plants. Phytochemicals provide plants with colour, flavour and natural protection against pests. Numerous epidemiological studies have indicated that a diet rich in fruit and vegetables offers considerable health benefits to humans. Among these benefits are:
- Reduction of the risk of developing many forms of cancer (lung, prostate, pancreas, bladder and breast).
- Reduction of the risk of cardiovascular diseases.
The majority of these beneficial effects are at least in part due to the presence of phytochemicals in vegetables and fruits. In this context phytochemicals may be defined as "non-nutrient" chemicals found in plants that have biological activity against chronic diseases (1).
While numerous factors have been shown to affect phytochemical levels in foods, to date the mechanism of action, stability during food processing, effect of agricultural factors and stability in post harvest storage on these compounds is not well understood. Therefore, there is a need to assemble existing knowledge and provide holistic information on the fate of these compounds up to their site of action. In Ireland at the present time there are a significant number of nationally and internationally funded research projects examining the effects of many different factors on levels of biologically active compounds in vegetables. A principal objective of the Irish Phytochemical Food Network (IPFN) is to collate existing information on levels of phytochemicals in Irish grown fruits and vegetables thereby gaining holistic knowledge on their fate as they pass through the food chain. This approach will also facilitate the identification of knowledge gaps.
Although the classification of this huge range of compounds is still matter of debate, most of them can be classified as a function of their chemical structure:
- Terpenoids
Carotenoid terpernoids
Non-carotenoid terpenoids - Polyphenolics
Phenolic acids
Flavonoid phenolics
Other non-flavonoid phenolics - Glucosinolates
Aliphatic and Alkenyl glucosinolates
Indolyl glucosinolates
Aromatic glucosinolates - Phytosterols
- Thiosulfanates
- Others
(Anthaquinones, capsaicin, piperine, chlorophylls and derivatives, betaines, polyacetylenes, sequiterpene lactones, glycoalkaloids)
At the present time work in the IPFN is focussed on three main phytochemicals in three Irish grown vegetables; onions, broccoli and carrots. These phytochemicals are respectively:
Polyacetylenes (PA)
Polyacetylenes are examples of bioactive secondary metabolites that were previously considered undesirable in plant foods due to their toxicant properties (2). However, a low daily intake of these "toxins" may be an important factor in the search for an explanation of the beneficial effects of fruit and vegetables on human health. For example, polyacetylenes isolated from carrots have been found to be highly cytotoxic against numerous cancer cell lines (3). Over 1400 different polyacetylenes and related compounds have been isolated from higher plants. Falcarinol a polyacetylene with anti-cancer properties is commonly found in the Apiaceae, Araliaceae and Asteraceae plant families (3):
Figure 1: Chemical structure of Falcarinol, a polyacetylene commonly found in carrots and parsnips.
Figure 1: Chemical structure of Falcarinol, a polyacetylene commonly found in carrots and parsnips.
As mentioned above the PA are common in plants of the family of carrots and parsnips, however other vegetables are rich in other phytochemicals different from polyacetylenes, for example, many fruits and vegetables are rich in polyphenols.
Polyphenols (PPh)
Polyphenols are a class of compounds characterized by the presence of one or more phenol unit or building block per molecule. Polyphenols are a widely investigated and diverse range of compounds. Their biological action is most likely related to their ability to scavenge harmful free radical s in biological systems (4). For example, there is growing evidence that Quercetin (a flavonol) might inhibit the growth of tumour cells containing type II estrogen binging sites, including breast, colon, ovarian, leukaemia, gastrointestinal and meningioma cancer cells (5). Quercetin and related compounds might also have protective effect against cardiovascular diseases and stroke by participating in the reduction of platelet aggregation and vasoconstriction (4).
Many different types of polyphenols have been found in plant. For example, phenolic acids and esters are characterised by only one phenol residue (Figure 2) with an acid or ester group (6).
Figure 2: Gallic acid is an organic acid, also known as 3,4,5-trihydroxybenzoic acid, found in as acid or esterified with sugar residues in gallnuts, sumac, witch hazel, tea leaves, oak bark, and in many other plants.
Figure 2: Gallic acid is an organic acid, also known as 3,4,5-trihydroxybenzoic acid, found in as acid or esterified with sugar residues in gallnuts, sumac, witch hazel, tea leaves, oak bark, and in many other plants .
Flavonoid phenolics are derivatives of the "flavone" (2-phenyl-1,4 benzopyrone) polyphenolic structure (6):
Figure 3: Flavones are a class of flavonoid based on the backbone of 2-phenylchromen-4-one (2-phenyl-1-benzopyran-4-one) shown above.
Figure 3: Flavones are a class of flavonoid based on the backbone of 2-phenylchromen-4-one (2-phenyl-1-benzopyran-4-one) shown above.
Distinguished sources of polyphenols include berries, tea, beer, grapes/wine, olive oil, chocolate/cocoa, coffee, walnuts, peanuts, pomegranates, and other fruits and vegetables (7, 8). High levels of polyphenols can generally be found in the fruit skins. The plants from the Allium family, such as onions, garlic or leek are particularly rich in polyphenols (in quantity and in variety). The plants from cruciferous family are particularly rich in other type of phytochemicals called glucosinolates.
Glucosinolates (GLS)
Structurally glucosinolates (β-thioglucoside-N-hydroxysulfates) are characterised by the presence of nitrogen and sulphur groups and they are derived from glucose and an amino acid (Figure 4). Glucosinolates act as secondary metabolites in cruciferous vegetables (such as Brassicaceae, Capparideceae and Caricaceae and also in the genus of Drypetes in the Euphorbiaceae family (9)). Although their role in plants is unclear, their potent odour and taste suggests a role in hervivore and microbial defence. Glucosinolates are not bioactive until they have been enzymatically hydrolysed to the associated isothiocyanate by the endogenous myrosinase enzyme that is released by disruption of the plant cell though harvesting, processing, or mastication. More than 120 types of glucosinolates with varying side chains have been isolated, but not all of these are present in edible plants (10).
Figure 4: Chemical structure common to all glucosinolates (β-thioglucoside-N-hydroxysulfate).
Figure 4: Chemical structure common to all glucosinolates (β-thioglucoside-N-hydroxysulfate).
These three phytochemicals; PA, PPhs and GLS are only a fraction of the huge range of phytochemicals present in all the fruit and vegetables varieties. However, they represent three main chemical structures, which are used to classify phytochemicals, as detailed at the beginning of this introduction.
References
- Kushad, M.; Masiuymas, J.; Smith, M.; Kalt, W.; Eastman, k., Health promoting phytochemicals in vegetables. Horticultural reviews 2003, 28, 125-185.
- Czepa, A.; Hofmann, T., Quantitative Studies and Sensory Analyses on the Influence of Cultivar, Spatial Tissue Distribution, and Industrial Processing on the Bitter Off-Taste of Carrots (Daucus carota L.) and Carrot Products. J. Agric. Food Chem. 2004, 52, (14), 4508-4514.
- Zidorn, C.; Johrer, K.; Ganzera, M.; Schubert, B.; Sigmund, E. M.; Mader, J.; Greil, R.; Ellmerer, E. P.; Stuppner, H., Polyacetylenes from the Apiaceae Vegetables Carrot, Celery, Fennel, Parsley, and Parnsnip and Their Cytotoxic Activities. J. Agric. Food Chem. 2005, 53, 2518-2523.
- Boots, A. W.; Haenen, G. R. M. M.; Bast, A., Health effects of quercetin: From antioxidant to nutraceutical. European Journal of Pharmacology 2008, 585, (2-3), 325-337.
- B. S. Patil, L. M. P. B. K. H., Changes in quercetin concentration in onion (Allium cepa L.) owing to location, growth stage and soil type. New Phytologist 1995, 130, (3), 349-355.
- Wrolstad, R. E., Handbook of Food Analytical Chemistry. First ed.; John Wiley & Sons, Inc.: Hooboken, New Jersey, 2005; Vol. 2.
- Belitz, H.-D.; Grosch, W.; Schieberle, P., Food Chemistry. Third Edition ed.; Springer-Verlag: Berlin, 2004; p 1070.
- Fennema, O. R., Food Chemistry. Third ed.; Marcel Dekker, Inc.: New York, 1996.
- Halkier, B. A.; Gershenzon, J., BIOLOGY AND BIOCHEMISTRY OF GLUCOSINOLATES. Annual Review of Plant Biology 2006, 57, (1), 303-333.
- Tian, Q.; Rosselot, R. A.; Schwartz, S. J., Quantitative determination of intact glucosinolates in broccoli, broccoli sprouts, Brussels sprouts, and cauliflower by high-performance liquid chromatography-electrospray ionization-tandem mass spectrometry. Analytical Biochemistry 2005, 343, (1), 93-99.
