Tag Archives: antioxidants

Green tea: definition, processing, properties, polyphenols

What is green tea?

Green tea is an infusion of processed leaves of tea plant, Camellia sinensis, a member of the Theaceae family.
It is the most consumed beverages in Asia, particularly in China and Japan.
Western populations consume black tea more frequently than green tea. However, in recent years, thanks to its health benefits, it has been gaining their attention.
Currently, it accounts for 20% of the tea consumed worldwide.

“You can never get a cup of tea large enough or a book long enough to suit me.”C.S. Lewis

Processing and properties of green tea

Green Tea
Fig. 1 – Camellia sinensis

As all other types of tea, it is produced from fresh leaves of Camellia sinensis.
The peculiar properties of the beverage depend on the type of processing that the leaves undergo. In fact, they are processed in such a way as to minimize both enzymatic and chemical oxidation processes of the substances contained in them, in particular catechins.
Therefore, among the different types of tea, it undergoes the lowest degree of oxidation during processing.
At the end of the processing, tea leaves retain their green color, thanks to the little chemical modifications/oxidations they have undergone. The infusion shows off a yellow-gold color.
Finally, the processing of tea leaves ensures that green tea flavor is more delicate and lighter than black tea.

The three main steps in the processing of green tea

After harvesting, tea leaves are exposed to the sun for 2-3 hours and withered/dried; then, the real processing starts.
It consists of three main steps: heat treatment, rolling and drying.

Heat treatment

Heat treatment, short and gentle, is the crucial step for the quality and properties of the beverage.
It occurs with steam (the traditional Japanese method) or by dry cooking in hot pans (a large wok, the traditional Chinese method). Heat treatment has the purpose of:

  • inactivate the enzymes present in the tissues of the leaves, thus stopping enzymatic oxidation processes, particularly of polyphenols;
  • eliminate the grassy smell in order to stand out tea flavor;
  • evaporate part of the water present in the fresh leaf (water constitutes about 75% of the weight of the leaf), making it softer, so as to make the next step easier.

Rolling

The rolling step follows the heat treatment of the leaves; this step has the purpose of:

  • facilitate the next stage of drying;
  • destroy the tissues of the leaves in order to favor, later, the release of aromas, thus improving the quality of the product.

Drying

The drying is the last step, which also leads to the production of new compounds and improves the appearance of the product.

Green tea polyphenols

Gree Tea
Fig. 2 – EGCG

All types of tea are rich in polyphenols, compounds that are also present in fruits, vegetables, extra virgin olive oil, and red wine.
Fresh tea leaves are rich in water-soluble polyphenols, especially catechins (or flavanols) and glycosylated catechins (both belonging to the class of flavonoids), molecules which are believed to be the responsibles of the benefits of green tea.
The major catechins in green tea are epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate, epicatechin, epicatechin, but also catechin, gallocatechin, catechin gallate, and gallocatechin gallate are present, even if in lower amount. These polyphenols account for 30%-42% of the dry leaf weight (but only 3%–10% of the solid content of black tea).
Green tea caffeine accounts for 1,5-4,5% of the dry leaf weight.

How to maximize the absorption of green tea catechins

In vitro studies have shown the high antioxidant power of catechins, greater than that of vitamin C and vitamin E. In vitro, EGCG is generally considered the most biologically active catechin.
In vivo studies and several epidemiologic studies have shown the possible preventive effects of green tea catechins, especially EGCG, in preventing the development of:

  • cardiovascular disease, such as hypertension and stroke;
  • some cancers, such as lung cancer (but not among smokers) and oral and digestive tract cancers.

For these reasons, it is essential to maximize the intestinal absorption of catechins.
Catechins are stable in acidic environment, but not in non-acidic environment, as in the small intestine; also for this reason, after digestion, less than 20% of the total remains.
Studies with models of the digestive tract of rat and man, that simulate digestion in stomach and small intestine, have shown that the addition of citrus juice or vitamin C to green tea significantly increases the absorption of catechins.
Among tested citrus juices, lemon juice is the best, followed by orange, lime and grapefruit juices. Citrus juices seem to have a stabilizing effect on catechins that goes beyond what would be predicted solely based on their ascorbic acid content.

References

Clifford M.N., van der Hooft J.J.J., and Crozier A. Human studies on the absorption, distribution, metabolism, and excretion of tea polyphenols. Am J Clin Nutr 2013;98:1619S-1630S [Abstract]

Dwyer J.T. and Peterson J. Tea and flavonoids: where we are, where to go next. Am J Clin Nutr 2013;98:1611S-1618S [Abstract]

Green R.J., Murphy A.S., Schulz B., Watkins B.A. and Ferruzzi M.G. Common tea formulations modulate in vitro digestive recovery of green tea catechins. Mol Nutr Food Res 2007;51(9):1152-1162 [Abstract]

Huang W-Y., Lin Y-R., Ho R-F., Liu H-Y., and Lin Y-S. Effects of water solutions on extracting green tea leaves. ScientificWorldJournal 2013;Article ID 368350 [Abstract]

Sharma V.K., Bhattacharya A., Kumar A. and Sharma H.K. Health benefits of tea consumption. Trop J Pharm Res 2007;6(3):785-792 [Abstract]

Green tea benefits for health

Benefits of green tea: science and myths

Green Tea Benefits
Fig. 1 – Green Tea Benefits

Tea drinking, particularly green tea, has always been associated, at least in East Asia cultures (mainly in China and Japan) with health benefits. Only recently, the scientific community has begun to study the health benefits of tea consumption, recognizing its preventive value in many diseases.

Green tea benefits in preventing cancer

Several epidemiological and laboratory studies have shown encouraging results with respect to possible preventive role of tea, particularly green tea and its catechins, a subgroup of flavonoids (the most abundant polyphenols in human diet) against the development of some cancers like:

  • oral and digestive tract cancers;
  • lung cancer among those who have never smoked, not among smokers.

Tea polyphenols, the most active of which is epigallocatechin-3-gallate (EGCG), seem to act not only as antioxidants, but also as molecules that, directly, may influence gene expression and diverse metabolic pathways.

Green tea benefits in cardiovascular disease

Cardiovascular disease is the main cause of deaths worldwide, particularly in low- and middle-income countries, with an estimate of about 17 million deaths in 2008 that will increase up to 23.3 million by 2030.
Daily tea consumption, especially green tea, seems to be associated with a reduced risk of developing cardiovascular disease, such as hypertension and stroke.
Among the proposed mechanisms, the improved bioactivity of the endothelium-derived vasodilator nitric oxide (NO), due to the action of tea polyphenols that enhance nitric oxide synthesis, and/or decrease superoxide-mediated nitric oxide breakdown seem to be important.

Drinking a daily cup of tea will surely starve the apothecary.” Chinese proverb

Green tea benefits and antioxidant properties

Tea polyphenols may act, in vitro, as free radical scavengers.
Since radical damage plays a pivotal role in the development of many diseases such as atherosclerosis, rheumatoid arthritis, cancer, or in ischemia-reoxygenation injury, tea polyphenols, particularly green tea catechins, may have a preventive role.

Green tea benefits in weight loss and weight maintenance

Green tea, but also oolong tea, that is, catechins and caffeine rich teas, has a potential thermogenic effect. This has made them a potential tool for:

  • weight loss, by increasing energy expenditure and fat oxidation;
  • weight maintenance, ensuring a high energy expenditure during the maintenance of weight loss.

Indeed, it has been shown that green tea and green tea extracts are not an aid in weight loss and weight maintenance, since:

  • they are not able to induce a significant weight loss in overweight and obese adults;
  • they are not helpful in the maintenance of weight loss.

Green tea benefits in preventing dental decay

Animal and in vitro studies have shown that tea, and in particular its polyphenols, seems to possess:

  • antibacterial properties against pathogenic action of cariogenic bacteria, as Streptococcus mutans, particularly green tea EGCG;
  • inhibitory action on salivary and bacterial amylase (it seems that black tea thearubigins and theaflavins are more effective than green tea catechins);
  • it is able to inhibit acid production in the oral cavity.

All these properties make green tea and black tea, beverages with potential anticariogenic activity.

References

Polyphenols: definition, structure and classification

Polyphenols: contents in brief

What are polyphenols?

Polyphenols are one of the most important and certainly the most numerous among the groups of phytochemicals present in the plant kingdom.
Currently, over 8000 phenolic structures have been identified, of which more than 4000 belonging to the class of flavonoids, and several hundred occur in edible plants.
However, it is thought that the total content of polyphenols in plants is underestimated as many of the phenolic compounds present in fruits, vegetables and derivatives have not yet been identified, escaping the methods and techniques of analysis used, and the composition in polyphenols for most fruits and some varieties of cereals is not yet known.

They are present in many edible plants, both for men and animals, and it is thought to be their presence, along with that of other molecules such as carotenoids, vitamin C or vitamin E, the responsible for the healthy effects of fruits and vegetables.
In the human diet, they are the most abundant natural antioxidants, and the main sources are fruits, vegetables, whole grains, but also other types of foods and beverages derived from them, such as red wine, rich in resveratrol, the extra virgin olive oil, rich in hydroxytyrosol, chocolate or tea, in particularly green tea, rich in epigallocatechin gallate (EGCG).

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Chemical structure of polyphenols

Polyphenols: Phenolic Skeleton
Fig. 1 – Phenol

The term polyphenols refers to a wide variety of molecules that can be divided into many subclasses, subdivisions that can be made on the basis of their origin, biological function, or chemical structure.
Chemically, they are compounds with structural phenolic features, which can be associated with different organic acids and carbohydrates. In plants, the most part of them are linked to sugars, and therefore they are in the form of glycosides. Carbohydrates and organic acids can be bound in different positions on polyphenol skeletons.
Among polyphenols, there are simple molecules, such as phenolic acids, or complex structures such as condensed tannins, that are highly polymerized molecules.

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Classification of polyphenols

Polyphenols: Flavonoid Skeleton
Fig. 2 – Flavonoid Skeleton

They can be classified into different classes, according to the number of phenolic rings in their structure, the structural elements that bind these rings each others, and the substituents linked to the rings.
Therefore, two main groups can then be identified: the flavonoids group and the non-flavonoid group.
Flavonoids share a structure formed by two aromatic rings, indicated as A and B, linked together by three carbon atoms forming an oxygenated heterocycle, the C ring; they can be further subdivided into six main subclasses, as a function of the type of heterocycle (the C ring) that is involved:

Non-flavonoids can be subdivided into:

  • simple phenols
  • phenolic acids
  • benzoic aldehydes
  • hydrolyzable tannins
  • acetophenones and phenylacetic acids
  • hydroxycinnamic acids
  • coumarins
  • benzophenones
  • xanthones
  • stilbenes;
  • lignans
  • secoiridoids

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Variability of polyphenol content of plant and plant products

Polyphenols: Quercetin
Fig. 3 – Quercetin

Although several classes of phenolic molecules, such as quercetin (a flavonol), are present in most plant foods (tea, wine, cereals, legumes, fruits, fruit juices, etc.), other classes are found only in a particular type of food (e.g. flavanones in citrus, isoflavones in soya, phloridzin in apples, etc.).
However, it is common that different types of polyphenols are in the same product; for example, apples contain flavanols, chlorogenic acid, hydroxycinnamic acids, glycosides of phloretin, glycosides of quercetin and anthocyanins.
The polyphenol composition may also be influenced by other parameters such as environmental factors, the degree of ripeness at harvest time, household or industrial processing, storage, and plant variety. From currently available data, it seems that the fruits with the highest content of polyphenols are strawberries, lychees and grapes, and the vegetables are artichokes, parsley and brussels sprouts. Melons and avocados have the lowest concentrations.

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References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Han X., Shen T. and Lou H. Dietary polyphenols and their biological significance. Int J Mol Sci 2007;9:950-988. doi:10.3390/i8090950

Manach C., Scalbert A., Morand C., Rémésy C., and Jime´nez L. Polyphenols: food sources and bioavailability. Am J Clin Nutr 2004;79(5):727-47 [Abstract]

Tsao R. Chemistry and biochemistry of dietary polyphenols. Nutrients 2010;2:1231-1246. doi:10.3390/nu2121231


Human health and carotenoids

Benefits of carotenoids for human health

Carotenoids belong to the category of bioactive compounds taken up with diet, that is, molecules able to provide protection against many diseases such as cardiovascular diseases, cancer and macular degeneration. They are also important for the proper functioning of the immune system.
Among the mechanisms that seem to be at the basis of their human health-promoting effects have been reported (Olson, 1999, see References):

  • the capability to quench singlet oxygen (see above);
  • the scavenging of peroxyl radicals and reactive nitrogen species;
  • the modulation of carcinogen metabolism;
  • the inhibition of cell proliferation;
  • the enhancement of the immune response;
  • a filtering action of blue light;
  • the enhancement of cell differentiation;
  • stimulation of cell-to-cell communication

Carotenoids, antioxidant activity and human health

Human Health and Carotenoids
Fig. 1 – Free Radical

Carotenoids, with the adaptation of organisms to aerobic environment, and therefore to the presence of oxygen, have offered protection against oxidative damage from free radicals, particularly by singlet oxygen, a powerful oxidizing agent (see also below).
Carotenoids stabilize singlet oxygen acting both chemical and physical point of view:

  • chemical action involves the union between the two molecules;
  • in physical action, the radical transfers its excitation energy to the carotenoid. The result is a low energy free radical and an excited carotenoid; later, the energy acquired by the carotenoid is released as heat to the environment, and the molecule, that remains intact, is ready to carry out another cycle of stabilization of singlet oxygen, and so on.

The capability of carotenoids to quench singlet oxygen is due to the conjugated double-bond system present in the molecule, and the maximum protection is given by those molecules that have nine or more double bonds (moreover, the presence of oxygen in the molecule, as in xanthophylls, seems to have a role).
Carotenoids are involved not only in singlet oxygen quenching, but also in the scavenging of other reactive species both of oxygen, as peroxyl radicals (therefore contributing to the reduction of lipid peroxidation) and nitrogen. These reactive molecules are generated during the aerobic metabolism but also in the pathological processes.

Lycopene, xanthophylls and human health

Lycopene, a carotene, canthaxanthin and astaxanthin, two xanthophylls present in foods of animal origin, are better antioxidants than beta-carotene but also than zeaxanthin that, with lutein, is involved in prevention of age-related macular degeneration.
Lycopene, in addition to act on oxygen free radicals, acts as antioxidant also on the radicals of vitamin C and vitamin E, that are generated during the antioxidant processes in which these vitamins are involved, “repairing them”.
Finally, lycopene exerts its antioxidant action also indirectly, inducing the synthesis of enzymes involved in the protection against the action of oxygen free radicals and other electrophilic species; these enzymes are quinone reductase, glutathione S-transferase and superoxide dismutase (they are part of the enzymatic antioxidant system).

Vitamin A and human health

Human Health and Vitamin A
Fig. 2 – Provitamin A Activity

Vitamin A, whose deficiency affects annually more than 100 million children worldwide, causing more than a million deaths and half million cases of blindness, is a well-known carotenoid derivative with many biological actions, being essential for reproduction, growth, vision, immune function and general human health.
In the human diet, the major sources of vitamin A are the preformed vitamin, which is found in foods of animal origins (meat, milk, eggs, etc), and provitamin A carotenoids, present in fruits and vegetables. In economically deprived countries, fruits and vegetables are the main source of vitamin A being less expensive than food of animal origin.
Of the more than 750 different carotenoids identified in natural sources, only about 50 have provitamin A activity, and among these, beta-carotene (precisely, all-trans-beta-carotene isomer) is the main precursor of the vitamin A.
Among the other carotenoids precursors of vitamin A, alpha-carotene, gamma-carotene, beta-cryptoxanthin, alpha-cryptoxanthin, and beta-carotene-5,6-epoxide have about half the bioactivity of beta-carotene.
Spinach, carrots, pumpkins, sweet potatoes (yellow) are example of vegetables rich in beta-carotene and other provitamin A carotenoids.
Acyclic carotenes, such as lycopene (the main carotenoid in the human diet), and xanthophylls, except those mentioned above (beta-cryptoxanthin, alpha-cryptoxanthin, and beta-carotene-5,6-epoxide), cannot be converted to vitamin A.

References

de la Rosa L.A., Alvarez-Parrilla E., Gonzàlez-Aguilar G.A. Fruit and vegetable phytochemicals: chemistry, nutritional value, and stability. 1th Edition. Wiley J. & Sons, Inc., Publication, 2010

Johnson E.J. The role of carotenoids in human health. Nutr Clin Care 2002;5(2):56-65 [Abstract]

Olson, J.A. 1999. Carotenoids. p. 525-541. In: Shils M.E., Olson J.A., Shike M., Ross A.C. “Modern nutrition in health and disease” 9th ed., by Lippincott, Williams & Wilkins, 1999

Ross A.B., Thuy Vuong L., Ruckle J., Synal H.A., Schulze-König T., Wertz K., Rümbeli R., Liberman R.G., Skipper P.L., Tannenbaum S.R., Bourgeois A., Guy P.A., Enslen M., Nielsen I.L.F., Kochhar S., Richelle M., Fay L.B., and Williamson G. Lycopene bioavailability and metabolism in humans: an accelerator mass spectrometry study. Am J Clin Nutr 2011;93:1263-73 [Abstract]

 

Fruits and vegetables in season

Health benefits of seasonal fruits and vegetables

Numerous studies showed that seasonality plays a key role in optimizing the antioxidant properties of fruits and vegetables. For example, a recent Chinese study have investigated the influence of growing season (summer vs winter) on the synthesis and accumulation of phenolic compounds and antioxidant properties in five grape cultivars. The study showed that both phenolic compounds and antioxidant properties in the skin and seed of winter berries were significantly higher than those of summer berries for all of the cultivars investigated. Finally, to choose seasonal fruits and vegetables also ensures considerable saving of money.

List of fruits and vegetables in season

Fruits and Vegetables: Fruits in Season
Fig. 1 – Fruits in Season

 

 

 

 

 

Fruits and Vegetables: Vegetables in Season
Fig. 2 – Vegetables in Season

 

 

 

 

 

 

Xu C., Zhang Y., Zhu L., Huang Y., and Jiang Lu J. Influence of growing season on phenolic compounds and antioxidant properties of grape berries from vines Grown in Subtropical Climate. J Agric Food Chem 2011:59(4);1078–1086