Antioxidant Capacity of Kenyan Tea

Processed tea is one of the most popular beverages. It is manufactured from the young tender leaves of the plant Camellia sinensis. Two types of tea products are most widely consumed; green and black tea.

In both cases, it is the chemical composition of the tea shoots and the reactions that occur during processing that determine the nature of the finished product and its quality. Though most of the tea produced in the world can be classified as non-aerated (green tea), partially aerated (oolong) tea and aerated black tea, processing has diversified to the production of specialty tea products such as white, flavored, organic, decaffeinated, herbal, scented and various other blends of tea. The manufacturing techniques of the above types of tea products, which may either be orthodox or non-orthodox, vary considerably and have a pronounced impact on the formative and degradative patterns of various biochemical components.
In the preparation of green tea, the tea leaves are steamed and then dried relatively rapidly after plucking to minimize chemical and enzymatic reactions. This stops the enzyme polyphenol oxidase [PPO] from oxidizing the tea leaf chemicals called catechins. In contrast, during black tea processing, tea shoots are macerated to initiate oxidation by PPO before firing. This reaction enables the catechins to form theaflavins (TFs) and thearubigins (TRs). TFs are homogenous substances, which give a yellow red coloration in fermented black tea and contribute to the briskness and brightness of tea liquor. TRs are responsible for the color, body and taste of tea. The manufacturing process for semi-fermented oolong tea consists of solar withering, panning, rolling and drying. Oolong tea is obtained by a partial oxidation of the leaf, an intermediate process between green and black tea manufacture. During this process, the characteristic floral aroma of oolong tea is produced. White tea is a rare specialty tea that gets its name from specific hairy tea cultivars, as well as a particular post-harvest processing method that raises small silvery trichome hairs on the dried buds. White tea contains a higher proportion of the buds that are covered with fine “silvery” hairs that impart a light white colour to the product. The brew from white tea is pale yellow in colour with no “grassy” undertones sometimes associated with green tea. True white tea is lightly fermented, rapidly steamed and dried leaving the leaves “fresh”. Unlike black, green and oolong teas, white tea is not rolled or crushed but it is steamed rapidly and air-dried to preserve most of the chemicals in the leaf. This unique processing produces a rare and expensive but highly refreshing drink. The differences between the various processes of tea manufacture result in differences in the biochemical profile between black, green, oolong and white tea.

 

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The chemical composition of tea is complex and includes polyphenols, alkaloids (caffeine, theophylline and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile compounds, minerals, trace elements and other unidentified compounds. Polyphenols however constitute the most interesting group and are the main bioactive molecules in tea. The major polyphenolic compounds are the catechins which include: (-)-epicatechin (EC), (-)-Epigallocatechin (EGC), (-)-epicatechin gallate (ECG), (-)-epigallocatechingallate (EGCG), (-)-Gallocatechins (GC) and (-)-gallocatechin gallate (GCG). Catechins are present in large amounts in green tea and account for 6-16% of the dry weight of tea leaves with EGCG constituting 10-50% of that proportion. TFs and TRs are another group of polyphenolic compounds found in both black and oolong teas.
Because of its polyphenols, tea has continued to be considered a ‘medicine’ since the ancient times. Research on the effects of tea on human health continues to be fuelled by the growing need to provide naturally healthy diets that include plant-derived polyphenols. In line with this, there is need to elucidate how known functional components in foods could expand the role of diet in disease prevention and treatment. There is already growing evidence that tea polyphenols have anti-heart disease activity in humans. Tea has also been shown to have anti-allergic action, anti-inflammatory and antimicrobial properties, potential anti-helmintic properties, antidiarrhoeal properties, antidiabetic activity and also antihyperglycaemic activity. In addition to many other potential health enhancing properties, studies have also shown that some catechins (EC and EGCG) inhibit glucoronidation and sulfonation of orally administered drugs thereby increasing the bioavailability of such drugs. Moreover, some epidemiological studies have associated consumption of tea with a lower risk of several types of cancer including those of the stomach, oral cavity, oesophagus and lungs. Tea however also contains caffeine, which is known for its stimulant effects. Caffeine causes mild addiction to some individuals, and can cause nervousness, anxiety, insomnia and headache.
The potential health benefits of tea have largely been ascribed to the antioxidant properties of its polyphenols. Tea polyphenols act as anti-oxidants by directly scavenging reactive oxygen species (free radicals) which are produced in the body. Free radicals are constantly generated in vivo due to exposure to environmental pollutants, radiation, chemicals, toxins, physical stress and the oxidation process of drugs and food. An imbalance between anti-oxidants and reactive oxygen species generated through the above factors results in oxidative stress leading to cellular damage and ultimately inflammation. The ability of tea polyphenols to scavenge for free radicals has been associated with teas’ therapeutic action against free radical mediated diseases thereby attracting tremendous research interest. Free radicals are known to contribute to numerous disorders in humans including cancer, atherosclerosis, arthritis, ischemia, central nervous system (CNS) injury, gastritis, dementia, renal disorders, Alzheimer’s disease, Parkinson’s disease, premature body ageing and Acquired Immune Deficiency Syndrome (AIDS). Many plant phenolics have been reputed to have antioxidant properties that are even much potent than vitamins E and C. In addition, currently available synthetic antioxidants like butylated hydroxyl anisole (BHA), butylated hydroxytoluene (BHT) and gallic acid esters have been suspected to cause or prompt negative health effects and hence the need to substitute them with naturally occurring antioxidants. Polyphenols which occur both in edible plants and foodstuffs derived from plants such as fruits, vegetables, wine and tea form a substantial constituent of the human diet and are thought to have the capacity to scavenge for the reactive oxygen species in the body.
Overally, there has been an increase in the quest to obtain naturally available antioxidants with broad-spectrum action. The upsurge of interest in the therapeutic potential of plants as sources of natural antioxidants has led to studies on the antioxidant potential of different types of tea products. Numerous such studies have been carried out by scientists of the Tea Research Foundation of Kenya (TRFK). In a study to evaluate antioxidant potency in different types of tea products, the TRFK established that overall, green and white teas’ had significantly higher antioxidant activity compared to black tea. In the study, there was no significant difference in the antioxidant capacity of black tea manufactured using orthodox and non-orthodox methods. Correlation analysis between tea polyphenols contents and the antioxidant activity of different types of tea products revealed that total catechins significantly correlated with antioxidant activity. EGCG was identified as the most potent antioxidant. EC, EGC, +C and ECG contents also showed significant influence on the antioxidant activity. Therefore, the antioxidant activity was higher in tea extracts containing high levels of EGCG, EC, EGC, +C and ECG.
Results from studies carried out in Kenya and elsewhere have also revealed that despite their lower antioxidant activity compared to green and white teas, black teas do also exhibit some antioxidant activity (see African Journal of Biotechnology,  Vol. 6 (19), pp. 2287-2296, 4 October 2007). During black tea manufacture, the catechins are oxidized and dimerised to TFs and TRs because of their high oxidation potential and high concentration in the leaves. TFs and TRs have also been shown to contribute significantly to the radical scavenging activity in black tea. Interestingly, TFs, which are the major phenolic product in black tea, have a higher radical scavenging activity compared to some of its precursors ECG, EGC and EC. This observation confirmed that conversion of catechins to TFs during black tea processing did not affect the radical scavenging potency of black tea. Additionally, studies carried out by the TRFK provided evidence of the contribution of TRs towards the antioxidant activity of black tea. The TRFK scientists also established that Oolong (semi-fermented) tea which is intermediate between green and black tea has an antioxidant activity that is intermediate of that of green and black tea. This type of tea does not contain high levels of the major anti-oxidative catechins and also does not yet contain a great amount of theaflavins and thearubigins which are found in fully fermented black tea.
Because different tea clones or cultivars have different levels and profiles of polyphenols, antioxidant activity will vary between tea products derived from different cultivars. The TRFK scientists have carried out a comparative study of the antioxidant activity of the Kenyan commercial teas with those of Japan and China to determine the effect of the variety from which the tea products were processed from on antioxidant activity. This study revealed that Kenyan tea products both green and black were high in antioxidant activity due to their high levels of total polyphenols (Figure 1). The high polyphenol content in Kenyan tea products was not entirely unexpected since the tea breeding programme in Kenya has indirectly and consistently selected germplasm for high total polyphenol content to produce black teas with high levels of TFs and TRs. Indeed, a previous study had confirmed the superiority of Kenyan tea germplasm in its total polyphenol content (see Tea, Vol. 25, pp. 81-89, 2005). Like most black tea producing countries, the predominant cultivars grown in Kenya are of assamica type or are assam-china hybrids. In contrast, tea germplasm from Japan and China that is traditionally used for green tea manufacture is of sinensis type and is selected to be low in astringency and bitterness and consequently low in total polyphenols. The TRFK scientists also compared the antioxidant activity of tea and popularly consumed vegetables such as spinach and onion and established that the antioxidant activity of tea was significantly higher than that of the fresh unprocessed vegetables which demonstrated the potency of tea as a potentially health enhancing food. Despite the increasing studies on the antioxidant property of black, green and oolong tea, limited information has been available on white tea. In a study carried out in Kenya, the antioxidant capacity of white tea was shown to be similar to that of green tea. This can be attributed to the high levels of EGCG, which is the most potent antioxidant, and ECG that is present in large amounts in the young fresh leaves or the hairy buds traditionally used in the manufacture of this rare specialty type of tea.
Whereas the antioxidant activity of tea has largely been ascribed to catechins and their oxidation products, recent studies in Kenya have indicated that this activity could as well be due to other rare leaf chemicals such as anthocyanins in some novel purple pigmented tea cultivars. Such purple leaf coloured cultivars have recently been developed in Kenya. Analysis of the biochemical composition of the Kenyan purple coloured tea leaf has revealed that these cultivars have anthocyanins and anthocyanidins; Cyanidin-3-O-glucoside, Cyanidin-3-O-galactoside, Delphinidin, Cyanidin, Pelargonidin, Peonidin and Malvidin (Food Chemistry, 2011 in press). These chemicals are also found in a number of fruits including grapes used for making red wine. The TRFK scientists established the anthocyanins to be highly effective radical scavengers in vitro and in vivo. Because the profile and levels of polyphenols in plants is greatly influenced by the environmental conditions, the antioxidant potency of a tea product may also vary with cropping season and region of production due to environmental influences on the biochemical composition of the tea leaf and it will be important that such changes be tracked in Kenyan tea. However, unlike for the producers that make their tea from leaf grown far away from the equator where there are significant seasonal variations in weather, it can be assumed that Kenyan teas which are all produced near the equator will be consistent in their biochemical make up and antioxidant capacity.

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Figure 1: Antioxidant capacity (%) and content of EGCG (%) in different types of tea.