The effect of ascorbic acid on total antioxidant activity of black and green teas
Introduction
Tea is the most widely consumed beverage world-wide and is rich in polyphenolic compounds known as the tea flavonoids. Green tea, which is the favourite type consumed in Japan and China, has been increasingly used also in Western countries in recent years. Green tea is prepared from fresh tea leaves and contains mainly catechins. Black tea is popular in the West and India and it is rich in thearubigins and theaflavins, which are dimers of catechins, formed by enzymatic oxidation during the manufacture of black tea. Tea flavonoids (such as catechins, theaflavins and thearubigins) possess strong antioxidant properties, i.e. they may protect the body from damage caused by free radical-induced oxidative stress. Therefore, the consumption of tea has been associated with reduced risk of major diseases, including coronary heart disease, stroke and cancer (Benzie & Szeto, 1999; Langley-Evans, 2000; Leenen, Roodenburg, Tijburg, & Wiseman, 2000; Ramarathnam, Osawa, Ochi, & Kawakishi, 1995; Robinson, Maxwell, & Thorpe, 1997). The considerable antioxidant potential of tea has long been recognized and it is dependent on many factors involved in tea preparation. It has been shown that green tea extracts have higher antioxidant capacity than black tea solutions and the total antioxidant potential correlates strongly with the total phenolic contents of tea of all types (Benzie & Szeto, 1999; Henn & Stehle, 1998; Langley-Evans, 2000). The findings of the in vitro experiments suggest that maximum antioxidant capacity is associated with the drinking of green tea prepared at high temperatures (90 °C) and with long infusion times. Black tea should ideally be prepared, between 70 and 90 °C, from leaves rather than tea bags, and infusion times should not exceed 1–2 min to produce maximum antioxidant potential (Langley-Evans, 2000). Moreover, due to the flavonoid-binding capacity of milk proteins, the addition of milk can decrease the antioxidant potential of black tea preparations (Langley-Evans, 2000; Robinson et al., 1997). An increase of plasma antioxidant capacity was observed in humans following tea consumption without milk (Serafini, Ghiselli, & Ferro-Luzzi, 1996). It has been suggested that milk/polyphenol complexes resist gastric breakdown, rendering the polyphenols unavailable for absorption (Serafini et al., 1996). However, this is not consistent with more recent findings (Leenen et al., 2000; Van het Hof, Kivits, Weststrate, & Tijburg, 1998), which have shown that adding milk to black tea did not influence the absorption of tea catechins and antioxidant capacity in human plasma. The addition of milk to tea is common practice in the United Kingdom. In other countries it is popular to take tea with lemon. Lemon contains vitamin C (about 50 mg ascorbic acid/100 g), which also has antioxidative properties. Ascorbic acid, in vitro, protects some flavonoids, such anthocyanins, against oxidative degradation during processing and storage of juices (Kaack & Austed, 1998). Around 150 mg of pure ascorbic acid can provide the same antioxidant potential as one cup of green tea of usual strength (1.5%) (Benzie & Szeto, 1999). Studies in human volunteers showed intake of ascorbic acid and green tea consumption to have inhibiting effects on endogenous formation of N-nitroso compounds (Vermeer, Moonen, Dallinga, Kleinjans, & van Maanen, 1999). This indicates the importance of investigating the antioxidant potential of teas with addition of vitamin C. The present study has been designed to compare the antioxidant activity of different commercially available types of tea in vitro, prepared following the instructions provided on the package, and to evaluate the possible effects of different doses (5–40 mg/100 ml tea solution) of vitamin C (ascorbic acid) on the total antioxidant capacity (TAC) of the teas.
Section snippets
Chemicals
Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) and ABTS (2,2′-azino-bis-[3-ethylbenzthiazoline-6-sulfonic acid]), required for the measurement of TAC, were purchased from Fluka (Buchs, Switzerland) and Sigma-Aldrich (Vienna, Austria), respectively. l-ascorbic acid was obtained from Sigma-Aldrich (Vienna, Austria). All other chemicals were supplied by Riedel-de Haen (Seelze, Germany). The ascorbic acid – enzyme test was from Boehringer Mannheim (Darmstadt, Germany).
Tea samples
Seven
Antioxidant activity of commercially available black and green teas
The values of antioxidant activity of both black and green teas, prepared following the instructions provided on the package, were in similar ranges, 13.3–21.6 mmol TE in green tea and 10.4–17.6 mmol TE in black tea and showed only small SDs (see Table 1). The maximal antioxidant potential was measured in Java green tea and the lowest in Ceylon black tea leaf. Darjeeling black tea (Queen`s Blend), Assam Bherjan green tea and Green Blend (Sencha + Green Darjeeling) had nearly the same second
Discussion
For the green and black tea extracts investigated, the values of antioxidant activity were at a comparable level, which correlates with the results of Henn and Stehle (1998) and Licht, Böhm, and Bitsch (1997). The previously reported (Benzie & Szeto, 1999; Robinson et al., 1997) considerable variations in the antioxidant activities of different teas, were not observed in our study. The tea varieties on the Austrian market are probably different from those in other countries, e.g. England.
Conclusion
Following the packet instructions regarding tea preparation, the domestic way of preparation can contribute to the antioxidants needed to protect the body from degenerative diseases. Serafini et al., 1996, Serafini et al., 2000 reported that the consumption of tea produced an increase in total plasma antioxidant capacity. The addition of ascorbic acid to tea extract can improve the antioxidant potential of tea in vitro. Further studies are required to investigate whether the consumption of such
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