Agarwood leaves tea

Tea is the second most consumed drink after water. The tradition of drinking tea has been around in most parts of the world for the past 2000 years (Cabrera et al., 2006). Tea is often associated with pleasant aroma and good taste, acts as a refreshing beverage, and provides health benefits. Generally, tea is produced from the young leaves and leaf buds of Camellia sinensis, a cultivated evergreen plant belonging to the family Theaceae. Originating from China, the plant is now widely cultivated in countries/regions such as India, Indonesia, Japan, Malaysia, Sri Lanka, Taiwan, and Central Africa, due to favorable local conditions such as having high humidity, fair temperature, and acidic soil (Kuo et al., 2005).

Tea is categorized by the differences in tea plant variety, type of tea, grade, market name, and location of the tea production. There are various kinds of tea in the world, such as green, black, white, ‘oolong’, and ‘pu’erh’, which are categorised based on the process by which they were manufactured. For instance, green tea and white tea are unfermented and slightly fermented teas that have undergone a minimal process where no oxidation occurs; ‘oolong’ tea is a semi-fermented tea, which has been subjected to partial oxidation; and black tea is a fully fermented tea that has undergone complete oxidation. Meanwhile, the processing of ‘pu’erh’ tea is very different from the other types of tea because it is subjected to microbial fermentation by Aspergillus niger (Sharangi, 2009). In fermented teas, the color of the tea leaf turns brown due to the action of leaf oxidizing enzymes that alter the tannins and catechins in the leaves (Gupta et al., 2008). One type of tea produced from C. sinensis known as ‘kombucha’, is made through the fermentation of tea and sugar by a symbiotic association of bacteria and yeasts forming a ‘tea fungus’. It tastes like a sparkling apple cider that is slightly acidic but has sweet flavor (Jayabalan et al., 2007). The level of acceptance for ‘kombucha’ tea has increased across the world like many other traditional beverages, due to its beneficial effects on human health and ease of preparation (Jayabalan et al., 2014).

The worldwide arithmetic mean per capita consumption of tea is 120 ml brewed tea per day (Cabrera et al., 2006). Approximately 77% of the tea produced and consumed is black tea, the most popular drink in Western countries; 21% is green tea and is mainly consumed in Asian countries like China, Japan, Korea and Thailand; and less than 2% is ‘oolong’ tea, which is most popular in China and Taiwan (Lee, 2009). China is famous for its tea traditions and is referred to as the ‘homeland of tea’. Meanwhile, green tea is preferred by the Japanese and they usually serve ‘sencha’ green tea to visitors at home or at the workplace (Surak, 2012). In England, the tradition of enjoying afternoon or late-afternoon tea is increasing (Rose, 2010). In traditional medicine, teas treat insomnia, give a calming effect, break down oil and fats, relieve joint pains, improve digestion, blood circulation, and urine flow, speed up bowel evacuation, and serve as a detoxification agent (Gupta et al., 2008).

Tea composition differs by species, plant variety, season, climate, leaf age, plucking position and horticultural practices (Kuo et al., 2005). Chemical constituents of tea are mainly polysaccharides, amino acids, sterols, polyphenols, vitamins, minerals, proteins, triterpenoids, organic acids and volatile compounds (Chen et al., 2008, Xiao et al., 2011). Tea also contains caffeine (1–5%) and some amounts of other xanthine alkaloids and fats (4–16%). Green tea, when compared to black tea, contains higher amounts of tannins or phenolic substances (5–27%) consisting of catechin (flavanol) and gallic acid (Wang and Ho, 2009). Nowadays, tea is becoming more popular as it provides health-promoting effects to humans. For instance, polysaccharides in tea possess several pharmacological effects such as antioxidant, antiviral, hepatoprotective, immune stimulating, antitumour, anti-obesity and antidiabetic activities (Chen et al., 2016). Catechin and the flavins are two examples of major active polyphenols in tea. Polyphenols in tea are known to have fungal inhibitory (Sitheeque et al., 2009) and anti-inflammatory properties (Cao et al., 2007). Polyphenols also possess anti-mutagenic activity as shown from their inhibitory effect on spontaneous mutations in Salmonella typhimurium TA100 strain (Zhao et al., 2014). In addition, polyphenols have anti-oxidative effect as demonstrated from the DPPH radical scavenging activity on a dose-dependent manner of tea extract (Cao et al., 2007), anti-carcinogenic effect as shown from the inhibition of the development of cancer for oral, esophageal, stomach, intestinal, colon, skin, liver, bladder, prostate, and breast cancer in in vivo studies (Yang et al., 2009), antitumor activity against SKOV-3 cells (Fan et al., 2011), lowering of plasma cholesterol and triglyceride levels as well as reduction of blood pressure and platelet aggregation in several systems (Bursill et al., 2007). Studies on ‘kombucha’ tea demonstrated that it has antibiotic properties, intestinal and glandular activities, regulation of gastric ailments, relief of joint rheumatism, gout and hemorrhoids, positive influence on the cholesterol level, arteriosclerosis, toxin excretion and blood cleansing, diabetes, nervousness, and aging problems (Jayabalan et al., 2007). In vitro studies proved that hibiscus tea, or commonly known as roselle tea, can reduce high blood pressure in pre- and mild-hypertensive adults due to the existence of major flavonoid components, delphinidin-3-sambubioside and cyanidin-3-sambubioside (McKay et al., 2010). Meanwhile, peppermint tea possesses significant antimicrobial and antiviral activities, strong antioxidant and antitumor actions, and some antiallergenic potential. Despite the numerous health benefits, herbal tea like any other herbal product needs to be consumed in a safe amount since depending on the plant used side effects may be experienced especially if taken in high doses. An allergic reaction to chamomile for example is not unusual. Hence, a thorough consideration on the effects of tea will be required for better understanding of its toxic effects on humans (Jain et al., 2013).

Due to the high demand for tea from people who are trying to look for new ways to improve their health rather than depending on modern medicine, tea production has been increasing, and includes several other kinds of plant materials besides C. sinensis. For instance, the leaves of Mentha piperita (peppermint), Hibiscus sabdariffa (roselle), Gingko biloba (gingko), Orthosiphon aristatus (‘misai kucing’), Psidium guajava (guava), Cymbopogon citratus (lemon grass), Momordica charantia (bitter gourd), Ficus deltoidea (‘mas cotek’), flowers of Chrysanthemum morifolium, Matricaria chamomilla (chamomile), Jasminum sambac (jasmine) and many more have been used in tea preparation (Chan et al., 2010). In the last ten years, a new source of plant has emerged. Tea is now also being derived from Aquilaria leaves and the consumption is growing rapidly, due to the popularity of agarwood, the main product from Aquilaria tree. Here, we review the chemical composition and pharmacological effects of Aquilaria leaves, which are commonly known as agarwood tea.

The emergence of agarwood tea