Introduction Generally functional foods may be classified in

Introduction Generally, functional foods may be classified into three categories: food to which a component has been added, food in which a component has been modified in nature and/or bioavailability, and conventional food containing naturally occurring bioactive substance [1]. For the later class, tea from Camellia sinensis is one of the most important functional foods and has held the second most popular beverage in consumption among all beverages except water worldwide. Tea contains over 4,000 chemicals and some of which have health promoting properties [2]. Based on different degree of calcium sensing receptor during tea process, tea can be simplistically divided into three major types: green (unfermented) tea produced from fresh tea leaves and enzymatic oxidation is inhibited using steaming or pan-frying; oolong (partially-fermented) tea made by wilting fresh leaves by sun, then slightly bruising; and black (fully fermented) tea made by crushing tea leaves to release the polyphenol oxidase and peroxidase for fully catalyzing the enzymatic oxidation and polymerization of original tea catechins [3–6]. Many potential health promotion properties associated with these three types of tea consumption have been reported [7,8]. Comparing with green tea having only monomeric catechins, i.e. (−)-epicatechin (EC), (−)-epicatechin-3-gallate (ECG), (−)-epigallocatechin (EGC), and (−)-epigallocatechin-3-gallate (EGCG), fully fermented black tea and semi-fermented oolong tea contain a mixture of catechins and their oxidized polymeric substances such as theaflavins and thearubigins [9]. Comparison of catechin contents in different fermented and semi-fermented teas is illustrated in Table 1. Theaflavins (TFs) and thearubigins (TRs), main secondary polyphenols formed during fermentation process by enzymatic oxidation, have been extensively studied on bioactivities and formation mechanism. The orange red or brown color and astringent taste of black tea infusion is attributed to TFs as TRs contribute to rusty color and richness taste [4,10]. There are four major TFs in black tea and oolong tea, that is, theaflavin (TF1), theaflavin-3-gallate (TF2a), theaflavin-3′-gallate (TF2b), and theaflavin-3,3′-digallate (TF3) [6,11,12]. Chemical structures of various types of tea catechins and theaflavins are shown in Fig. 1. Among three types of tea, black tea is the most popular tea produced and consumed preferentially in the United State, England, and other Western countries with 78% of global market, followed by 20% of green tea consumed primarily in Asian and Northern African countries, and about 2% of oolong tea consumed mainly in Taiwan, southern China, and most Eastern countries [13,14]. It has been noted that production and consumption of oolong tea worldwide have increased over the past decades. For example, the production of oolong tea in China from 2000 to 2014 had been nearly doubled and increased from 67.6×103 to 254×103 metric tons [15]. Catechin prior to oolong tea is oxidized in the range of 10–80% during processing depending on the demand of customers [9,16]. The taste quality of oolong tea depends on several properties, such as smell of volatile fragrance, taste sensation of sweetness, umami, and intensity of astringency. The differentiation of green tea, black tea and oolong tea, regardless of degree of fermentation, is also depended on their contents of free amino acids, mainly -theanine and several natural amino acids including glutamic acid, asparagine, serine, alanine, leucine, and isoleucine [17]. Major contents of oolong tea infusion are listed in Table 2, which has two categories: monomeric polyphenols and polymeric substances. Oolong tea have been demonstrated to possess various pharmacological activities such as antioxidant activity by reducing oxidative stress, anti-cancer, anti-obesity, anti-diabetes, preventive effect of atherosclerosis, heart disease, hypertension, anti-allergic effect, and antiseptic effects [18–27]. However, all of the studies were applied to oolong tea extract, not single characteristic oolong tea polyphenols. Until 1984, a new group of polymeric oxidized flavan-3-ols was isolated and identified from oolong tea as theasinensins A, B, C, and later in 1988 for D, E, F and G, have been confirmed from oolong tea by the Japanese scientists Nonaka and Hashimoto [28–33]. As a matter of fact, this group of compounds was formerly discovered by Robert in 1958 [34,35] and was known as bisflavanol A, B, and C, which were formed by coupling of EGCG [36]. Based on the literatures listed above, theasinensins were implied as the bioactive flavonoids in oolong tea.