Sensorial properties vs chemical composition of green and black teas

INTRODUCTION The nature and quantity of chemicals contained in a cup of tea and its sensorial properties are due to a number of factors related to the characteristics of the product but also to the preparation method, that differs according to the varying cultures and traditions (type of water, use of teabag or loose-leaf, tea/water ratio, water temperature, steeping time). The effects of the different conditions of tea preparation on the its sensory attributes are scantly investigated, though preference and choice of consumers are mainly based on sensory quality [1]. The objectives of the present study were to determine the effects of the infusion time: (a) on the amount of the major catechins (EC, EGC, ECG, EGCG) and of the total polyphenols in green and black teas; (b) on the sensorial properties of teas by means of devices such as the electronic nose (e-nose) and the electronic tongue (e-tongue). The sensory quality of food is generally determined by a panel of tasters, which is an expensive method in terms of time and labour, and sometimes gives inaccurate or subjective results. The e-nose and e-tongue are fast, simple and non-destructive devices applied successfully in the food and pharmaceutical field. EXPERIMENTAL METHODS Materials and preparation of tea infusion Tea samples (2.5 g) were extracted in 250 mL of water with low mineral content, heated to the temperature reported in Table 1. The loose leaves were dipped for 3 and 5 min for green (G) and black (B) teas, respectively, and for the prolonged time of 10 min. After extraction, the infusions were filtered, then immediately analyzed. Table 1: Type of teas and brewing conditions. HPLC analysis of catechins An HP 1100 Chemstations (Agilent Technologies) equipped with a C18 column was used. A step gradient solvent system comprising of acetonitrile and 0.1% orthophosphoric acid in deionized water was employed (flow rate: 1 mL/min; wavelentgh: 210 nm; temperature: 35°C). Samples were analysed in triplicate. Chemical analysis Total polyphenols were determined in triplicate by the Folin-Ciocalteau method [2]. Electronic nose A portable electronic nose (PEN2-Win Muster Airsense Analytics Inc, Schwerin, Germany) was used. It consists of a sampling apparatus, a detector unit containing 10 Metal Oxide Sensors, and a pattern recognition software (Win Muster v.1.6) for data recording and elaboration. Electronic tongue Analyses were performed with the Taste-Sensing System SA 402B (Intelligent Sensor Technology Co., Ltd., Japan). The detecting part of the system consists of four detecting sensors whose surface is combined with artificial lipid membranes having different response properties to chemical substances on the basis of their taste. Statistical treatment of data Data were subjected to one-way analysis of variance (ANOVA) and comparison among means was determined according to Fisher’s least significant difference (LSD) test. Significant differences were accepted at p<0.05. E-nose and E-tongue data were statistically elaborated by Principal Component Analysis (PCA). RESULTS AND DISCUSSION Table 2 reports the total catechin and polyphenol content of the different teas. The statistical treatment of data showed significant differences among samples for all parameters. B teas had a mean content of polyphenols higher than G teas, which had a higher catechin concentration. For the prolonged infusion time a significantly higher amount of polyphenols and cathechins were extracted from G and B teas. Table 2 - Total content of catechins (CAT) and polyphenols (PP) in B and G teas (mean value±std dev., n=3) Fig. 1 shows the score plot and the loading plot of e-tongue data. In Fig.1a, the discriminative ability of e-tongue in distinguishing the tastes of G and B teas is shown. G teas were grouped at the left of the first principal component (PC1) and were discriminated from B teas, more dispersed along the PC1 and located in the lower part of the plane. The loading plot (Fig.1b) shows that the G teas were characterised by the astringency and aftertaste-astringency, while black teas were perceived as more bitter, sour and salty. As shown by the arrows in the e-tongue score plot, the prolonged extraction time (10 min) shifted the G samples on the second principal component (PC2) towards an increase of aftertaste-astringency; concerning B teas, the shift was along the PC1 in the direction of increased astringency sensation. Fig.1: E-tongue PCA-score plot (a) and loading plot (b) The e-nose score plot (Fig.2a) shows that G teas were grouped at the left of PC1 and were discriminated from B teas, more dispersed along PC1. From the loading plot (Fig.2b) it can be noticed that the volatile components of G teas were perceived by WC sensors (specific for aromatic and aliphatic compounds). These sensors are able to detect the vegetative tones of G teas. For B teas the aroma evolution towards fruity/floral/spicy characters was perceived by the WS sensors (characterized by a broad range sensitivity) and by WW sensors (sensitive to many terpenes). As shown by the arrows in the e-nose score plot, G and B teas infused for 10 min were shifted on PC1 and PC2 in the direction of increasing response of WS and WW sensors. Fig.2: E-nose PCA-score plot (a) and loading plot (b) CONCLUSION Slightly variations in the time of infusion greatly affected the chemical composition of teas and consequently their sensorial characteristics. These differences can be easily evaluated by using sensory instrumental devices such as e-tongue and e-nose, which demonstrated to be able to discriminate the different types of tea and to reveal changes related to the infusion conditions. REFERENCES: [1] Sharma V, et al., Food Chem., 93, 141-148 (2005). [2] Singleton VL et al., Am. J. Enol. Viticult.,16, 144-158 (1965).