Effect Of Water Quality On Flavor Quality Of Longjing Tea Infusion And Its Mechanism

Water is the main carrier of tea flavor quality delivery and formation. Water quality influences the sensory quality of tea infusion. Though many people have been convinced of the importance of water quality for tea infusion, however, the mechanism for water quality affecting tea flavor quality is still unknown. And it is expected to suggest a standardized water for making tea infusion. It was practically proved that water quality is the important factor which leads to three problems of tea beverage, i.e. color change, sediment produce and taste change, also the affecting mechanism is still unclear.To solve the problems, the present study aims to analyze the effect of water quality on the sensory quality and pHysiochemical indexes of Longjing tea infusions by using different water samples with different quality to prepare tea infusion in order to clarify the key factors of the water quality which influence tea flavor and discuss how the key factors influence the flavor of the tea infusion. The main results were listed as follows:1. Eight kinds of water samples from 34 places, including city tap water (HZ), pure water (CJ), distilled water (ZL), mineral water (KW), springwater or normal drinking water (TR), natural mineral water (TK), soda water (SD) and mineral water containing gas (KQ) were analyzed. The results showed that, (1) The sensory quality, the pHysical and chemical properties, and the thermal stability of different water samples were very different, and the water samples with low concentration of ions, such as distilled water, pure water, native water, and natural spring water had better sensory quality; (2) Ca2+, Mg2+, K+ and Na+ were the main positive ions and the contents of these four ions were much different in various water samples, and also there were three negative ions Cl", NO3- and SO42- which showed a larger difference among the water samples; (3) The pH of the most water samples was between 4.0 and 8.0; (4) After heating treatment, the difference in sensory quality of different water samples diminished, the concentrations of ions changed, the pH significantly increased, and the electrical conductivity of most water samples decreased.2. The effect of water quality on the sensory quality and the main chemical components of Longjing tea infusion were analyzed. The results showed that, (1) The aroma quality of tea infusion gradually changed from clean aroma to tender flavor, nut aroma, and bad aroma with the increasing of total ions concentrations; (2) the taste quality also gradually changed from fresh and brisk to bad taste; (3) The concentrations of polyphenols, catechins, amino acids and organic acids were significantly influenced by the ions in the water, and the concentrations of EGCG and oxalic acid in the tea infusions prepared with natural mineral waters were lower 50% than those prepared with pure waters; (4) The ions Na+, Ca2+, Cl- and NO3- in tea infusion mainly came from the water, the Mg2+ and SO42-came partly from water, and there were obvious interactions between Ca2+, SO42-and other components in the infusion; (5) The pH of the tea infusion ranged from 6.0-7.0 which was narrow relative to the original water used for preparing the tea infusion.3. Correlations between ions/pH and tea taste and flavor was analyzed by adding the ions in tea infusion and changing the pH of tea infusion. The results showed that, (1) Ca2+, Mg2+, Na+ were the main positive ions in water which affected the flavor quality of tea infusion, and some negative ions could adjust the effectiveness of the positive ions. The high pH induced by HCO3- could intensively cause flavor change; (2) The water with pH 5.0-6.5 was beneficial to improve the flavor of green tea infusion, and lowering pH of the water with CO2 could improve the fresh degree and stability of taste quality, and the purity and thickness of aroma quality. Consequently, Ca2+, Mg2+, Na+ and pH were the main factors of water quality which influenced the flavor of tea infusion. The quality of Longjing tea infusion brewed with low salinity natural water is better, and the main water-quality indicators are:total ion concentration<50mg/L, in which Ca2+, Mg2+ concentration<5mg/L, and pH 5.5-6.8.4. Correlations between main taste components and main water quality factors were analysed and multiple simulation tests were performed to investigate the effect of the water quality factors on the taste of the tea infusion. The results showed that the main water quality factors could cuase a composition change of the taste compounds to change the taste characteristics of the tea infusion, which contributed for the difference in the taste of Longjing tea infusions prepared with different water. The key ions could cause the pH change of the water which affected the concentrations of polypHenols, organic acids and amino acids in the tea infusion. For example, Ca2+ and Mg2+ easily combined with polypHenols (catechins) which affected the taste of the tea infusion. Ca enhanced the astringent taste and lowered the bitter taste from EGCG, and weakened the umami and sweet taste from theanine, and the bitter and brisk taste from caffeine, and the sweet taste from sucrose, and strengthened blunt sense of various tastes. Moreover, the results showed that Ca2+ reinforced the combination between EGCG and protein to enhance the astringent taste of the tea infusion.5. Correlations between main aroma components and main water quality factors were analysed and multiple simulation tests were performed to investigate the effect of the water quality factors on the aroma of the tea infusion. The results showed that the delivery of aroma was influenced by the main water quality factors. The different composition of ions in the water affected the delivery of aroma compound. On the basis of the analytical data, Ca decreased the amount of dimethyl sulfide, benzaldehyde, beta cyclocitral, cis-3- hexenyl hexanoate and β-ionone, and increased the amount of linalool and nerolidol from the tea infusion. Mg2+ decreased the amount of benzaldehyde and beta cyclocitral, and Na+ decreased the amount of beta cyclocitral and limonene. Meanwhile, Ca2+ could changed aromatic characteristics of some compounds. For example, Ca2+ made dimethyl sulfide smell blunt and stuffy. EGCG, sucrose, theanine and caffeine influnced the volatility of aroma components. Clearly, EGCG increased the amount of dimethyl sulfide, benzaldehyde, caprylaldehyde, linalool, nonanal and (3-ionone, which are the key aroma components of green tea. And the thermal degradation of EGCG may cause the change of the aroma constituents, which improved the aroma of green tea infusion.The results in the present dissertation will provide some knowledge on the tea-flavor chemistry, processing technology of tea beverage and selection of the water used for tea infusion in daily life.