Studies On Instrumental Assessment Of Pu-erh Tea Quality

Pu-erh tea is a unique microbial fermented tea produced using sun-dried leaves of large-leaf tea species(Camellia sinensis (Linn.) var. assamica (Masters) Kitamura) in Yunnan province of China and it has become increasingly popular in the Southeast Asia for its special quality properties as well as its potential health function, such as hypolipidemic and anti-obesity effects. The special sensory characteristics of Pu-erh tea arise from enormous chemical and biochemical changes and transformations of the constituents of sun-dried green tea leaves during the post-fermentation process in which high temperature, high humidity and microbial activities were essential. Therefore the chemical constituents of Pu-erh tea are more complicated than those of green tea, black tea and Oolong tea, leading to difficulty in studying. Few study was focused on the chemical quality of Pu-erh tea in the past, which has posed a major obstacle for recognizing and evaluating Pu-erh tea scientifically and is unfavorable to the Pu-erh tea industry. In this paper, the aroma constituents, the major taste compounds, mineral elements and biological activities of the representative products of Pu-erh tea were analyzed. Furthermore, the key constituents which are responsible for the special flavor and taste were explored based on above analysis combined sensory evaluation and some chemometrics methods including principal component analysis (PCA) and partial least square (PLS), and some models were constructed to predict the sensory quality as well as the content of some chemical constituents of Pu-erh tea and to discriminate the origins and grades of Pu-erh tea. Moreover, objective evaluation methods of Pu-erh tea quality were also studied using electronic tongue, electronic nose and near infrared spectrometry. The main results were listed as follows:1. Key constituents of Pu-crh tea aromaBased on the reference peak of1,2,3-trimethoxybenzene,26peaks in HS-SPME/GC-MS chromatograms were selected as the characterization peaks and the fingerprint was therefore constructed. The important indicator for distinguishing the origins of Pu-erh teas seemed to be benzaldehyde,3,4.5-trimethoxy-, butanoic acid,2-ethyl-,2-propenyl ester, benzene,1,2,3-trimethoxy-5-methyl-,3,4-dimethoxytoluene, citral and2-isopropyl-4-methylhex-2-enal, and alcohols compounds and methoxy-phenolic compounds were the important indicators for identifying the grade of Pu-erh tea. In addition, the important indicator for distinguishing the score of aroma quality of Pu-erh teas were found to be nerol, nerolidol, benzaldehyde,3,4,5-trimethoxy-. dimethyl itaconate, n-hexadecanoic acid,3,4-dimethoxytoluene and benzene,1,2,3-trimethoxy-5-methyl-, and on the basis of these10compounds, a PLS model with correlation coefficient of0.740and cross-validated correlation coefficient of0.480was constructed to predict the aroma quality of Pu-erh tea. Pu-erh tea samples with different aroma quality could be classified as several groups using eletronic nose technology, and the tea samples were regularly distributed to some extent in the diagram of PC A.2. Major taste constituents of Pu-erh teaThe major chemical constituents related to taste in Pu-erh tea, including tea polyphenols, catechins, flavonoids, water extracts, free amino acids, soluble sugar, tea pigments, alkaloid and organic acids, were determined. Pearson’s linear correlation analysis showed that seven chemical indicators, i.e., free amino acid, DL-C, GCG, tartaric acid, citric acid and total organic acid, were significantly correlated(p<0.05or p<0.01) to the score of taste in Pu-erh tea sensory evaluation, and PCA showed that catechins, free amino acids and organic acids were considered as the key determinant of taste quality of Pu-erh tea. On the basis of these7compounds, a PLS model with correlation coefficient of70.7%was constructed to predict the taste quality of Pu-erh tea. Pu-erh tea samples with different taste qualities could be classified in several groups in general using electronic tongue technology, and the samples produced in different manufacturer could also be well classified. However, different grades of Pu-erh tea samples, i.e., high, medium and low grade, has poor discrimination effects.3. Mineral elements of Pu-erh teaThe contents of sixteen mineral elements of Pu-erh tea and sun-dried green tea were determined by ICP, and their differences between different grades and between different producing areas were also compared. The contents obtained in mg kg-1for the elements analyzed in Pu-erh tea were16.68(B),3306.64(Ca),272.18(Fe),806.05(Al),14522.80(K),2054.96(Mg)682.09(Mn),0.25(Mo),487.71(Na),3628.98(P),2350.90(S),173.78(Si),61.77(Zn),2.48(Ni),0.15(Co) and undetectable (Hg). The contents of the Mn, Na, Zn, Ca, Fe, Al, K, Mg, Ni and Co in Pu-erh tea were observed to be much higher than those of sun-dried green tea (p<0.05). Significant differences in the content of Ca, Fe, Al, K, Mg, S, Ni and Co were observed between different producing areas of Pu-erh tea (p<0.05). However, there were no significant differences in the content of the studied elements except Ni in Pu-erh tea between different grades(p>0.05). Moreover, as far as some elements potentially dangerous to health were concerned, the ranges obtained for the detected elements in Pu-erh tea were80.2-151.6mg kg (fluoride),0.66-4.66mg kg-1(lead),14.8-19.3mg kg-1(copper),1.95-4.98mg kg-1(chromium),0.07-0.25mg kg-1(arsenic) and0.023-0.130mg kg-1(cadmium). The mean leached analytes concentrations in tea infusion were523.86μg L-1for fluoride,5.70μg L-1for lead,43.18μg L-1for copper,13.67μg L-1for chromium,0.43μg L-1for arsenic and0.17μg L-1for cadmium after the tea leaves were brewed twice with boil water in a ratio of1g/50ml for5minutes. The mean dissolving rates of fluoride, lead, copper, chromium, arsenic, and cadmium were45.8%,24.6%,26.2%,35.2%,30.8%and27.4%, respectively. Significant differences in the content of the studied elements except copper were observed between different Pu-erh tea’s habitats. Compressed Pu-erh tea had lower lead levels than loose tea. Based on a body weight of70kg consuming15g of Pu-erh tea daily or105g Pu-erh tea weekly per person, the intake of the studied elements were below the safe limits recommended by various authorities, suggesting that under the current dietary intake, there are no health risks to Pu-erh tea drinking consumers.4. Application of NIRS in the predication of sensory quality and chemical constituents of Pu-crh teaThis study tentatively established the quantitative analysis models of Pu-erh tea both for chemical constituents (free amino acids, tea polyphenols, theabrownins and water extract) and for sensory quality (liquor color, aroma, taste and infused leaf) by using near infrared spectroscopy combined with partial least squares (NIRS-PLS). Results showed that the model for predicting tea polyphenols had acquired a high fitting degree when the number of principal components was7with the values of Re, RMSEC, Rp and RMSEP of this model being93.52%,0.591,92.24%, and0.627, respectively. However, the prediction abilities of other seven models were dissatisfied.5. Biological activities of Pu-erh teaThe antioxidant activity of sun-dried green tea and three grades of Pu-erh tea, i. e. high, medium and low grade, were studied and compared. The results showed that sun-dried green tea had much higher antioxidant activity than Pu-erh tea (p<0.05), and the significant difference (p<0.05) was also shown in evaluation of the total antioxidant activity, scavenging hydroxyl free radical activity and scavenging DPPH free radical activity between high and low grades. Moreover, the results of high throughput screening showed that the receptor of PPARδ and FXR in biological cell could be effectively activated by Pu-erh tea ethyl acetate extract to some extent, which has provided a preliminary scientific basis and technological support for the hypolipidemic and anti-obesity effect of Pu-erh tea. However, the activation of Pu-erh tea ethyl acetate extract on PPARy and LXR was not significant.6. Comparison of three kinds of instrumental assessment of Pu-erh tea qualityIn general, results showed that the electronic tongue technology has better effect on the instrumental assessment of Pu-erh tea quality than the electronic tongue technology. However, near infrared spectrometry is not suitable for the analysis of Pu-erh tea quality.