Comparative study of the volatile fingerprints of roasted and unroasted oolong tea by sensory profiling and HS-SPME-GC-MS

Abstract

Roasting plays important roles in shaping the volatile profile of oolong tea. In this study, the sensory attributes and volatile compositions of 153 roasted or unroasted oolong tea samples, belonging to four typical types, namely, High Mountain oolong tea (HMT), Tieguanyin tea (TGYT), Dongding oolong tea (DDT) and Wuyi rock tea (WRT), were studied in detail. Based on the sensory evaluation by tea evaluation experts, their respective sensory profiles were established and compared. Unroasted teas had more pronounced fresh and green flavors, while roasted teas had higher scores in pungent and caramel flavors. In particular, WRT demonstrated a unique fragrance of floral fruity flavors. By using HS-SPME-GC-MS analysis, a total of 128 compounds were identified across all samples. Notably, it was found that roasting largely increased the variety of volatile compounds in oolong tea. Furthermore, the characteristic volatile compounds of each type of tea were identified by PLS-DA modeling. Linalool and geraniol were the characteristic volatiles of HMT. Four volatiles, including (E)-nerolidol, jasmin lactone, benzeneacetaldehyde, and 4-methyl benzaldehyde oxime were identified as the characteristic volatiles of TGYT. Seven volatiles, including N-ethyl pyrrole, 3-(hydroxy methyl) pyridine, 4-pyridylcarbinol, 1-methyl pyrrole-2-carboxaldehyde, 2-ethyl-3,5-dimethyl pyrazine, 4-amino-2,3-xylenol, and 4,6-dimethyl pyrimidine were the characteristic volatiles of DDT. For WRT, 2,2,6-trimethyl cyclohexan-1-one, hexanoic acid, benzaldehyde, benzyl alcohol, β-cyclocitral, (E)-β-ionone, α-ionone, and octanoic acid were the characteristic volatiles. These findings expand our knowledge of the volatile fingerprints of oolong tea.

Keywords: Characteristic volatile compounds; HS-SPME-GC-MS; Oolong tea; Sensory profiling; Volatile compounds.

Graphical abstract

Fig. 1

Sensory description and scoring radar map of four types of oolong tea. Different letters of a–c indicate significantly different sensory scorings (P < 0.05).

Fig. 2

The number (A) and content (B) of chemical classes in oolong tea, including HMT, TGYT, DDT, and WRT. Different letters of a–c indicate significantly different numbers of chemical classes (P < 0.05).

Fig. 3

Principal component analysis (PCA) of the volatile compounds in tea leaf samples from the typical oolong tea.

Fig. 4

Comparison of the volatile compounds in HMT and other types of oolong tea by using PLS-DA analysis (A); volatile compounds with the top 10 highest VIP values in PLS-DA modeling (B); the concentrations of geraniol (C) and linalool (D) in the four types of oolong tea. Different letters of a–c indicate significantly different concentrations (P < 0.05).

Fig. 5

Comparison of the volatile compounds in TGYT and other types of oolong tea by using PLS-DA analysis (A); volatile compounds with the top 10 highest VIP values in PLS-DA modeling (B); the concentrations of (E)-nerolidol (C), jasmin lactone (D), benzeneacetaldehyde (E), and 4-methyl benzaldehyde oxime (F) in the four types of oolong tea. The concentration of 4-methyl benzaldehyde oxime was calculated based on 2-octanol. Different letters of a–c indicate significantly different concentrations (P < 0.05).

Fig. 6

Comparison of the volatile compounds in DDT and other types of oolong tea by using PLS-DA analysis (A); volatile compounds with the top 10 highest VIP values in PLS-DA modeling (B); the concentrations of N-ethyl pyrrole (C), 3-(hydroxy methyl) pyridine (D), 4-pyridylcarbinol (E), 1-methyl pyrrole-2-carboxaldehyde (F), 2-ethyl-3,5-dimethyl pyrazine (G), 4-amino-2,3-xylenol (H), and 4,6-dimethyl pyrimidine (I) in the four types of oolong tea. The concentration of N-ethyl pyrrole, 3-(hydroxy methyl) pyridine, 4-pyridylcarbinol, 1-methyl pyrrole-2-carboxaldehyde, 4-amino-2,3-xylenol, and 4,6-dimethyl pyrimidine were calculated based on 2-octanol. Different letters of a–c indicate significantly different concentrations (P < 0.05).

Fig. 7

Comparison of the volatile compounds in WRT and other types of oolong tea by using PLS-DA analysis (A); volatile compounds with the top 10 highest VIP values in PLS-DA modeling (B); the concentrations of 2,2,6-trimethyl cyclohexan-1-one (C), hexanoic acid (D), benzaldehyde (E), benzyl alcohol (F), β-cyclocitral (G), (E)-β-ionone (H), α-ionone (I), and octanoic acid (J) in the four types of oolong tea. The concentration of 2,2,6-trimethyl cyclohexan-1-one, hexanoic acid and octanoic acid were calculated based on 2-octanol. Different letters of a–c indicate significantly different concentrations (P < 0.05).