Analysis of volatile compounds in large-leaf yellow tea during manufacturing processes using gas chromatography-ion mobility spectrometry

Abstract: Large-leaf yellow tea (LYT) is a classical beverage with a unique caramel taste similar to the fried rice flavor in Asian areas. LYT processing substantially determines the quality grade of tea, volatile composition, and aroma profile. However, the aroma property and volatile compounds of LYT still remain unclear during processing. This study aims to clarify the volatile compositions and variations in the processing of LYT using the gas chromatography-ion migration spectrometry (GC-IMS), coupled with principal component analysis (PCA). The reason was that the GC-IMS technology was characterized by fast, simple, efficient, and environmentally friendly. A total of 40 volatile compounds were identified, including alcohols, ketones, aldehydes, esters, and heterocyclic compounds. Among the identified volatile compounds, the main volatile compounds in LYT were heterocyclic compounds and aldehydes. Moreover, the relative numbers of heterocyclic compounds increased significantly with the process of tea after fixing treatment. Meanwhile, the relative content of aldehydes enhanced dramatically, whereas those of alcohols and ketones compounds decreased remarkably after primary roasting. In addition, the linalool oxide (trans-furanoid) with floral odor was only identified in the previous samples with primary roasted treatment, where the highest relative amount accounted for 21.98 % of the total found in fresh tea leaves (FTL). The 1-octen-3-one and acetophenone were only identified in tea samples after primary roasted processing. Furthermore, the differential profiles and chromatographic fingerprint of LYT during processing were established using the volatile compounds detected by GC-IMS technology. The characteristic chromatographic regions consisted of benzaldehyde, 2,5-dimethylfuran, furfurol and dimethyl disulfide in the full-fire processing of LYT. A PCA analysis was performed on the identified volatile compounds to further distinguish LYT processes, where the accumulated contribution rate of the first two principal components (PC 1 for 65.0 %, PC 2 for 15.5 %) was 80.5 %, indicating that the GC-IMS was useful to discriminate the LYT samples of different processes. The tea samples were separated according to the processes, where there was a far distance between FTL and LYT, indicating that the tea processing was vitally important to the formation of aroma characteristics of LYT. The findings can provide new insight into the determination of volatile compounds for better monitoring process and quality control of tea.