Monitoring volatile compound profiles and chemical compositions during the process of manufacturing semi-fermented oolong tea

SummaryThe manufacture of semi-fermented oolong tea is a skilled task requiring “sniffing indices” during processing. An experienced “sniffer” uses the appearance of grassy and floral fragrances to monitor the progress of fermentation. In the present work, semi-fermented oolong tea was sampled at 13 Stages of processing. At each stage, volatile compounds were collected by solid-phase micro-extraction and investigated by GC-MS analysis. It was found that C6 aldehydes remained at constant levels throughout fermentation. Those volatiles responsible for the fragrance of oolong tea increased up to the end of fermentation. Trans- β-ocimene, α-farnesene, and 3-hexenyl hexanoate reached peak levels before heat fixation (“panning”). The fruity volatiles were C10 or C12 esters. Significant variations in trans- β-ocimene, linalool (and its oxides), 1H-indole, and α-farnesene accompanied the appearance of the floral fragrance. There was a 14.7% decrease in total catechins between the fresh tea shoots (Stage 1) and th...