Objective Effects of shaking the tea leaves during withering in processing the floral-fruity Mingguancha Black Tea on sensory quality were determined and an evaluation indexing system proposed.

Method Either one-bud-3 or 4-leaves or two to three partially curled young shoots plucked from ‘Mingguancha’ tea bushes in the spring were processed without (CK) or with one, two, or three rounds of shaking under the unified control of light shaking intensity (drum speed: 8 r·min⁻¹) conditions during withering in the process of making the floral-fruity Mingguancha Black Tea. Sensory quality and main biochemicals with a widely targeted metabolomic profiling using the ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) combined with the headspace solid-phase microextraction/gas chromatography-tandem mass spectrometry (HS-SPME/GC-MS/MS) were determined on the prototype products.

Result Leaves-shaking during withering significantly impacted the sensory quality of the resulting tea, especially when it was carried out three times consecutively. The floral and fruity aroma with the sweet and mellow taste of the brewed tea became outstanding as a result. Multiple rounds of shaking significantly decreased the contents of water extracts and total free amino acids in the tea, but the soluble sugars and catechins bitterness/astringency index remained largely unchanged. The polyphenols, polyphenols/free amino acids ratio, caffeine, and catechins increased by shaking once, but decreased when more rounds of shaking took place. Based on the profile of non-volatile and volatile compounds in them, the experimental tea samples could be classified into four categories. There were 47 differential non-volatiles and 51 differential volatiles detected in all samples. Among the predominant non-volatiles, methyl jasmonate, jasmonic acid, and 2'-deoxyadenosine increased with repeated shakings, whereas the abundant volatiles including chlorogenic acid, luteolin-8-C-glucoside, and gallocatechin decreased. On the chemicals that contributed to the olfactory characteristics of the tea, 29 were esters, such as γ-hexalactone, δ-decalactone, hexyl hexanoate, and phenethyl isovalerate, in addition to indole, 2-methyl-4-phenyl-2-butanol, α-caryophyllene alcohol, etc. Aside from limonene, these compounds increased significantly with repeated shaking.

Conclusion Multiple rounds of shaking during tea-withering boosted aromatics formation in Mingguancha Black Tea. The floral and fruity note was attributed mainly to the esters and other volatiles contained in the tea. And the decline of phenolic substances and the accumulation of compounds like methyl jasmonate by the shaking might have given the brewed tea a sweet connotation and mellow taste.