Effects of the leaf-spreading process on the quality of white tea during the withering of fresh tea shoots

This study aims to investigate the effect of the leaf spreading process on the flavor quality and biochemical components of white tea during withering. The static withering on bamboo sieves was used with 0.5 kg fresh leaves per sieve, and the diameter of the sieve is 90 cm. The indoor air conditions were preset at 20-22 ℃, and 55%-65% relative humidity (RH) as the control (Immobility). Tea samples were acquired by thin or thick piling-up, namely Bingshai or Duiqing when the weight loss rate of fresh tea shoots reached 65%, 70%, and 75%. The results showed that there were the similar sensory qualities in all tea samples. However, the freshness of the aroma decreased, and the sweetness of taste increased in the white tea samples with Duiqing, compared with the Immobility and Bingshai. The quality of tea samples with the late Bingshai (4 to 8 sieves of work in process (WIP) merged into 1 sieve when the weight loss rate arrived at 70%) was slightly better than those with the early Bingshai (2 to 6 sieves of WIP merged into 1 sieve when the weight loss rate reached 65%). Furthermore, the best sensory qualities of the tea samples were produced from the thinnest Bingshai than Immobility. The best quality white tea sample was acquired with the thickest Duiqing (6 sieves of WIP merged into 1 basket) when treated for the same length of time (2 days) and different thicknesses (2 to 6 sieves of WIP merged into 1 bamboo basket: diameter (Φ) = 46 cm, height (H) = 72 cm) of Duiqing at the weight loss rate of 75%. Additionally, the WIP was preferred for the shortest duration (2 days), if treated with a different length of time (2 to 6 days) at such thickness of Duiqing. Although no regular changes were seen in the contents of catechins, alkaloids, and the main free amino acids among different treatments, the levels of proline (sweet) and some floral fruit components (phenylethanol, nerol, and geraniol) were significantly reduced in the white tea samples from the Duiqing process group, compared with the Immobility/Bingshai process group. The biochemical pattern recognition showed that all white tea samples were divided into two groups, i.e., Immobility/Bingshai and Duiqing process groups. The Duiqing process effectively regulated the biochemical compositions in the white tea samples, with an increase in the content of fatty acids, alkanes, norisoprenoids, and aldehydes. The 23 chemical compounds (such as nerol, citral, and vanillin) were considered the most important markers to distinguish them from each other. These findings can provide a strong reference to regulating the flavor quality of white tea using leaf spreading.