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The yellowing process is the crucial step to form the characteristic sensory and chemical properties of yellow tea. To investigate the chemical changes and the associations with sensory traits during yellowing, yellow teas with different yellowing times (0–13 h) were prepared for sensory evaluation and chemical analysis. The intensities of umami and green-tea aroma were reduced whereas sweet taste, mellow taste and sweet aroma were increased under long-term yellowing treatment. A total of 230 chemical constituents were determined, among which 25 non-volatiles and 42 volatiles were the key chemical contributors to sensory traits based on orthogonal partial least squares discrimination analysis (OPLS-DA), multiple factor analysis (MFA) and multidimensional alignment (MDA) analysis. The decrease in catechins, flavonol glycosides and caffeine and the increase in certain amino acids contributed to the elevated sweet taste and mellow taste. The sweet, woody and herbal odorants and the fermented and fatty odorants were the key contributors to the characteristic sensory feature of yellow tea with sweet aroma and over-oxidation aroma, including 7 ketones, 5 alcohols, 1 aldehyde, 5 acids, 4 esters, 5 hydrocarbons, 1 phenolic compound and 1 sulfocompound. This study reveals the sensory trait-related chemical changes in the yellowing process of tea, which provides a theoretical basis for the optimization of the yellowing process and quality control of yellow tea.

Highly stable nanopores with precise size and film thickness are essential for ultrasensitive molecular biosensors and efficient ion filters. However, the stability of both solid-state nanopores and biological nanopores is of a few hours. Even for two-dimensional (2D) materials with excellent properties, it is very difficult to prepare nanopores with stability for more than 1 day. Here, in mechanically exfoliated 2D materials with excellent quality, highly stable nanopores have been fabricated. Through the manual control of defect density and laser spot irradiation, the breakdown process is more controllable. The nanopore diameter can be precisely adjusted from 1 to 10 nm. The 2 nm thick graphene nanopore (with a diameter of 2 nm) maintains good consistency in conductivity during the 1-month immersion in KCl (1 M) solution. The pore size changes less than 0.7% per day. We found that 2D material nanopores can produce a strong rectification ratio (~50) due to the morphology undulation. In addition, our devices can recognize modified 5 nm Au nanoparticles, showing great potential for single-molecule detection. This work can guide the development of high-quality ion rectifier devices and biosensors based on 2D material nanopores. [ABSTRACT FROM AUTHOR]