## 1 Introduction  
Dark tea, a type of post-fermentation tea that has increasingly become a popular and a preferred choice by consumers, is produced by microbial fermentation. Dark tea has distinct sensory characteristics, such as a unique stale, fungal flavor and a mellow, smooth taste; it also has weak astringency [1]. Furthermore, dark tea has multiple health benefits that are associated with its potent antioxidant, hepatotoxicity, hyperglyic and hyperlipidemic activities [2]. The microbial fermentation process is considered the main factor responsible for the formation of color, aroma and taste characteristics of dark tea. Catechins, which have an stringent taste, are rich in green teas and are oxidized during microbial fermentation. They contribute to the increase in pigments that cause a reddish-brown color of tea infusions. Theabrownins, the most abundant pigments in dark tea, can alter the gut microbiota and reduce hepatic cholesterol; they can also decrease lipogenesis by inhibiting the intestinal FXR-FGF15 signaling pathway [3]. Theabrownins have also been demonstrated to exhibit a strong umami taste [4]. Additionally, free amino acids also play a vital role in the taste of tea. The levels of most free amino acids are significantly reduced after fermentation [4, 5]. Aroma is also one of the most important criteria determining the quality and flavor of dark tea. The quantitative sensory description has been found to change from a \"green\" attribute to \"fungal flower\", \"flower\", \"mint\" and \"woody\" attributes during the manufacturing process of Fuzhuan brick tea [6]. The aromas of different types of dark teas can vary depending on the types of raw leaves and the species of microorganism [7]. _Aspergillus niger_, an important fungal species in biotechnology, is often used to produce various extracellular enzymes, such as amylase, aminopeptidase, pectinase, catalase, cellulase, esterase and lipase [8]. During the microbial fermentation process, _A. niger_ is one of the predominant microbes in dark tea that contribute to the formation of the unique quality of dark tea [2]. Specifically, _A. niger_ can produce high contents of theabrownins in dark tea and enhance the contents of volatiles including linaloid oxides, \(\beta\)-ionone, gerani and 9,12-octadecadienoic acid [7; 9]. The composition of flavonoids, glycerophospholipids, organo-oxygen compounds and fatty acids could also be altered during _A. niger_ fermentation. Meanwhile, enzymes produced by _A. niger_ could change the concentrations and compositions of metabolites in tea leaves during fermentation [10]. Dark tea is usually prepared from old, coarse and rough leaves of _Camellia sinensis_ var. _sinensis_ or _Camellia sinensis_ var. _assamica_, and the enzyme deactivation is conducted by pan-fired treatment [11; 12]. Steamed green tea with a needle shape is produced in Meitan County of Guizhou Province, China, using leaves of _Camellia sinensis_ cv. _Qianmei_ 601; the enzyme deactivation is conducted by steam treatment. Steamed green tea usually has a typical green note, which is distinct from the roasted taste of pan-fired green tea [13]. The typical green note generated as a result of the steam treatment may endow fermented tea with its characteristic flavor. However, research on the development of fermented steamed green tea is scarce. Therefore, _A. niger_ PW-2 was applied to produce of a novel, unique fermented loose tea using Guizhou steamed green tea as a raw material. The characteristic metabolites such as volatiles, catechins, pigments, amino acids and alkaloids, which are responsible for the flavor and taste of tea, were determined to reveal how the tea quality is improved by _A. niger_ PW-2 fermentation. To elucidate the mechanism by which fermentation affected the conversion of metabolites, proteomics technology was used to analyze enzymes produced by _A. niger_ PW-2 during different fermentation stages.