# Enhancing the Flavor Profile of Summer Green Tea via Fermentation with _Aspergillus niger_ RAF106
## 1 Introduction  
In China, green tea is classified based on the seasons it is grown in: spring, summer, and autumn. It goes through a process of fixing, rolling, and drying without fermentation [1]. The production of SGT accounts for approximately half of total annual tea production; however, it exhibits a more pronounced bitter and stringent flavor compared to spring green tea [2]. The presence of this characteristic imposes limitations on the processing and utilization of SGT, resulting in substantial resource inefficiency. Catechins, including ester catechins and non-ester catechins, are the main bitter and asringent substances in green tea. The ester catechins, including (-)-epigallocatechin gallate (ECG) and (-)-epicatechin gallate (EGCG), exhibit the highest content, constituting approximately 75% of the total catechin content. Notably, SGT demonstrates the elevated content of ester catechins in comparison to spring tea [3, 4]. Ester catechins exhibit a higher degree of astringency and bitterness compared to non-ester catechins. Additionally, there is a positive correlation between the ratio of non-ester catechins/ester catechins and the sensory evaluation score of the tea broth flavor [5, 6]. Microbial fermentation is a conventional technique in which the combined action of microorganisms, increased temperature, and optimal moisture content triggers a series of biochemical reactions involving decomposition, oxidation, consolidation, structuralalteration, methylation, and glycosylation. As a result, this complex process gives rise to unique flavors and bioactive compounds [7]. Microbial fermented tea exhibits distinct organoleptic characteristics, including a vibrant infusion, unique aroma, natural sweetness, velvety taste, minimal bitterness, and astringency compared to non-microbially fermented tea [8]. The effects of fermentation can vary depending on the microbiota, including bacteria, yeast, and mold. In recent years, there has been a significant research focus on the bacterial fermentation of tea, particularly in relation to kombucha. The process of fermentation leads to changes in the sugar and acid content, resulting in a unique flavor profile. Additionally, the production of aromatic compounds enhances the aroma of the tea [9]. However, tea polyphenols have an inhibitory effect on the growth and reproduction of bacteria, resulting in the relatively prolonged fermentation period of bacteria, which lasts between 1 and 2 weeks. Yeast has often been used to ferment Pu-erh tea, which leads to an increase in tea polyphenols, theaflavins, and catechins while causing a decrease in amino acids, caffeine, flavonoids, thearubigin, and theaflavin [10]. _A. niger_ possesses a rich enzyme system, which is not inhibited by tea polyphenols. The fermentation process generates an enzyme system that triggers a cascade reaction, which can lead to the production of volatile and non-volatile metabolites that enhance the flavor and functional properties of tea. Metagenomics has recently facilitated the discovery of _A. niger_'s glycoside hydrolase (GH) genes, which play a pivotal role in pile fermentation and aroma production in dark tea [11]. After the fermentation of steamed green tea by _A. niger_ PW-2, the content of catechins significantly decreases, while the content of volatile compounds, including linalool, (-)-\(\alpha\)-terpineol, and geraiol, also decrease significantly [12]. Therefore, the different strains of _A. niger_ and variations in raw materials result in diverse fermentation outcomes. In our previous investigation, _A. niger_ RAF106, isolated from Pu-erh tea, biocontrol ester catechins of green tea polyphenols in the aqueous solution to non-ester catechins and gallic acid, improving the bioavailability of tea polyphenols and their free radical scavenging activity in vitro was comparable to that of the control group [13]. Additionally, SGT fermented with _A.