# Enhancing the Flavor Profile of Summer Green Tea via Fermentation with _Aspergillus niger_ RAF106
## 4 Discussion  
In general, the content of volatile compounds was found to be higher after fermentation compared to before, and the aroma characteristics exhibited a greater intensity post-fermentation. The content of alcohol, alkanes, and nitrogen oxides in tea showed an increase following fermentation, with the alcohol content reaching 54.80% after a 6-day fermentation period (Figure 3A). Linalool and oxidized linalool are among the most prominent aromatic compounds in tea, imparting a floral fragrance (Figure 3D, Table 1) [18]. The tea in question bear resemblance to renowned Chinese spring teas, such as Keemun black tea and Xihu Longjing tea [19; 20]. The alkane compound n-dodecane exhibited significant alterations in its content when subjected to the influence of _A. niger_ RAF106 (Figure 3D, Table 1). This characteristic facilitates the generation of a distinct _Lonicera japonica_ aroma in tea and enhances its freshness following fermentation [21]. The predominant nitrogen-oxygen compounds found in fermented tea are known as tea pyrroles, which contribute to the roasted aroma and nutty undertones in the tea's aromatic profile. Elevating the content of tea pyrroles enhances fragrance characteristic of fermented tea (Figure 3D, Table 1) [22]. Catechin, a crucial component responsible for the bitterness and astringency found in tea, is also an essential bioactive compound. The fermentation of _A. niger_ RAF106 results in a reduction in the content of ester-type catechins while also promoting an increase in the content of non-ester-type catechins and gallic acid (Table 2). During the process of fermentation, microorganisms demonstrate growth and metabolic activities that result in the release of various hydrolytic enzymes such as cellulase, amylase, tannase, and protease, among others. These enzymes aid in breaking down polyphenic compounds present in tea into smaller molecular entities. As a consequence, ester-type catechins undergo hydrolysis to produce non-ester-type catechins and gallic acid [23; 24]. Tea flavor quality greatly relies on the presence of organic acids, which also serve as crucial building blocks or transitional compounds in the development of various flavor components and aromatic substances. These factors significantly contribute to overall tea quality [25]. Following the fermentation of _A. niger_ RAF106, the content of lactic acid, acetic acid, and citric acid in tea exhibited a significant increase, with acetic acid and lactic acid being the most predominant acids (Table 2). These findings align with those observed in black tea following Ottoman fermentation [26]. Tea quality can be assessed by examining various factors such as tea polyphenols, total flavonoids, caffeine, free amino acids, and soluble sugars. Among these factors, tea polyphenols and caffeine are known for their contribution to the bitter and as stringent taste of tea. Additionally, flavonoids play a significant role in determining the color and flavor characteristics of tea infusion [27]. Free amino acids are primary contributors to the freshness and aroma of tea broth, while soluble sugars serve as the main source of sweetness in tea broth [28; 29]. The fermentation of _A. niger_ RAF106 did not have a significant impact on tea polyphenols. There was a slight variation in the caffeine content during the initial stages of fermentation, followed by a decrease observed on the sixth day (Table 2). Despite caffeine's chemical stability, it can be converted into theobromine or other theophylline through microbial enzymatic activity and under hot and humid environmental conditions. Additionally, caffeine can form compounds with polyphenols and acids to generate salts, resulting in a reduction in the caffeine content after fermentation [30; 31]. The total flavonoid content can be significantly enhanced within a short fermentation period by _A. niger_ RAF106, with the peak level achieved on the fourth day of fermentation (Table 2). This phenomenon could be attributed to the capacity of _A.