## 1 Introduction  
Mulberry leaves are a functional food, as well as traditional Chinese medicine [1]. The leaves or their extracts have pharmacological effects, such as hypoglycemic effects, hypolipid-demic effects, antioxidant effects, antiinflammatory effects, anticancer, and so on [2, 3]. The medicinal value of mulberry leaves lies in their multiple bioactive ingredients [2, 4]. Because of their various bioactive ingredients and pharmacological activities, mulberry leaves are made into mulberry leaf tea (MT) and are accepted by the public. Processing MT can increase its active ingredient content, enhance its function, and change its sensory characteristics. Fermentation is a widely used food processing method. Mulberry leaves fermentation by _Ganoderma lucidum_ can improve the content of 1-deoxynojirimycin to 0.402%, which is 1.2-fold of unfermented mulberry leaves [5]. Fermentation with different microbes of mulberry leaves increased the total phenolics and flavonoids contents, which, in turn, enhanced the antioxidant activities and hypoglycemic activities [6, 7]. Therefore, the fermentation of MT with microbes is an effective approach to improve its qualities. _Eurotium cristatum_ is a kind of probiotic fungus isolated from Chinese traditional post-fermented tea [8; 9; 10]. _E. cristatum_ secretes extracellular enzymes to promote a series of complex chemical reactions such as oxidation, condensation, and polymerization of ingredients in substances [10]. Nowadays, it has been widely used in the fields of tea, beans and grains processing, and deep processing of traditional Chinese medicine. Fermentation with _E. cristatum_ gave the substrates unique sensory qualities and enhanced their bioactivities [11; 12; 13; 14]. Several studies have focused on _E. cristatum_ fermentation of mulberry leaves. After solid-state fermentation of mulberry leaves with _E. cristatum_, the release of total flavonoids was enhanced, and 2,2\({}^{\prime}\)-azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) radical scavenging activities and \(\alpha\)-glucosidase inhibitory activities were increased [15]. [PERSON] et al. produced a kind of mulberry leaf Fu loose tea via _E. cristatum_ solid-state fermentation of mulberry leaf raw dark green tea. The mulberry leaf Fu loose tea showed higher contents of tea pigments and decreased contents of polyphenols, flavonoids, and polysaccharides. The fermented tea exhibited better \(\alpha\)-glucosidase inhibitory activity [16]. However, as a potential new tea product with health benefits, there is no research on the flavor characteristics and aroma substances of fermented mulberry leaf tea, which plays an important role in the evolution of tea qualities. The unique flavors, aroma compounds, and transformation mechanisms have been researched using multi-omics. The odor-active compounds of different aroma types (FBT) were detected by HS-SPME combined with GC-MSO [17]. The influence of pile-fermentation on the aroma characteristic of dark tea was studied by GC\(\times\)GC-QTOFMS combined with E-nose and GC-O techniques. The aroma compounds such as naphthalene, 2-methylnaphthalene, and dibenzofuran increased the woody flavor; at the same time, (Z)-4-heptenal, 2-nonenal, and 1-hexanol enhanced the mushroom, fatty, and sweet flavors [18]. Microbial fermentation imparted the unique flavor of Pu-erh tea, as the volatile compounds were detected by the GC-MS technique. The odor activity value was used to investigate the key aroma-active compounds and the flavor attributes of Pu-erh tea [19; 20; 21]. In this study, fermented mulberry leaf tea (FMT) was made by _E. cristatum_ solid-state fermentation of mulberry leaf tea, and the sensory qualities of MT and FMT were systematically analyzed in this study. The differences in taste and color properties of tea infusions between MT and FMT were analyzed by electronic tongue and colorimeter.