Substrate-Mediated Raw Material Grade Affects Sensory Quality, Chemical Composition, and Fungal Community of Fu Brick Tea

Fu brick tea (FBT) develops its characteristic qualities through fermentation, yet how variation in the chemical composition of raw dark tea (RDT) is associated with microbial succession and final tea quality remains unclear. In this study, three grades of RDT (premium-grade (1M), first-grade (2M), and second-grade (3M)) were processed into FBT under identical conditions to examine the relationship between initial composition, microbial community structure, and sensory attributes. Results revealed that high-grade RDTs (1M) contained higher levels of water extracts (WE, 36.35 ± 0.14 (%), p < 0.05), total polyphenols (TP, 14.93 ± 0.19 (%), p < 0.05), and free amino acids (FAA, 2.90 ± 0.03 (%), p < 0.05), promoting Aspergillus (96.06% in C1M, compared with 66.43% in C2M and 55.01% in C3M) dominance and resulting in brighter liquor with enhanced body and smoothness. Correlation analyses demonstrated a coherent sequence from substrate composition to microbial assembly and then to quality-related chemistry. WE, TP, and FAA were positively correlated with Aspergillus abundance and body and smoothness (p < 0.05), whereas soluble sugars correlated with Rhodotorula and sweetness (p < 0.05). These findings support a substrate-mediated association framework in which the chemical composition of RDT is closely aligned with microbial community structure and sensory differentiation during FBT fermentation, providing a scientific basis for raw material grading and fermentation management in dark tea production.