The substrates of kombucha typically consist of tea and sugar. In this study, the effect of initial sugar concentration on volatile compound and sensory characteristics of raw Pu-erh tea (RAPT) kombucha was investigated. Compared to tea free (S1) and sugar free (S2) samples, the sugared tea (39 g/L sucrose in S3 and 78 g/L sucrose in S4) revealed better sensory quality and higher liking scores after the fermentation process. Hence, high-throughput sequencing analysis was performed to determine the variation in microbial composition between S3 and S4. The result showed that S4 exhibited higher abundances of Komagataeibacter and Brettanomyces as compared to S3. In addition, S4 presented the most favorable sensory qualities characterized by higher intensities of fruity, alcoholic, and fatty aromas, and the highest overall liking score. The metagenomic and metabolomic analysis was employed to further explore the metabolic pathways of RAPT kombucha under the optimal sugar concentration. The metagenomic and metabolomic analyses revealed that the pathways related to carbohydrate and amino acid metabolism were highly active under optimal sugar content, with compounds including glucose 6-phosphate, pyruvate and glutamate suggested to be important metabolites in regulating the sensory quality of the kombucha beverage. This paper provides a scientific basis for optimizing sugar addition in kombucha production.

Selenium (Se) biofortification is considered an effective approach to enhance the nutritional and functional properties of fungal polysaccharides. In this study, Pleurotus geesteranus was biofortified with different Se sources including sodium selenite [Se(IV)], sodium selenate [Se(VI)], potassium 2-selenocyanatoacetate (PSeCA), and selenium ore powder (SeOP) to obtain Se-enriched polysaccharides by ultrasound-assisted extraction (UAE). UAE parameters were optimized via a Box–Behnken design; the optimized UAE model exhibited high predictability (R² = 0.9886, CV = 1.71%), enabling reliable scale-up for industrial extraction, with the PSeCA group achieving the highest polysaccharide content (75.12 mg/g) and Se level (13.85 mg/kg) compared to the control and other Se-fortified groups. The monosaccharide composition analysis on polysaccharides revealed that Se(IV) and Se(VI) primarily increased mannose and fructose contents, whereas PSeCA and SeOP exhibited characteristic glucose-dominant profiles. Furthermore, the molecular weight (Mw) distribution of fungal polysaccharides could be altered under biofortification conditions. In addition, polysaccharides of each group revealed different antioxidant bioactivities among tested free radicals. The result indicated that PSeCA as an organic Se source, rarely studied, has promising potential in P. geesteranus biofortification for obtaining antioxidant Se-polysaccharides.