Carbohydrate conversion and microbial community restructuring following Saccharomyces cerevisiae inoculation during...

Microbial fermentation profoundly reshapes the carbohydrate matrix of ripened Pu-erh tea, yet the chemical mechanism underlying sweetness formation remains poorly defined. Here, we show that controlled inoculation with Saccharomyces cerevisiae at the turning stage of pile fermentation induces targeted cleavage of tea cell-wall polysaccharides and redirects carbon flux toward sweetness-active carbohydrates. Compared with naturally fermented controls, yeast-inoculated teas exhibited a marked decrease in cell-wall cellulose, concomitant increases in hemicellulose-, pectin- and lignin-derived fragments, and significantly elevated levels of glucose, fructose, galactose, trehalose, ribitol and arabitol. Integrated targeted and widely targeted metabolomics, together with metatranscriptomic CAZy annotation, revealed a yeast-specific activation of β-glucosidases, α-trehalases, β-1,3-glucanases and trehalose-6-phosphate synthase, accompanied by enhanced expression of hexokinase and phosphoglucomutase. This enzyme ensemble accelerates depolymerization of β-1,4-glucan and α-1,6-glycosidic linkages in cellulose and pectin side chains, liberating neutral sugars and polyols that chemically account for the enhanced sweetness, thickness and smooth mouthfeel of inoculated teas. We propose a yeast-driven polysaccharide-monosaccharide conversion mechanism that provides a molecular basis for carbohydrate transformation in Pu-erh tea and offers a general framework for modulating carbohydrate-derived flavor attributes in fermented plant-based foods.