From waste to flavor enhancer: differential pressure explosion puffing modifies spent coffee grounds for microbial consortia-driven flavor synthesis in craft beer systems

The high lignocellulosic encapsulation of spent coffee grounds (SCG) severely restricts their substrate accessibility and microbial responsiveness. This study elucidated the regulatory mechanism by which differential pressure explosion puffing (DPEP) optimizes microbial-flavor metabolic networks in SCG beer through the structural remodeling of SCG. Results demonstrated that DPEP significantly enriched Klebsiella abundance, enhancing sugar-phenol synergistic metabolism to promote the synthesis of key flavor compounds, including butyl propionate (fruity), 4-hydroxycinnamic acid (bitter), D-Mannose (caramel), and Indanone (floral). In contrast, untreated groups exhibited Aeromonas and Enhydrobacter dominated proteolytic and alkane oxidation pathways, generating off-flavor compounds such as 2-heptanone (musty). DPEP created a three-tiered regulatory axis (substrate deconstruction-microbial adaptation-metabolic redirection) to transform SCG from a carbon‑nitrogen imbalanced matrix into a flavor-enhancing agent. This process synergistically enhanced the harmonious integration of "coffee-malt" richness and "floral-fruity" aftertaste in the brewed beer. Our findings provide a technical paradigm for flavor-oriented valorization of lignocellulosic waste, advancing sustainable strategies for biomass upcycling.