Synergistic effects of ultrasound and yeast co-fermentation on microstructural, metabolic, and sensory properties of Robusta coffee.

The study investigated an approach combining ultrasound with co-fermentation of P. fermentans and K. marxianus to improve the microstructural, metabolic, and sensory characteristics of Robusta coffee. Ultrasound treatment at 100, 200, and 300 W modified the surface microstructure of green coffee beans. Among them, 200 W exhibited the most pronounced promotion of yeast growth during fermentation (FU2). Compared with the control, the combined ultrasound and fermentation treatment increased total flavonoid and polyphenol contents while reducing caffeine and chlorogenic acid contents. In cupping tests, FU2 received the highest score (82.16 ± 0.58), noted for roasted, cocoa, floral, and honey aromas. Headspace solid-phase microextraction/gas chromatography-mass spectrometry (HS-SPME/GC-MS) analysis showed reduced undesirable volatile compounds (VCs) (pyrazines, pyridines and phenolic volatiles) and increased furans, alcohols, and esters in fermented samples. Metabolomics revealed significant alterations in metabolite profiles after ultrasound treatment. These metabolic changes were further regulated during fermentation, accompanied by significant enrichment of flavor precursor pathways, including phenylalanine metabolism, pyruvate metabolism, and the citrate cycle. These findings provide a potential strategy for improving the sensory quality and value-added utilization of Robusta coffee.