Two-Stage Fermentation Turns Spent Coffee Grounds into Yeast Biomass and Active Hydrogenases

Researchers have developed a two-stage fermentation process that converts spent coffee grounds into a usable feedstock, first growing yeast on hydrolyzed coffee waste and then using the leftover medium to cultivate Escherichia coli and generate catalytically active [NiFe]-hydrogenases.

Spent coffee grounds, an abundant byproduct of coffee production, were first processed by thermal and acid hydrolysis to release fermentable sugars. When the yeast Saccharomyces cerevisiae ATCC 13007 was grown on these hydrolysates at different substrate concentrations, acid hydrolysis clearly outperformed thermal treatment in supporting yeast growth. The team selected a non-diluted hydrolysate from 10% acid-hydrolyzed spent coffee grounds as the optimal substrate for the first fermentation stage.

Performance in that stage was strong. Within 24 hours, the yeast consumed essentially all of the available glucose, roughly 150 millimolar, and produced about 240 millimolar of ethanol, which the authors interpret as evidence of efficient fermentative metabolism and carbon conversion from the coffee waste into a valuable product.

The yeast cultivation also substantially changed the chemistry of the remaining liquid. Total carbon in the hydrolysate rose from 0.1 to between 4.4 and 5.0 grams per liter, and the carbon-to-nitrogen ratio shifted dramatically, from 1:7 in the initial hydrolysate to 16:1 in the cell-free supernatant, where yeast cells had been removed, and 4:1 in the lysed cell supernatant, produced by disrupting the yeast cells. These altered media then served as the growth substrate for the second stage of the process.

In the second stage, the researchers used these residual media to cultivate E. coli BW25113 and to analyze hydrogenase activity. The lysed cell supernatant, diluted twofold, gave the best results on both counts: the highest biomass formation, at 0.58 grams of cell dry weight per liter, and the highest hydrogenase activity, measured at 110 to 120 milliunits per milligram after dilution. The cell-free supernatant, by contrast, supported lower bacterial growth and lower enzyme activity. The authors attribute the superior performance of the lysed cell supernatant to its more favorable carbon-to-nitrogen ratio and to improved nutrient availability released by breaking open the yeast cells.

Importantly, the hydrogenase activity measured was hydrogen-oxidizing activity, which confirmed that the enzymes produced were catalytically active [NiFe]-hydrogenases. According to the authors, this demonstrates that the two-stage approach can generate functional hydrogenases with potential applications in bioenergy and aerobic biocatalysis, alongside the microbial biomass produced in the second stage.

Overall, the study presents yeast-pretreated spent coffee ground hydrolysate as a substrate that supports ethanol production, bacterial biomass formation, and sustainable hydrogenase generation within a single integrated workflow. The work was carried out on laboratory strains in controlled cultures, and the reported outcomes are specific to the strains, hydrolysate preparations and dilutions tested. The article was published as an open-access paper on 9 October 2026, and the version shared at publication is subject to further editorial edits before the final version of record appears.

Subject of Research: Bioconversion of spent coffee grounds via two-stage fermentation with Saccharomyces cerevisiae and Escherichia coli

Article Title: Bioconversion of coffee waste via two-stage fermentation using Saccharomyces cerevisiae and Escherichia coli

Article References: Patvakanyan, N., Shirvanyan, A., Vanyan, L., Vassilian, A., Yegizbay, K., Poladyan, A., Bekbayev, K., & Trchounian, K. (2026). Bioconversion of coffee waste via two-stage fermentation using Saccharomyces cerevisiae and Escherichia coli. AMB Express. https://doi.org/10.1186/s13568-026-02126-1

Image Credits: AI Generated

DOI: 10.1186/s13568-026-02126-1

Keywords: spent coffee grounds, bioconversion, two-stage fermentation, Saccharomyces cerevisiae, Escherichia coli, hydrogenase, ethanol, hydrolysate, waste valorization, biomass, bioenergy, AMB Express