Bioconversion of Spent Green Tea Residues via Gamma Irradiation and Yeast Fermentation: Effects on Ruminal Fermentation, Degradability, and Methane Emissions

Spent green tea residues are an abundant lignocellulosic by-product that poses environmental disposal concerns, while its use in ruminant nutrition is constrained by low ruminal degradability associated with high fiber and phenolic contents. This study investigated the effects of gamma irradiation and solid-state fermentation using Saccharomyces cerevisiae, and their combination, on chemical composition and ruminal fermentation. Gamma irradiation reduced neutral detergent fiber (379 to 336 mg g−1 dry matter), acid detergent fiber (277 to 244 mg g−1 dry matter), and total phenolics (89 to 75 mg gallic acid equivalents g−1 dry matter) while increasing non-fiber carbohydrates (234 to 269 mg g−1 dry matter) and ash (32 to 44 mg g−1 dry matter). These modifications enhanced the gas production rate (3.26 to 3.37% h−1), neutral detergent fiber degradability (38.9 to 46.1%), and the acetate-to-propionate ratio (2.25 to 2.43) while reducing ammonia nitrogen (108 to 96 mg L−1), crude protein degradability (55.1 to 48.8%), and increasing rumen exoglycanase activity by 11% and xylanase activity by 15%. However, the methane proportion in total gas increased from 12.4 to 13.3% and methane yield per unit of degraded dry matter increased from 36.5 to 39.2 mL g−1 degraded dry matter. Solid-state fermentation using Saccharomyces cerevisiae alone did not change the chemical composition or ruminal fermentation of non-irradiated biomass. However, when applied to irradiated biomass, it increased the crude protein from 320 to 383 mg g−1 dry matter and ether extract from 31 to 49 mg g−1 dry matter, improved dry matter degradability (53.3 to 63.1%), organic matter degradability (55.3 to 64.3%), crude protein degradability (48.8 to 54.6%), total volatile fatty acids (49 to 57 mmol L−1), and net energy for lactation (2.43 to 3.02 MJ kg−1 dry matter), and maintained ammonia nitrogen at levels similar to the irradiated biomass. Although the combined treatment increased the methane yield per unit of incubated dry matter (19.2 to 23.4 mL g−1 dry matter), the methane yield per unit of degraded dry matter was lower than that of the irradiated biomass alone (37.0 vs. 39.2 mL g−1 dry matter degraded) and comparable to the untreated control (36.5 mL g−1 dry matter degraded). Overall, the integration of gamma irradiation as a pretreatment followed by solid-state fermentation with Saccharomyces cerevisiae enhances the nutritional value of spent green tea residues, supporting its valorization as a sustainable alternative ruminant feed resource.