Biotechnological Valorization of Almond Hulls via Solid-State Fermentation with <i>Saccharomyces cerevisiae</i> and Fibrolytic Enzyme Supplementation: Enhancing Ruminal Fermentation and Reducing Greenhouse Gas Emissions

Valorization of agricultural by-products is a key component of circular strategies aimed at enhancing the sustainability of livestock systems. Almond hulls (AHs), a major residue of the almond-processing industry, are characterized by their high non-fiber carbohydrate (NFC) content, but low crude protein (CP) content and ruminal fermentation. This study evaluated the effects of treating AHs with exogenous fibrolytic enzymes (EFEs) and <i>Saccharomyces cerevisiae</i> (SC) via solid-state fermentation. Treatments were applied individually or in combination (SC + EFEs). The effects on chemical composition and ruminal fermentation were assessed. EFEs reduced the fiber content and increased the NFC content. This accelerated ruminal fermentation and reduced the lag time. However, it did not change the overall fermentation extent. SC increased the CP content and ether extract but reduced the NFC content. This modification promoted the growth of ruminal bacteria. As a result, the ruminal fermentation extent, ruminal degradability and volatile fatty acid (VFA) content improved. However, methane (CH₄) and carbon dioxide (CO₂) emissions relative to the substrate, degraded substrate and total gas emission were not affected. SC + EFEs had synergistic effects. This further increased the CP content and ether extract and reduced the NFC and fiber contents. The treatment modulated ruminal microbiota by decreasing protozoa and increasing bacteria. It also reduced the fermentation lag time and enhanced the fermentation extent, degradability and VFA production favoring propionate formation. Additionally, it reduced CH₄ and CO₂ emissions per unit of degraded substrate and the total gas emission. Overall, the SC + EFEs represent an effective approach to enhance the nutritional value of AHs while partially mitigating greenhouse gas emissions relative to substrate utilization and fermentation pathways.