Turning Bourbon Distillery Waste into Mycelium Materials: A Scalable Approach from Kentucky

Distilleries produce more than just spirits. In Kentucky—home to 95% of the world’s bourbon—booming production has led to a surge in byproducts, particularly stillage: a nutrient-rich slurry of spent grains and moisture left after distillation. Traditionally used as animal feed or disposed of via wastewater systems, stillage is now gaining attention as a promising input for bio-based materials.

A recent study from the University of Kentucky published in Fungal Biology and Biotechnology, explores how this overlooked waste stream can be transformed into pure mycelium materials (PMM) using solid-state fermentation. By feeding stillage solids to fast-growing fungi like Rhizopus oligosporus, researchers were able to produce tunable, leather-like sheets of aerial mycelium. The process not only valorises distillery byproducts but also opens up new avenues for sustainable material production at scale.

Bourbon stillage. Credits: Depositi photos

Fast-Growing Fungi, Flexible Materials

Most commercial mycelium materials are made using Basidiomycota fungi such as Pleurotus ostreatus, known for their ease of cultivation and controllable growth. But the Kentucky study tested R. oligosporus, a Mucoromycota species used in tempeh fermentation. Its advantage? Speed. The fungus grew up to four times faster than P. ostreatus, producing dense mats of mycelium in just three to five days under controlled conditions.

The team focused on growing aerial mycelium—the fluffy, thread-like structures that rise above the substrate. These fibres can be harvested and processed into flexible, leather-like sheets. Their quality and structure depended on several factors: substrate packing density, support height, and nutrient composition.

Optimising Growth Without Chemicals

To guide upward growth and increase yield, researchers adjusted how tightly the stillage substrate was packed and introduced vertical supports. Denser packing helped direct fungal growth toward the surface, while taller supports encouraged fungi to climb and form longer aerial structures.

Interestingly, the use of unmodified stillage (without added carbon) helped suppress sporulation—the formation of spores that usually marks the end of vegetative growth. This was attributed to its naturally low carbon-to-nitrogen ratio (C:N ~11.9), which prolonged the production of usable mycelium. In contrast, adding sawdust and other carbon-rich materials (raising the C:N to ~33) increased sporulation, reducing material quality.

Humidity also played a key role. Experiments showed that high relative humidity (95%) combined with nitrogen-rich substrate suppressed sporulation even further, supporting longer vegetative growth.

The researchers also tested the effect of applying water and glycerol sprays to the fungal mats to improve growth and texture. While 1% glycerol slightly improved moisture retention, 20% glycerol increased the density of the resulting PMM. These treatments could become part of future protocols to fine-tune material feel and mechanical properties.

Representative R. oligosporus pure mycelium material. Credits: Roy & al.

Leather-Like Sheets from Stillage

After a few days of fermentation, the aerial mycelium could be peeled off from the substrate using a separatory membrane, forming thick, fluffy sheets. These early-stage PMMs were flexible and soft to the touch—indicating strong potential for further development into textile applications.

The findings suggest that with the right operational conditions, distillery waste can be transformed into scalable biomaterials without expensive additives or energy-intensive processes. By tapping into fast-growing fungi like R. oligosporus and controlling growth through physical rather than chemical means, the process remains low-cost, tunable, and sustainable.

Next Steps

The study highlights key parameters for optimising PMM production and offers a roadmap for scaling up. Future work will focus on material performance—tensile strength, flexibility, and chemical resistance—as well as adapting the process for open-air or industrial settings. Given that fungal leather alternatives are gaining traction across fashion and design sectors, this waste-to-materials approach could play a major role in decentralised biomanufacturing.