## 1 Introduction  
The increasing demand for renewable alternatives to replace fossil resources has driven research into exploring alternative feedstocks for the synthesis of fuels and chemicals. Lignocellulosic biomass has been identified as a promising feedstock with great potential to serve as a carbon-neutral source of fuels and chemicals due to its abundance and composition [1]. Lignocellulosic biomass is typically defined as plant-derived materials, such as agricultural residues (e.g., barley straw, corn stover), byproducts of industrial processes (e.g., brewer's spent grain (BSG), distiller's spent grain (DSG), sugarcane bagasse), or dedicated energy crops (e.g., Miscanthus) [2]. All lignocellulosic plant sources share the same fundamental building blocks, with the main structural components--cellulose, hemicellulose, lignin, and protein--varying depending on the source [3]. Malted barley is a common ingredient in most whiskey, although other unmalted cereal grains can also be included in certain variants. For example, in Ireland, Irish pot still whiskey is made from a mash containing a minimum of 30% malted barley, a minimum of 30% unmalted barley, and other unmalted cereals, such as rye and oat (up to 5%), depending on the distillery's process. Meanwhile, the mash for Irish Malt Whiskey contains 100% malated barley. Furthermore, Irish Grain Whiskey is produced from malated barley (not exceeding 30%) and can include whole unmalted cereals, such as maize, wheat, or barley. On the other hand, Irish Blended Whiskey is a combination of Grain Whiskey with added Malt Whiskey and/or pot still whiskey [4]. Distiller's spent grain (DSG) is the wet, unhydrolyzed solid fraction left after the separation of word from the mash grain. It is primarily composed of kernel husk, pericarp, and seed coat, making it a rich source of carbohydrates, proteins, lignin, phenolic compounds, lipids, and minerals. Large quantities of DSG are generated during mashing, with additional forms, such as distiller's dried grains with solubles (DDGS), also available. Other waste streams from whiskey production include spent lees and pot ale [5]. Figure 1 illustrates the process leading to the generation of DSG from malated barley during whiskey production. Briefly, ground malt is mixed with water to produce a mash, which is then heated. During a resting period, enzymes transform the protein and starch in the mash into fermentable sugars and amino acids. After draining the mash, a liquid filtrate (wort) and a solid residue (DSG) are obtained [5]. The wort is subsequently fermented, distilled, aged for a minimum of three years in wooden casks, and bottled as whiskey. The structural components of DSG, such as cellulose, hemicellulose, lignin, and proteins, make it a valuable lignocellulosic feedstock for developing high-value bioproducts and chemicals [6]. Cellulose, which constitutes up to 25.4% of DSG, is a key precursor for producing nanocellulose, widely used in bio-based polymers, composites, and sustainable packaging [7]. Lignin (27.8%) offers diverse applications, including adhesives, polyurethanes, and phenolic resins, due to its unique aromatic structure, while hemicellulose (28.4%) and proteins (26.9-34.9%) expand the potential for creating biodegradable films, bioethanol, and other bioproducts [7; 8]. Additionally, the high organic content and oil (10%) in DSG further enhance its suitability for diverse biorefinery operations [7; 9]. Brewer's spent grain, a similar by-product, has already been successfully valorized for bioplastics and nanocellulose production, underscoring the feasibility of converting DSG into sustainable materials. Despite its primary use as animal feed due to its high protein content, DSG presents environmental challenges because of its high moisture content, which makes it prone to spoilage. Optimizing its use in biorefineries and addressing these management challenges could significantly contribute to a low-carbon, circular bioeconomy [6].