## 3 Results and Discussion
### Composition Analysis
[48] reported a protein composition of 22.13%, and [PERSON] et al. [49] reported a value of 14.2%. Although the protein content of DSG in this study is not as high as that of protein-rich legumes such as soybeans, the protein content of DSG is appreciable as a waste material. It could be explored for various industrial applications and has the potential to serve as a cost-effective source of plant protein, offering benefits for nutrition, sustainability, and the economy. Cellulose is a major component of spent distillers' grains (DSG), originating from the cereals used in mashing. In this study, cellulose content ranged from 11.75% to 32.75%. The highest cellulose content (32.75%) was obtained from Distillery 9, which used 100% malted barley, while the lowest cellulose content (11.75%) was observed in Distillery 6, which used 60% malted barley and 40% unmalted barley. The cellulose content differed significantly, irrespective of similarities in the starting grain composition. However, operational technicalities between distilleries (e.g., craft vs. large-scale distilleries) may account for these differences. The cellulose content in DSG can vary between batches from the same distillery and across different distilleries [26; 44]. Factors such as the extent of starch removal during mashing and wort filtration can also influence cellulose content. The range of cellulose content observed in this study compares favorably with previously reported values: 16.8% by [PERSON] [46], 19.21% by [PERSON] et al. [50], 13.8% by [PERSON] et al. [42], and 21.84% by [PERSON] et al. [51]. However, the cellulose content of DSG is lower than that of some lignocellulosic biomass sources, such as sugarcane bagasse (40-50%) and barley straw (31.09%) [52]. Nevertheless, for a waste product, DSG presents an affordable cellulose source for synthesizing bioproducts. Hemicellulose is the second most abundant polysaccharide in DSG after cellulose. In this study, hemicellulose content ranged from 6.97% to 19.47%. Significant differences in hemicellulose composition were observed among DSG samples from the various distilleries, despite similarities in the starting grain. Xylose was the most abundant hemicellulose backbone in DSG (70-80%), followed by arabinose (20-30%) and a small amount of galactose. These findings are consistent with values reported by [PERSON] et al. [53], who documented a 9.8% hemicellulose content in brewer's spent grain. Similarly, [PERSON] and [PERSON] [45] reported a hemicellulose content of 9.44% in brewer's spent grain from a single brewer. However, other studies have reported higher hemicellulose content for spent grain from similar sources. For example, [PERSON] et al. [54; 55], [PERSON] et al. [43], [PERSON] et al. [50], and [PERSON] et al. [56] reported hemicellulose content values of 28.4%, 22.7%, 26.94%, and 26.4%, respectively. The variation in hemicellulose content is attributed to factors such as grain type and variety, processing conditions, and malting techniques. Lignin is one of the main components of plant cell walls, a natural phenolic, and the second most abundant biopolymer in plants after cellulose [57]. In this study, the lignin content of DSG ranged from 8.44% to 15.71% across the nine samples. The results show that most samples were significantly different (\(p<0.05\)) in lignin content, irrespective of the starting grain type. The highest lignin content (15.71%) was observed in DSG from Distillery 3 (single pot whiskey), while the lowest lignin content (8.44%) was found in DSG from Distillery 8 (single malt). The lignin content of lignocellulosic biomass is influenced by the biomass type. In this study, the lignin in DSG showed a higher proportion of acid-insoluble lignin compared to acid-soluble lignin. However, no specific relationship could be established between the starting grain type and the lignin content, as similar grains exhibited variations in lignin levels.