## 1 Introduction
Data on the global volume of spent grain generated from distilleries are scarce in the literature. However, DSG is reported to account for about 85% of the waste generated in distilleries. According to [PERSON] et al. [10], approximately 2.5-3.0 kg of wet DSG is generated for every 1 L of whiskey produced, amounting to about 2400 tons of spent grain  
Figure 1: Illustration of DSG generation process in a distillery. per million liters of alcohol generated annually [11]. DSG is very similar to brewer's spent grain (BSG), a byproduct of beer production. DSG and BSG are generated during the initial stage, prior to fermentation, in whiskey and beer production, respectively. However, they differ in composition due to the differences in the types of grain used in the mash bill between breweries and distilleries. Recently, there has been considerable research interest in the processing of spent grain into value-added products, with a significant number of studies focusing on BSG. For example, BSG has been used to produce butanol and methane [12], arabinvarlan [13], single-cell protein [14], lignin [15], protein [16], bioethanol [17], nanocellulose [18], phenolic compounds [19], and food products [20]. The various studies on brewer's spent grain primarily focus on its utilization in food and feed applications [6; 21; 22; 23; 24]. However, there is an inadequacy of studies on the composition and utilization of DSG. This lack of robust data on DSG is one of the major barriers to its exploitation as a viable lignocellulosic biomass feedstock for the synthesis of high-value products [25]. Therefore, the availability of scientific data will provide deeper insights into its components and aid in the development of tailored processes to maximize DSG's potential as a biorefinery feedstock. In addition to the lack of studies on DSG composition, there is also limited data on the compositional variability of DSG from different origins. Since DSG sources are highly diverse, significant variations in their composition make it challenging to standardize their utilization in large-scale biorefinery operations. Variations in the chemical composition of the distiller's spent grain have also been reported across different batches from the same distilleries and within the same production plants [26; 27]. This high compositional variation presents additional barriers to DSG utilization in various applications. This variation can be mitigated by understanding the sources and extent of variation among the different DSG components [28]. A comprehensive analysis of the composition of DSG obtained from various sources is, therefore, vital for its industrial applications. Such an analysis would provide insights into the compositional variability of DSG generated by different distilleries and further help optimize conversion processes and tailor product development for the maximum yield of value-added products from distillery waste. This approach would benefit the distillery industry by increasing economic viability through waste utilization and contribute to realizing an environmentally sustainable whiskey production process. To the best of our knowledge, studies on the composition analysis of distillery spent grain from multiple sources are very scarce. In this study, we investigate the composition of distiller's spent grains from nine different distilleries, focusing on variability in their structural composition and identifying the principal components of DSG as a potential lignocellulosic biomass feedstock for synthesizing value-added bioproducts and chemicals. The rationale for using DSG from various sources is to simulate real-life scenarios, as sourcing DSG from a single distillery for biorefinery operations may be challenging. Additionally, this study provides comparative data on DSG from various sources. Further, the utilization of food industry waste, such as DSG, in biopolymer production aligns with UN Sustainable Development Goal 12, which focuses on responsible consumption and production.