The impact of brewers' spent grain type on the structure and performance of poly(ε-caprolactone)-based composites

A drive towards sustainability of wood-polymer composites (WPCs) should assume maximizing the resource efficiency, hence exploiting all benefits yielding from the application of wood or other plant-based materials. It encourages using recycled raw materials from the food or agricultural sector. The brewing industry, with its inclusiveness and few geographical limitations, poses as an excellent source of fillers for WPCs. However, the diversity of beer styles yields a vast range of generated by-products, affecting the properties of final composites. Herein, the presented study investigated the unprecedented number of 22 different variants of brewers' spent grain (BSG) yielding from the production of various beer styles as fillers for poly(ε-caprolactone)-based composites. A strong correlation between the appearance (mainly lightness and hue) of BSG and resulting WPCs has been revealed. The increasing WPCs' browning index, affected by the products of non-enzymatic browning reactions, correlated with their surface wettability (an increase from 82 to 90°) and reduced the interfacial hydrophilicity gap. The thermal damage caused by the malting of barley, wheat, or rye reduced the final WPCs' tensile strength (from 9.3 to even 8.1 MPa) and thermal stability (from 247.1 to even 229.7 °C). On the other hand, oxidative stability was significantly stimulated, which was expressed by a 12 °C increase in oxidation onset temperature. None of the analyzed WPCs showed antimicrobial activity during disk diffusion assay tests. Reported findings enable adjustment of the performance of the final WPC by the proper selection of introduced BSG filler, providing auspicious insights into the potential applications, such as packaging materials, food containers, or cosmetic accessories like brushes, combs, or toothbrushes.