Two in one: combining brewers' spent grain and spent yeast as functional fillers for sustainable polymer composites to maximize the resource efficiency of brewing by-products

Brewers' spent grain (BSG) and spent yeast (SY) are sustainable by-products of brewing industry. The former is rich in cellulose and hemicellulose and residual lignin and starch, whereas the latter contains significant portion of proteins. They are known as sustainable additives that can be upcycled in developing polymer composites. By combination of BSG and SY functional additives, a series of Mater-Bi (blend of poly(butylene adipate-co-terephthalate) (PBAT), thermoplastic starch (TPS) and poly(ε-caprolactone) (PCL) biocomposites were developed and analyzed for morphological, rheological, thermal, and mechanical properties. An almost homogeneous distribution of BSG and SY in these composites was observed, with more voids at the interface between Mater-Bi matrix and BSG filler, due to high aspect ratio of BSG particles, but SY addition partially improved interfacial adhesion due to the plasticizing effect of proteins. This effect was differently reflected in a reduced interfacial hydrophilicity gap or enhanced compatibility, leading to slightly higher flexural modulus and hardness. Incorporation of higher amount of SY to composite favored rheological behavior featured by lowering complex viscosity regardless of angular frequency. Since SY and BSG possess particles in the range of 10–100 µm, such rheological differences originate from proteins and their potential conversion to melanoidins during melt blending leading to plasticized PBAT/TPS/PCL biocomposites. The outcome of this work supports Circular Economy and decarbonization principles as well as upcycling of industrial by-products which aligns with industrial-scale application readiness.