Cellulose-Based Upcycling of Brewer´s Spent Grains: Extraction and Acetylation

Abstract

As the main byproduct of beer production, huge amounts of brewer´s spent grain (BSG) are generated annually worldwide. BSG is being currently underutilized since they are mostly devoted to animal feeding. Nevertheless, BSG offers a wide range of upcycling possibilities due to its lignocellulosic nature. In this work, we have addressed the extraction of cellulose and its further conversion into cellulose acetate, a biodegradable polymer with potential industrial application, especially as food contact material. The effectiveness of both processes, cellulose extraction and acetate synthesis were monitored by performing a comprehensive physicochemical characterization of the products. The influence of reaction time (1–5 h) on acetylation extent and acetate properties was also assessed. The results reflected that the process effectively removed hemicelluloses and lignin from BSG, and yielded a cellulose pulp with 63% crystallinity and 351 °C maximum degradation temperature. However, around 28% of the cellulose was solubilized, with acid pretreatment as the most aggressive step (above 19% disappearance). Acetylation extent was practically not affected by reaction time, and cellulose acetate with degree of substitution approximately 2.60 was obtained already after 1 h. Infrared spectra and X-ray diffractograms were similar for all acetate samples. Nevertheless, the thermogravimetric analysis evidenced that at least 3 h were needed to obtain a product with high thermal stability. This work addressed for the first time the acetylation of cellulose isolated from BSG, and may serve as basis for the manufacturing of a biobased plastic with application in sectors such as food packaging.