Phenolic and Cellulose-Rich Fractions from Subcritical Water Treated Beer Bagasse

Of the three types of waste generated in beer processing, beer grain spent (BGS) or beer bagasse is the most abundant and has a high potential for valorisation. In this work, defatted BGS was subjected to an extraction process with subcritical water (SWE) at different temperatures (110, 130, 150 and 170° C) to obtain extracts rich in phenols and the cellulosic fractions. Furthermore, the obtained cellulose fractions were also purified by means of a greener methodology using hydrogen peroxide (H ₂ O ₂ ). The results showed that the extraction conditions affected the composition and properties of the fractions. The dry extracts obtained at 170°C were richer in phenolics (24 mg GAE. g ^− 1 defatted beer bagasse (DB), but with lower antioxidant capacity (71 mg DB.mg ^− 1 DPPH). This extract (E-170) also showed the highest antibacterial potential (lower MIC values) against L. innocua (80 mg·mL ^− 1 ) and E. coli (140 mg·mL ^− 1 ) than those obtained at lower temperatures. The purification of cellulose from the SWE residues, using hydrogen peroxide revealed that DB is not a good source of cellulose material since the bleached fractions showed low yields (20–25%) and low cellulose purity (42–67%), even after four bleaching cycles (1 h) at pH 12 and 8% H ₂ O ₂ . Despite this, the subcritical water extraction method highlights the potential of a simple processes as technological option to convert underutilized side streams like beer bagasse into added-value, potential ingredients for innovative food and pharmaceutical applications.