Extraction of valuable compounds from brewing residues: Malt rootlets, spent hops, and spent yeast

Brewing generates large amounts of residues including barley malt rootlets, spent hops, and spent yeast. These residues are sustainable and rich sources of compounds with valuable nutritional and functional properties such as proteins, polyphenols, and polysaccharides. The growing scarcity of natural resources and the increasing unsustainability of waste management, in the context of a rising global population, stress the need for reusing these wastes. This requires the development of suitable methodologies to recover interesting compounds. This work reviews the characteristics of brewery residues, the methodologies employed for the recovery of their valuable compounds, their functionalities, and their potential applications. A special consideration was focused to those methods employing innovative and sustainable approaches such as the use of deep eutectic solvents, microwave-assisted extraction, pulsed electric fields, high intensity focused ultrasounds, pressurized liquids, and supercritical fluids. Key findings and conclusions: Despite the general use of conventional solid-liquid extraction, works using emerging techniques are becoming more usual. While the benefits of these techniques in relation to sustainability are clear, yields are not always as high as expected. Polyphenols and proteins were mostly extracted from barley malt rootlets, while spent hops are more appreciated for their polyphenols, especially xantohumol and essential oils, and spent yeast is rich in polysaccharides, especially β-glucan, proteins, fatty acids, and flavour enhancers nucleotides. Some of these compounds exert significant antioxidant, antiproliferative, and immunoactivity and are potentially interesting for the food, cosmetic or pharmaceutical industries.