Beer is a widely consumed carbonated beverage made from natural ingredients, including malted cereal, hops, yeast and water. It is rich in nutrients and contains carbohydrates, minerals, vitamins, amino acids and polyphenols. The brewing process, however, generates a significant amount of solid waste, including hot trub, a slurry of entrained wort, hop particles and mainly unstable high-molecular-weight colloidal proteins that coagulate during the boiling of the wort. Given the environmental impact of agro-industrial waste, finding sustainable methods to reuse this waste by transforming it into bio-products is crucial.

The aim of this study was to assess the potential of converting these by-products into biologically active extracts suitable for use as functional ingredients in cosmetic and pharmacological formulations. The extracts consisted of one alcoholic and one hydroalcoholic extract prepared by Soxhlet extraction and one hydroalcoholic extract prepared by maceration. The extracts' antibacterial activity against the Gram-positive bacteria S. aureus, S. aureus (MRSA), S. epidermidis, B. cereus, E. faecalis, S. mitis, S. pyogenes, and S. mutans, as well as against the Gram-negative bacteria E. coli and P. aeruginosa and against the yeast C. albicans, was determined by the agar diffusion method and the minimum inhibitory concentration (MIC) by the microdilution method. The minimum inhibitory concentration and inhibition zones values obtained showed significant inhibitory effects against the Gram-positive bacteria tested, with the Soxhlet extracts presenting the best inhibitory results for both hydroalcoholic and alcoholic extracts. In both methods, the extracts showed the best antibacterial activity against the microorganisms S. epidermidis and B. cereus, with an MIC of 0.63 and 0.31 mg/mL, respectively. The tested extracts seem to be promising, low-cost antibacterial agents that can be incorporated in cosmetic and pharmacological formulations. Additional studies will be conducted to assess their antioxidant activity and safety by using in vitro and in vivo models and investigating the chemical composition of the extracts.