Polyphenolic Content and Antimicrobial Effects of Plant Extracts as Adjuncts for Craft Herbal Beer Stabilization

Extracts from locally grown aromatic plants can enhance the geographical characteristics and microbial stability of craft beers, which are often not pasteurized or filtered. Here, the chemical and antimicrobial properties of aqueous extracts from leaves of <i>Myrtus communis</i> L., <i>Pistacia lentiscus</i> L., <i>Artemisia arborescens</i> L., and floral wastes of <i>Crocus sativus</i> L., all cultivated in Sardinia (Italy), were assessed. <i>P. lentiscus</i> extract had the highest polyphenol content (111.20 mg GAE/g), followed by <i>M. communis</i> (56.80 mg GAE/g), <i>C. sativus</i> (32.80 mg GAE/g), and <i>A. arborescens</i> (8.80 mg GAE/g). Notably, only the <i>M. communis</i> extract demonstrated significant inhibitory activity against pathogenic and spoilage microorganisms, with minimum inhibitory concentrations of 0.18, 0.71, and 1.42 mg GAE/mL against <i>Staphylococcus aureus</i>, <i>Lactiplantibacillus plantarum</i>, and <i>Lacticaseibacillus casei</i>, respectively. Additionally, it reduced the growth of <i>Levilactobacillus brevis</i> and <i>Fructilactobacillus lindneri</i> at concentrations of 0.35 and 0.71 mg GAE/mL, respectively. Based on its significant antimicrobial activity, the <i>M. communis</i> extract was further characterized using high-resolution mass spectrometry, revealing high abundances of nonprenylated phloroglucinols, flavonoid derivatives (myricetin), and quinic acids. Lastly, adding <i>M. communis</i> extract (2.84 mg GAE/mL) to commercial beer effectively prevented the growth of <i>L. brevis</i> and <i>F. lindneri</i>, showing its potential to avoid beer's microbial spoilage.