A Free Exhaustive Literature Review on Hops (Humulus lupulus L.)

Abstract This article presents a background on hop chemistry, methods of addition during the brewing process to provide aroma and bitterness, and it discusses the optimal conditions for dry-hopping to maximize aroma extraction while minimizing bitterness and other issues. Essential oil extraction occurs much faster than what is commonly believed by brewers, reaching a maximum within only 2–3 days, and it is not negatively impacted by lower temperatures. This review discusses the potential problems of refermentation that may occur in packaged dry-hopped beers from the degradation of unfermentable carbohydrates by hop dextrinases, and the limits of the IBU scale in dry-hopped or sour beers to correlate perceived bitterness with iso-α-acids levels. While hop varieties can vary in bitter acids and essential oil levels and composition, the aroma of raw hops may not correlate well with the ‘hoppy aroma’ of the final product due to biotransformation of geraniol and enzymatic release of geranyl esters and polyfunctional thiols by brewing yeast. Timing of addition, temperature, alcohol levels, agitation, yeast strain, and even fermentation vessel dimensions are all factors that can influence the extraction, transformation, haze formation or loss of volatiles by the scrubbing effect of carbon dioxide and adsorption to yeast cells. Finally, the newest products available for brewers are presented, from new hop products to genetically-modified yeasts with enhanced thiol cleavage ability, to potential plant alternatives to hops. A link is provided in the ‘Background Section’ of the paper that takes the reader to the full paper and list of detailed references.