CAMBRIDGE PRIZE LECTURE - STUDIES ON THE SENSITIVITY OF LACTIC-ACID BACTERIA TO HOP BITTER ACIDS

Hop bitter acids act as mobile-carrier ionophores. They inhibit the growth of beer-spoilage bacteria by dissipating the transmembrane pH gradient. Their activity is pH dependent. Low pH favours antibacterial activity but high pH reduces it. Resistance to hop bitter acids is a stable character, associated only with beer-spoilage lactic acid bacteria. Hop-resistant organisms can maintain a larger transmembrane pH gradient and ATP pool than can hop-sensitive organisms. Prior exposure of bacteria to trans-isohumulone does not influence the degree of resistance to hop bitter acids. However, in some strains, exposure to trans-isohumulone does induce the ability to spoil beer. The chemistry of these compounds is more complex than previously thought. In aqueous solutions, such as beer, hop acids bind to metal ions and may be covalently hydrated.