Molecular structure and antibacterial function of hop resin materials
Spoilage of beer by lactic acid bacteria (Lactobacillus spp., Pediococcus spp.) has considerable economic significance. Compounds derived from the hop (Humulus lupulus) possess antibacterial activity but their role in prevention of beer spoilage has been disputed. Prior to the present study, the mechanism by which such compounds inhibited growth of beer-spoilage lactic acid bacteria was unknown. A range of hop compounds (colupulone, (-)-humulone, (-)-cohumulone) and hop-derived compounds (trans-isohumulone, trans-isocohumulone, trans-humulinic acid, dehydrated humulinic acid) was prepared and analysed using several spectroscopic techniques (UV, IR spectrometry; mass spectroscopy; 1 H-NMR f 13 C-NMR spectroscopy). The ability of these weak acids to ionise in solution was studied. The lack of agreement between pKa values measured using potentiometric, conductimetric, spectroscopic and solubility methods suggests that, in aqueous solution, the compounds formed covalent hydrates. Antibacterial activity of the compounds against Lactobacillus brevis IFO 3960 was pH-dependent: the undissociated form of each compound was the active antibacterial moiety and the ionised form had little activity. The antibacterial activity of the undissociated forms of each of the compounds was similar. In the case of the undissociated form of trans-isohumulone, 0.1-0.4ptM was sufficient to inhibit growth of Lact.brevis IFO 3960 in a modified version of de Man Rogosa Sharpe medium over the pH range 4.0-7.0. Trans-isohumulone acted as an ionophore of the mobile-carrier type. In lactic acid bacteria, H + was exchanged for Mn 2+ in an electroneutral process that required the presence of a second monovalent cation (e.grK + , Na + ).