Importance of tetrahydroiso alpha-acids to the microbiological stability of beer

While beer provides a very stable microbiological environment, a few niche microorganisms are capable of growth in malt, wort, and beer. The production of off-flavors and development of turbidity in the packaged product are due to the growth and metabolic activity of wild yeast, certain lactic acid bacteria (LAB) and anaerobic Gram-negative bacteria. Beer also contains bitter hop compounds, which are toxic to Gram-positive and Gram-negative bacteria, and contribute to preventing the spoilage of this beverage. In the boiling process, the hop alpha-acids (humulones) are isomerized into iso alpha-acids. These products are responsible for the bitter taste of beer, but they also play an essential role in enhancing foam stability. Antibacterial activity of iso alpha-acids and their hydrogenated derivates (rhoiso alpha-acids and tetrahydroiso alpha-acids) in MRS broth and beer have been evaluated against different LAB (Lactobacillus and Pediococcus) for the determination of their beer-stabilizing capabilities. Besides this, we have determined the minimum inhibitory concentration and the bacteriostatic effect of each compound against Pediococcus. We found that tetrahydroiso alpha-acids (added directly to beer during production processes) are the compounds that present the greatest antibacterial activity against the main agents implicated in beer spoilage.