Fermentation is becoming increasingly popular. The shelf life of foods can be extended by means of fermentation with lactic acid bacteria. This process produces aroma substances and acids that give the fermented products a fresh, slightly sour flavor. Fermented foods are considered natural and healthy. The fermenting trend takes in not only homemade sauerkraut and kimchi, or pickled tomatoes and cucumber, but also commercially produced beverages such as kombucha, kefir, special lemonades, and beer.
Beers made from mixed fermentation using Saccharomyces cerevisiae with lactobacilli or other microorganisms such as Lachancea thermotolerans, Wickeramhomyces anomalus, Torulaspora delbrueckii or Pichia kluyveri have long been well established in the craft beer portfolio. However, bacteria and hops have a difficult relationship: Hops are known for their antibacterial properties. This is due to their bitter compounds: Even in minute quantities (0.001 to 0.1 wt.%) , the iso-alpha acids, alpha acids, and beta acids have antimicrobial and preservative properties. And they don’t spare lactic acid bacteria either. The bitter acids can penetrate the bacteria’s cells and cause a pH drop that effectively starves the bacteria. Hop-resistant bacteria, on the other hand, have means of preventing penetration by hops.
A recent study attempted to combine the probiotic properties of lactobacilli with the anti-inflammatory properties of hops in the form of new types of sour beers. However, probiotic lactobacilli in particular are said to be hop-resistant. The researchers analyzed the viability of Lacticaseibacillus paracasei subsp. paracasei F19 (F19) and 431 (L431) in a sour beer with a bitterness value of 22 IBU that they had brewed themselves and at the same time examined the expression of a number of genes that are associated with hop resistance. The fermentation was a two-stage process in which first the lactobacilli were active and subsequently an ale yeast was added.
As it turned out, both the probiotic strains F19 and L431 proved to be unaffected by the presence of hop bitter acids in the wort and are therefore ideally suited for producing sour beers with a higher hop content. Responsibility for hop resistance has for many years been attributed to various genes (horA, horB, horC, hitA, bsrA, recA). The ones considered particularly important for keeping hops out of the cell are horA and horC, but the current evidence is by no means clear.
In the event of hop resistance, there is nothing to stand in the way of the desired lactic acid fermentation, and presumably even much higher bitter acid contents are tolerable here. This opens up entirely new opportunities for integrating hops in sour beer fermentations. In this case, however, it is advisable to follow the authors’ lead and skillfully combine controlled fermentations using a specific lactobacillus with selected yeast fermentation, rather than relying on spontaneous fermentation.
Quite irrespective of whether fermentation is possible or not in the presence of hops, the hops can still make a pleasing contribution to the design of any sour beers. And that is thanks to the hop oils. They consist of monoterpenes (40 %), sesquiterpenes (40%), carboxylic acid esters (15 %), and minor quantities of numerous important groups of aroma substances. Depending on the hop variety and product, they produce expressive aromas with impressions of citrus, fruits, berries, herbs, spices, woods, or menthol. Many different hop oil products can be used as downstream products and enter into a unique interplay with other aroma compounds in the beverages. In the process, they are also able to mask or transform unpleasant off-flavors that are often produced in alternative fermentations. This makes them ideally suited for flavor design in fermented beverages such as kombucha, kefir, and special lemonades.
Herkenhoff, M.E. (2024). The Probiotic Paradox: Thriving in High-Hopped Sour Beer. Journal of the American Society of Brewing Chemists, 82(4), 422–430. https://doi.org/10.1080/03610470.2024.2350108
