Biotransformation

Biotransformation sounds like something Bruce Banner goes through when he turns into the big, green Incredible Hulk. In a beery context, we’re talking about slightly smaller green things: hops. Biotransformation is the modification of a compound by an organism, and in brewing it occurs when active yeasts gobble up hop oils compounds and create entirely different flavour and aroma molecules in the process. It’s a new concept but not a new practice: biotransformation and the well-practised art of dry-hopping go hand in glove. The brand new flavours conjured up by this special interaction play a big role in the ‘juiciness’ of a New England IPA, and may well be the future of craft beer innovation.

“Biotransformation is a phenomenon that can take flavour intensity to another level,” states Robert Percival, Regional Sales Manager for Lallemand Brewing in Europe, and Certified Doemens beer sommelier. Lallemand is involved in the production and marketing of yeast, and is studying its interaction with hop oils to figure out what varieties work best, and in what conditions, for biotransformation. It’s a process that involves enzymes in yeast that are only sometimes present, and oftentimes in forms that are inefficient. But when it works right, Robert says “It’s an excellent tool, another part of the jigsaw of increasing diversity in beer, particularly useful in the current craft beer climate where we’re seeing trends like the evolution of modern IPAs, the style of hop-forward beers that are very fruity and aromatic.”

There are two ways that biotransformation can occur in brewing. The first involves glycosides, one of the many compounds that hops gift to beer. These are compounds made up of two components: a single sugar in ring formation, and a sister compound that could be anything else, bonded together at a specific position. In the glycosides useful for brewing, the sister compound is a tasty, aromatic oil. But, and this is the big but, in their native form, glycosides have no flavour, sweetness or aroma. This is because their two components are bonded together so tightly that they cannot become volatile enough to interact independently. But thankfully for us beer lovers, yeast cells are attracted to sugar rings in the same way that Homer Simpson is attracted to a ring doughnut. As the yeast gobbles up the sugar, its ß-glycosidase enzyme breaks the bond that fuses the two glycoside components together, liberating one from the other and allowing the aromatic compound to float free. The second way is the conversion of a hop compound into another, entirely new compound.

In this way, linalool becomes terpineol, a woody, lilac and citrus smell; geraniol converts to ß-citronellol, with its sweet, rose scent. It’s these essential oil compounds that add the complexity to beers that have undergone biotransformation. The science may also go some way to explaining why many people find bottle conditioned or unfiltered beer superior to filtered beer, where the yeast is available to breakdown remaining glycosides after primary fermentation.

So how do brewers get it to work? The most important point is that biotransformation can only occur when the yeast is active. This means that hops need to be added during fermentation, but not at the beginning, since the vigour of fermentation will strip away the hop oils, and yeast membranes still present will adsorp them. “The addition point and the timing becomes critical,” explains Robert. “We have to add these late hops in just before the end of fermentation when the yeast is still active but the fermentation not nearly as volatile to minimise any stripping out of these aromatic oils. Obviously with ethanol being present towards the end, you’ll have higher solubility as well, so it’s getting that balance.”

But even with all these factors right, the magic might not happen. The problem is finding a yeast that will ‘express’ the specific enzyme required to chop the glycosides in half. “Some strains will express this better than others. For example, ordinary ale yeast, Saccharomyces cerevisiae, will typically express exo-1,3-ß-glucosidase, which is not as effective and a bit slower than exo-1,4-ß-glucosidase, expressed by something like Brettanomyces.” These versions of the enzyme differ only in their physical arrangement, but it makes a big difference to their efficiency of work.

“Brewers are increasingly looking for diverse yeasts to give novel characteristics and, in particular, novel flavours, and that’s very exciting in one sense,” says Robert, before sounding a note of caution. “Yeasts like Brettanomyces and S. diastaticus need to be handled with caution as the potential for cross-contamination increases. The message we need to take home is that the brewing community is about pushing boundaries and looking at things like biotransformation and increasingly interesting yeasts, but ultimately considering the safety as well.”

The research at Lallemand aims to help brewers identify which “fractions”, or sizes, of hop oils and which yeasts will work well together. “We’ve progressed the scientific understanding of biotransformation as a phenomenon in brewing and how we as suppliers and brewers can use the right conditions, varieties of hops strains and yeast, and the right production techniques to enhance this process. It’s really a lot of work to characterise and quantify - measuring the availability of glycosides is incredibly complex, and there are so many variables in the interactions - but we’re building up more information all the time - we’re being able to say this yeast will or won’t do this function very well, and then working with hop merchants to look at a specific hop.” Robert sees this as important work given the direction of innovation in beer style: “In recent years of hop-forward beer evolution, there’s been a move away from bitterness to more fruity and aromatic IPA and that’s very much what we’re seeing in the market as a yeast producer, away from the neutral, clean yeast of west coast IPAs and towards increasingly fruity, ester-y yeasts as used in the New England style to create a slightly more complex IPA.”

Cloudwater Brewery of Manchester, England put the power of biotransformation to the test in a recent experiment with a super hoppy, tropical tasting double India pale ale (DIPA, for short). The plan: brew two identical batches of their infamous DIPAv3, using the same recipe and same method except for one thing: when to add the hops. In the first batch, named DIPAv4, the blend of Citra, Amarillo, Simcoe and Mosaic was added during fermentation to ensure biotransformation occurred. Hops were added to the second batch, DIPAv5, only after fermentation was finished - the standard dry-hop method. The difference in the final results was huge. Tasting notes by Zach Knowle of Draft magazine indicate a “funky and herbal” nose to v4, with a “catty” smell backed up by tropical notes. v5, on the other hand, was “lemonade-citrusy and so full of additional orange peel, orange blossom, honey and lime notes, it should probably be planted in a verdant California orchard.”

Getting down to taste, v4’s hops came across “as woody, almost mossy, with notes of overripe orange, mango notes, chopped onions, fresh grass blades and tangerine pulp injected throughout.” Zach once more noticed citrus in the flavour of v5, “with spikes of lime and garlic cloves.” The finish seemed messier, the bitterness rougher, and the texture softer. The beers even looked different too, with v4 pouring “a pale tangerine hue—not perfectly clear, but you can certainly see through it” whereas v5 was almost completely opaque. “It’s as stark a visual contrast as you’d get putting a bright West Coast-style IPA up against a turbid Northeast variety.

Robert is every excited by this proof of concept, and other similar work: “A lot of commercial breweries at the minute are dry-hopping in the late stages of fermentation. Demonstrated by fact that a lot of brewers are trying it, I think there’s a lot more to come on biotransformation,” says Robert. “I think it has a lot of potential and can contribute a lot to beer. Our responsibility as a yeast producer is to do research into how we can select specific yeasts that can give brewers the best chance for this to contribute to their beer.” To do so requires a great amount of collaboration between hop merchants, hops scientists, fermentation experts and brewers. Ultimately, Robert wants to see the characterisation of yeasts and how they will express biotransformation enzymes at the genetic level.

Could yeast be genetically engineered for biotransformation? “Designing a yeast to have exactly the characteristics you want - that has already happened,” states Robert. “It’s not standard practice yet, as in the EU it’s illegal at this moment in time. But the restrictions are not there in North America and I know of a number of interesting GMO projects done on yeast. A lot of that work has been done and is ready. We could create some really interesting novel flavours and functions.” Beyond the law, there’s the stigma around GMOs (genetically modified organisms) that would need tackling before beer drinkers could accept the use of ‘mutant’ yeasts. “I think there’s an appetite for brewers to use them, so if the market was ready and accepting of them then they could be brought to market tomorrow.”

A change in the law notwithstanding, it could be a case of needs must. According to Lallemand, “Coupled with an 80% increase in hop usage by beer production over the past 10 years, the increasing hop demand has led to a corresponding increase in hop prices, depending on variety. There is also a noticeable increase in the use of aroma varieties… however… these particular varieties tend to be lower yielding which further increases cost and potential supply shortfalls. As craft brewers continue to increase their hop usage, they must weigh the overall process costs and desire to maintain sustainable natural resources.” Could a yeast designed to biotransform hop compounds with extreme efficiency pave the way for beers bursting with aroma but made with minimal hops? It’s an event already on the horizon, beyond which is a beautiful, tropical sunrise. Before that new day, however, it’s the research of groups like Lalleland and the experience of Cloudwater and other brewers that can unlock the secrets of biotransformation.