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Hop biotransformation - how yeast creates the tropical nose of a NEIPA

The juicy, tropical nose of a modern NEIPA, full of passion fruit, mango and citrus, does not come straight from the hop. That is, not entirely. Part of these aromas is created by the yeast, reworking hop compounds into new, far more fragrant ones. This phenomenon is called biotransformation and is one of the greatest discoveries of modern brewing. It works like this: when the hop is added during active fermentation, the enzymes of the working yeast modify the hop aromas, releasing and creating compounds of which there was little or none in the hop itself. This is why the moment of hopping relative to the work of the yeast matters so much. Here is a guide to biotransformation: what it is, the three mechanisms behind it, where it draws the tropical nose of a NEIPA and when to add the hop to use it.

What biotransformation is

Biotransformation is the transformation of hop aromas by the enzymes of actively working yeast during fermentation. The key word is transformation: the yeast not only ferments sugar into alcohol, but also modifies compounds coming from the hop, turning them into others. For this to happen, the hop has to be added while the yeast is still active, not after fermentation has finished. The yeast enzymes then act on the hop compounds, releasing hidden aromas and creating entirely new ones. This phenomenon changes the way we think about hopping: the hop stops being a passive addition of aroma and becomes a raw material that the yeast reworks. Understanding that yeast can transform hop aromas is the starting point for all the rest: the three mechanisms and the characteristic tropical nose of a NEIPA.

Mechanism one: thiol release

The most important mechanism of biotransformation is thiol release. In hops part of the aromatic thiols exist in a bound form, that is attached to a molecule, which makes them odourless and imperceptible. Yeast has an enzyme that can break this bond and release the free thiol, and free thiols are powerful compounds with an aroma of passion fruit, mango and tropical fruit. It is precisely this mechanism that is responsible for the strongest effect of biotransformation and is the main source of the intense, tropical character of well-made NEIPAs. In other words, in the hop slumbers a hidden potential of tropical aroma that only the yeast can unlock. Without its work these compounds would stay bound and odourless. Thiol release is the chemical heart of the juicy nose of modern IPAs, impossible to reach with hops alone without the involvement of yeast.

Mechanism two: terpene transformation

The second mechanism is the transformation of terpenes, the scent compounds of the hop. The yeast enzymes modify hop terpenes, turning some aromas into others. The flagship example is geraniol, a compound with a smell of rose and citrus, which the yeast can transform into citronellol, of a rose-citrus note, and into linalool, of a floral and lavender character. Other terpene transformations create compounds that were not present in the original hop at all. As a result the aroma of the beer after fermentation tends to be different, richer and more complex than the hop alone would suggest. This shows that yeast not only releases hidden aromas but also actively transforms them, creating a new palette of scents. Terpene transformation is the second layer of biotransformation, adding floral and citrus notes to the tropical core from the thiols.

Mechanism three: glycoside hydrolysis

The third mechanism is glycoside hydrolysis. In hops part of the aromatic terpene alcohols occur in the form of glycosides, that is bound to a sugar molecule, which makes them odourless. Yeast has enzymes that can break this bond and release the free, fragrant terpene alcohol. It is another way to increase the amount of perceptible aromatic compounds in the finished beer. It works similarly to thiol release: the aroma exists, but it is hidden, and the yeast unlocks it. Together these three mechanisms, thiol release, terpene transformation and glycoside hydrolysis, make up the whole phenomenon of biotransformation. Each of them in its own way increases or changes the aroma of the beer relative to what the hop itself brought. It is thanks to them that modern beers can smell so intense and so different.

A table of the three mechanisms

Let us gather the three mechanisms of biotransformation in one place:

The table shows that in all three cases the yeast releases or creates an aroma the hop alone would not give. It is the common denominator of biotransformation.

When to add the hop

Since biotransformation requires active yeast, the moment of adding the hop is crucial. To use it, the hop is added during active fermentation, not after it ends. The best moment is the so-called high krausen, the peak of yeast activity, usually a day or two after the visible start of fermentation. At that point the activity of the yeast enzymes is highest, and after fermentation finishes it drops clearly. That means the same hop added during active fermentation will give a different, often richer aroma than added to finished beer. This is why modern, heavily hopped beers are often hopped during fermentation, not just after it. The moment of hopping relative to the work of the yeast is the key to using biotransformation. We cover the hopping techniques themselves more in hopping.

Biotransformation versus classic dry hopping

It is worth distinguishing biotransformation from classic dry hopping. Classic dry hopping adds the hop after fermentation, when the yeast no longer works, so it gives a clean, fresh aroma of the hop itself, without transformation. Biotransformation adds the hop during fermentation, so to the hop aroma is added a layer created by the yeast: released thiols, transformed terpenes, hydrolysed glycosides. These are two different effects: one purely hoppy, the other hoppy-yeasty. Many brewers combine both, adding the hop both during fermentation for biotransformation and after it for freshness. We cover the chemistry of the hop itself, thiols and terpenes, more in hops up close. Understanding the difference between these two approaches is a higher level of understanding modern, aromatic beer.

The role of the yeast itself

Since it is the yeast that carries out biotransformation, its choice matters. Different yeast strains have a different ability to release thiols and transform terpenes, because they differ in their enzyme equipment. Some strains are downright selected and promoted for strong biotransformation, especially for NEIPA, where the tropical nose is the star. This shows that the aroma of modern beer is born from the cooperation of hop and yeast, not from the hop alone. The brewer therefore chooses not only the hop variety and the moment of addition, but also a yeast strain able to draw the maximum aroma from the hop. We cover the role of yeast in beer more in brewing yeast. Biotransformation is proof that yeast is not only the engine of fermentation but also an active creator of aroma.

How to sense it in the beer

The influence of biotransformation is easiest to sense in a well-made NEIPA. If a beer hits you with an intense, juicy aroma of passion fruit, mango and tropical fruit, going beyond what the hop alone would give, that is a sign of successful biotransformation. These downright juicy, fruity notes are often the effect of thiols released by the yeast. It is worth comparing a NEIPA hopped during fermentation with a classic IPA dry hopped only after it, to feel how different the character of the aroma can be. Over time you will start to recognise this intense, tropical profile and link it to the cooperation of hop and yeast, not just to the hop. It is a higher level of beer tasting, at which aroma stops being a simple sum of ingredients and becomes the result of a process in which yeast plays a creative, not just a technical, role.

The essentials in brief

Let us gather it up. Biotransformation is the transformation of hop aromas by the enzymes of actively working yeast during fermentation. Three mechanisms stand behind it: thiol release, giving the powerful aromas of passion fruit and mango; terpene transformation, for example geraniol into citronellol and linalool, giving rose, citrus and floral notes; and glycoside hydrolysis, releasing further hidden aromas. To use it, the hop is added during active fermentation, best at the peak of yeast activity, not after it. This is why modern NEIPAs smell so intense and tropical. The choice of yeast strain also counts. Now you know that behind the juicy nose of a NEIPA stand not only the hop but also the yeast that reworks it.

Note every beer in GustoNote - the style, the intensity and the character of the aroma. Over time you will start to recognise a tropical nose from biotransformation and understand more deeply how hop and yeast together build the flavour of beer. We cover the whole process more in how beer is made.