I first heard about thiols at the Great British Beer Festival’s Learn and Discovery Bar last year. I had ostensibly signed up to volunteer because if there’s one thing I love, it’s introducing people to new beer. Little did I know that I would do a fair bit of learning and discovering myself.

Thiols are sulphur-containing chemical compounds that deliver perceptible flavours despite only being present in tiny amounts. So small in fact that they are measured in parts per trillion. Some thiols we already know and don’t like too much. The skunky aroma of lightstruck beer is caused by a thiol.

Other thiols create deliciously fruity aromas and flavours. They can be free volatiles. These thiols add flavour to a beer when it is dry hopped. A great example is the characteristic white grape notes of Nelson Sauvin.

Some desirable thiols are bound. A hop might contain a lot of thiol precursors, but they need to be unlocked to make a difference to the finished beer. The Wild Beer Company team [a couple of months before the business was bought out of administration by Curious brewery] was staffing the affectionately named Disco Bar with me that day. It had brought along a beer called Thiology. As well as capitalising on the abundant free thiols in Hallertau Blanc to give rhubarb notes, which complemented the apricot-like esters from Southern Cross, the beer also accessed the bound thiols in Saaz by mash hopping and using specially selected yeasts. The fruit character of the beer, and the brewing process, really intrigued me.

Winemakers have been utilising thiols for decades. The most famous is arguably Marlborough Sauvignon Blanc from New Zealand, which has elevated levels of thiols. The action of the yeasts and low nitrogen levels during the winemaking process helps them to form from the precursors in the grapes. These aroma compounds work in harmony with others like esters, terpenes and methoxypyrazines to produce very distinctive wines. Thiols alone don’t necessarily make all the difference, but they are another tool in the flavour kit.

The potential of thiol precursors in hops, malt and other ingredients has only really undergone close scrutiny in recent years because accurate measurement of these tiny sulfur-containing compounds is so complex. Thiols only make up one per cent of the total composition of hop oil. The levels of thiol precursors in different types of malt vary, but research indicates that lower kiln and mash temperatures may help to preserve them. So very fruity ales (and I use the term ale advisedly here) can theoretically be made without any hops at all.

Lallemand Brewing, the US supplier of brewing yeast and bacteria, has been doing a lot of research and development in this area. It explains how yeast unlocks thiols better than I ever could (mainly because the word moiety was not in my layman’s lexicon).

“Because bound non-volatile thiols exist as an amino acid moiety, the liberation of thiols through beta-lyase activity plays an important role during periods of nitrogen scarcity for yeast.”

So, yeast strains that produce the most beta-lyase enzyme will extract thiols more effectively. In the US, this has led to genetically modified products for brewers hitting the market. Here in the UK, it means that yeasts are being selectively bred for their beta-lyase activity. In New Zealand, Jos Ruffell has created Phantasm – an extract made from local Sauvignon Blanc grape must. The product is loaded with thiol precursors and claims to boost the tropical fruit aromas in beer when used alongside a thiolised yeast.

So why do I find this all so interesting? Well, firstly, some UK-grown hops have been shown to contain decent levels of hop precursors. Challenger, UK Cascade (which has been grown here since 2002) and other British hop varieties can be used to add the same tropical fruit punch that we expect from New World varieties. This is good for our local supply chains and the environment, as I discussed recently, here on What’s Brewing. Check out Brew York’s Thioly Cyrus pale ale (5 per cent) to see UK Cascade used in this way, alongside Saaz.

Secondly, it's exciting that the curiosity and innovation of our brewers is leading to new methods of extracting flavour from those four basic building blocks of beer - water, yeast, malt and hops. I have nothing against flavoured beers per se. If you want to throw a load of fruit into the mix, then more power to you. I definitely have a particular weakness for the sour Morello cherries in a Belgian kriek. But there is something genuinely compelling about seeing the most traditional ingredients being brought to life in a new way.

Finally, one of the best beers I have tried that uses thiolised yeast to unlock phenomenal fruity aromas and flavours is actually a low-alcohol product. William Bros Brewing’s Alien Form AF IPA is 0.5 per cent but you might be none the wiser if you tasted it blind. I’m fascinated to see if more alcohol-free brewers take the opportunity to experiment with thiols to add an extra dimension to their beers.