This week I cover the major hop thiols and how they can be used to increase the aromatic and fruity flavors in beers like IPA.
What are Hop Thiols?
Hop thiols, also known as mercaptans are sulfur based organic compounds that are highly aromatic. Thiols are a tiny percentage (< 1%) of the essential oils in hops, but they pack a big punch. They exist in a wide variety of products including fruit, hops, grapes and even grain. Scientifically they were first identified in the wine making industry where they have been exploited for many years to make aromatic wines.
Over the last 10-15 years, scientists have started studying thiols in hops in an effort to support the Craft Beer IPA revolution, as well as the push to get more aromatic and fruity flavors into IPAs. They have found that hops provide the majority of free thiols, with grains and other adjuncts primarily contributing bound thiols. However it is the free thiols that are chemically important for brewers as these produce the largest impact in a finished beer.
While the thiols in hops are much more complex and varied than in grapes, the four major hop-derived thiols include (Ref: Scott Janish):
- 4MSP or 4MMP (4-mercapto-4-methyl-pentan-2-one) – Has the aroma of Boxtree, Ribes, Chives, Tomato plant and Blackcurrants
- 3MHA or 3SH (3 mercaptohexanol) – Aroma of Grapefruit, Citrus, White Grape and Gooseberry
- 3S4MP or 3M4MP (3-mercapto-4-methylpentan) – Passion fruit, Grapefruit and Rhubarb
- 3MHA or 3SHA (3-mercaptohexyl acetate) – Converted to 3MH during fermentation, and can also produce beta-lyase during fermentation – Aroma of Passion fruit and Guava.
The above compounds are actually very common in wines like Reisling and Sauvignon Blanc, even though they don’t occur in grapes. They are developed during fermentation of wine from precursors in the grape must. Interestingly, thiols are also biotransformed in beer as described below.
Analyzing thiols in beer is exceptionally challenging because they make up only parts per trillion in the finished beer, and are also highly reactive. So the equipment and processes needed to isolate them are very expensive. However, our nose can detect thiols even in at a threshold of a few parts per trillion, so they have a big impact on aroma and flavor even though they exist in an infinitely tiny concentration.
I’ve done a number of podcasts over the years focused on thiols. Here are a few if you are interested in learning more: Yeast Biotransformations with Chris White and Nick Impellitteri, Hop Thiols and Varieties with Stan Hieronymus, Thiols and Fruity IPAs with MoreBeer, Thiolized Yeast with Adam Mills.
Thiols and Biotransformations
Yeast and hops interact in a complex process during fermentation which we call hop biotransformation. A variety of changes occur during fermentation including (Ref: Hopqueries):
- The modification of terpenes. These include the reduction of popular hop oils like geraniol (floral aroma) and citronellol (citrus/lime-like), Terpenes make up about 80% of the essential oils in hops, so the selection of hops high in geraniol and citronellol can result in more floral and fruity aromas.
- Conversion of Organic Acids into Esters – Yeast converts organic acids into fruity ester compounds. Some yeasts, like many English ale yeasts produce more esters than others, but the classic ester aroma is a product of this process. As hops age during storage, they develop more branch chain acids, so older hops actually produce more esters.
- The Release of Thiols – Thiols exist in two forms – free and bound. Only free thiols produce the rich tropical aromas we desire in beer. Hops like Citra and Mosaic are higher in free thiols, and as a result have become staples in many Craft IPAs. Other hops like Calypso have a higher percentage of bound thiols, but some bound thiols can also be released during fermentation.
Obviously for a transformation to take place, you must have hop aroma oils in the wort while it is fermenting. Since most hop aroma oils are very volatile, you typically need to add them as whirlpool hops or add some dry hops during fermentation. You also want to select hops that are high in thiols or terpenes like geraniol if you want to achieve the desired effect.
To maximize your thiol output you should also select a thiolized or thiol enhancing yeast. These yeast strains are either naturally selected or genetically modified to activate an IRC7 gene in the yeast that works to free bound thiols during fermentation. I covered these strains in the BYO article here, and also in my podcast with Dr Chris White and Nick Impellitteri.
Mash hopping has also been shown to be an effective technique if you select hops high in bound thiols and combine this with thiol enhancing yeast. I wrote a detailed BYO article on this topic here. This article actually covers a lot of ground including hop and yeast selection if you are interested in learning more.
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