Experimenting with Thiols to Enhance the Aroma of Black Currant Mead

Thiols are a type of chemical compound that contains a sulfur atom bonded to a hydrogen atom. They are often responsible for the characteristic smell of foods, such as garlic, onions, and black currant berries. In black currant berries, thiols are responsible for the strong, distinctive aroma that is often described as fruity or black currant-like. They may have a characteristic pungent skunk-like, catty or almost a rotten egg odor.

There is a lot of activity in beer making world related to thiols from hops and yeast interaction (biotransformation) during fermentation. In beer these thiols are highly aroma-active compounds derived from hops that can give very intense fruity flavor to predominantly dry-hopped beers. 

In black currants these compounds include p-mentha-8-thiol-3-one C₁₀H₁₈OS (also known as thiomenthone), 4-methoxy-2-methyl-2-butanethiol C₆H₁₄OS (also known as 2-mercapto-4-methoxy-2-methylbutane), and 3-methylthiopropionaldehyde CH₃SCH₂CH₂CHO (also known as methional). These compounds are produced by the plant during ripening, and they are responsible for the characteristic aroma of black currants. Thiols are also found in other fruits, such as raspberries, strawberries, and grapes. However, the levels of thiols in black currants are much higher than in other fruits. This is why blackcurrants have such a strong, distinctive aroma.

In the beer making world there are a number of yeast strains now available that enhance the biotransformation of thiols from the hops during fermentation. For example, Omega Thiolized Yeasts or Lallemand on Thiols and yeast. The thiols most brewers are manipulating in their beer with thiolized yeast strains are slightly different compounds than the dominant ones in black currant berries.

For beer, there are free thiols in many varieties of hops, they make hops like Citra and Mosaic fragrant. The work on beer with thiolized yeast seems to focus on thiol precursors and having the yeast biotransform them into actual thiols.

Many years ago, the wine industry discovered that the distinctive passion fruit aromas in popular grape varieties like sauvignon blanc could be traced to thiol aroma. For wine makers there are a few yeast strains available that help emphasize the thiols from the grapes. For example, very high levels of varietal thiols characterize New Zealand Sauvignon Blanc wines. There are a variety of techniques used to increase the thiol content of some wines as discussed in a recent article. Recent approaches include thiol enhancing yeast strains and even additional nutrients to enhance thiol production during fermentation.

I decided to experiment with using a thiolized beer yeast strain to ferment a black currant mead. While black currant berries have different thiols than those in the hops that the thiolized yeast strains are used to biotransform during beer fermentation perhaps the yeast can work on the thiols in the black currants.

I love a good black currant mead. What if there was a way to increase the distinctive characteristics of black currants in a mead without increasing the amount of fruit needed to make the mead? Could I use a thiol enhancing beer yeast to convert some precursors in black currants into additional black currant like thiols?

My understanding is that the majority of the thiols in black currants are in the skin. However, I am going to conduct this experiment using black currant concentrate since that is what I have on hand. The concentrate I have is a 5:1 concentrate from Currant C that is stated to be 65° Brix or a huge specific gravity of 1.3227.

I am going to use Omega Star Party (OYL-404) a thiolized version of Omega West Coast Ale I (OYL-004). I will split a single batch of must and ferment a portion with Star Party and the other portion with West Coast Ale I. Star Party has a stated alcohol tolerance of 11% and a fermentation temperature range of 60–73° F (16–23° C).

The specification for Star Party calls for 73-80% attenuation but since that is for beer wort with a complex sugar makeup I am going to assume in a mead the yeast will ferment all the sugars until the alcohol tolerance limit is reached. I am planning for an OG of 1.102 which with the yeast finishing at 11% ABV should result in a FG of 1.020.

I couldn’t find anything about the nutrient needs of Star Party or the parent strain West Coast Ale I so I am going to treat them as if the require a medium level of nutrients.

Some Initial Results

I started by producing, for me, a typical mead must using my usual additives and nutrient schedule. I targeted approximately 7 gallons of must so I could split it into 2 separate fermenters after it was mixed, one with each yeast strain.

Amount	Ingredient
15.7 pounds	Wildflower honey
3 grams	Opti-White
3 grams	FT Blanc Soft
0.5 gallon	CurrantC brand black currant concentrate
25.1 grams	Fermaid O

My actual batch size was more like 6.667 gallons and the original gravity was just a tad higher than planned at 1.107. Given that yeast strain is supposed have an 11% ABV alcohol tolerance, I expected a final gravity of 1.026. I followed a normal nutrient addition schedule with additions at:

• 6.28 grams of Fermaid O at 24 hours
• 6.28 grams of Fermaid O at 48 hours
• 6.28 grams of Fermaid O at 96 hours
• 9.28 grams of Fermaid O after 9 days at SG 1.069

At the start of the process the two different yeast packets were both almost 3 months from when they were manufactured, the Star Party packet was 3 days newer than the West Coast Ale I packet.

The meads were fermented in identical plastic fermenters side by side and the temperatures measured during fermentation were identical within the accuracy of my infrared no contact thermometer. I measured the SG and temperature almost daily for the first few weeks of fermentation. Consistently, during the fermentation the batch fermented with the Star Party yeast was 2 to 4 SG points higher (slightly slower fermentation) than the batch with the West Coast Ale I. In the end, both batches ended up with a calculated 12.9% ABV and were drier than I expected due to the extra fermentation. A month after initially pitching the yeast the meads were racked to glass carboys. After almost a month in the glass they were again racked and at this point were only 1 SG point apart in their final SG and were both still at 12.9% ABV.

The meads finished dryer that I like my black currant meads. They were at SGs of 1.012 and 1.013. I first stabilized each carboy with 1.09 grams of potassium metabisulfite followed by 1.21 grams of potassium sorbate a day later. I then added 395 grams of honey to each carboy. The adjusted SG of each carboy was then 1.022.

After waiting a few days for some sediment to drop I filtered the meads and bottled them in 750 ml corked bottles.

I measured the SG virtually every day for the first few weeks after pitching the yeast. I have an Anton Paar Easy Dense for measuring the SG so the samples needed were only a few ml each day but that was still enough to get a sensory evaluation along the way. Both Janis and I found the thiolized version to be slightly more complex almost from the very start of fermentation.

After a couple of months in the bottles the thiolized version is still just a little bit more complex than the non-thiolized version. I’ve thought about getting a bunch of folks together to do a proper triangle test to get statistically valid assessments of the two versions.

At this point, the cost is nominally the same when making a black currant mead with or without a thiol enhancing yeast. Therefore, I will use a thiol-enhancing yeast for my future batches of black currant meads.

References

https://getollie.com/blog/the-essential-guide-to-thiolized-yeast

https://omegayeast.com/yeast/ales/star-party-ale

https://www.compoundchem.com/2015/07/23/blackcurrants/#:~:text=p%2Dmentha%2D8%2Dthiol,name%3A%20%E2%80%9Ccat%20ketone%E2%80%9D.