I’m not the biggest fan of sour beer, but I am still very interested in brewing with non-conventional microbes. I brewed a batch of sour beer using Wyeast’s Lambic Blend and some bottle dregs around 3.5 years ago. It turned out surprisingly nice, and it got complements from many of my friends who love sour beer. Together with one of these friends, I brewed up a fresh 30L batch of wort (a couple of weeks ago), which I split into two fermenters. These two sub-batches were pitched with two different cultures: The Yeast Bay’s House Sour Blend, which contains a mixture of several different yeasts and bacteria, and a pure culture of a strain of Lachancea thermotolerans, which we’ve (our lab at VTT) isolated from an oak tree here in Finland.
Lachancea thermotolerans is a very interesting yeast, because of its ability to produce lactic acid. From a brewing perspective, this means it can applied e.g. to sour beer production. This is particularly interesting, because it allows for the production of sour beer without the cross-contamination risks associated with the use of lactic acid bacteria. Lachancea thermotolerans has recently gathered some interest in the brewing science community as well, as last year there were some published studies and conference presentations on the topic.
We haven’t really characterized our isolate of L. thermotolerans yet, so I thought I’d try it out in a homebrew batch. The pre-culture was smelling promising, with a strong fruity aroma combined with a distinct lactic tartness. As I wasn’t sure how the isolate would handle high osmotic stress or high ethanol concentrations, we decide to play it safe by brewing a relatively low-gravity wort. Using a 50/50 blend of pale ale malt and wheat malt, we aimed for a specific gravity of 1.050. As my brew kettle doesn’t fit 30 liters of wort, we decided to dilute the ‘Sour Blend’ portion of the wort with water. Hence we ended up with 15 liters of 1.049 wort for the L. thermotolerans portion, and 15 liters of 1.037 wort for the ‘Sour Blend’ portion.
For hops, we went with some old Saaz hops from 2010 I found in the freezer. We only aimed for a couple of IBUs. The L. thermotolerans portion got an additional handful of Nelson Sauvin hops I also found in the freezer (from 2013, so it had definitely lost some punch) added to it after flameout. We were hoping this would complement the fruity aroma that the yeast seems to produce and add a slight bit of extra bitterness.
After 2 weeks of fermentation (starting from around 19C, rising to around 23C), I transferred the L. thermotolerans portion to a keg for carbonation. The gravity had dropped to 1.011 (for an attenuation of around 78%), suggesting the yeast is capable of using maltotriose. This is quite an important result, as maltotriose is typically the second most abundant sugar in wort. The pH of the beer had dropped to 3.48 and the beer was definitely tasting sour, suggesting that a considerable amount of lactic acid had been produced by the yeast during fermentation.
Overall I’m very happy with the flavour and the aroma. The aroma is very fruity, with hints of stonefruit and tropical fruits. In the flavor there is a nice balance between the fruity and the sour notes. The flavour is also very clean, and with this I mean that there are no strange or off-putting off-flavours. This L. thermotolerans isolate also doesn’t produce any phenolic off-flavours (POF-).
L. thermotolerans is definitely a promising candidate for sour beer production, and I’m sure we will see more acid-producing yeasts pop up in the near future.