Yeast: deliciously ancient
08 Aug 2019
No one who consumes bread, beer, wine, or chocolate can escape the use of yeasts. Yet, I feel most people’s knowledge of yeasts is restricted to the understanding that ‘yeasts turn sugars into useful foods/drinks and have been used by humans for a long time’.
I found myself wondering how food/drink created by yeasts in ancient times would compare to those made today. I imagined them to be different, so I was shocked to read about 6 strains of 5000-year-old yeasts that were still culturable! These yeasts were isolated from Egyptian flasks and grown to replicate the drinks of the time. I imagine the researchers were super excited to try Egyptian alcohol in the 21st Century.
Yeasts on tour
With my interest sparked, I decided to go on a journey to find out more about yeasts. This luckily coincided with a trip to Glasgow for the FEMS/SfAM conference. My journey started at the Tennent’s brewery famed for Tennent’s lager made since 1885. As someone who consumes 3 alcoholic drinks a year, I wasn’t overly delighted by the ad that the tour included a free pint, especially as I have never had a pint of anything stronger than water. Nonetheless, I couldn’t wait to see their fancy machines and hear about yeast fermentation.
The tour guide explained in detail the use of Scottish barley, hops and water to produce the lager. Firstly, the barley is ‘mashed’ in hot water to ferment the sugar and create a sweet porridge. For the next stage, the water is drained and combined with hops. From here, the product enters a fermentation vessel where Tennent’s add their secret ingredient – a specific (but undisclosed) strain of bottom-dwelling yeast. This yeast ferments the sugars to create the lager. For Tennent’s lager each yeast culture is replaced after 6 batches using a fresh culture from frozen stock known to only 3 people.
For my quest, I had to know more about the yeast, so I asked the tour guide about the fermentation process. He informed us that lager yeasts require lower temperatures and take longer to ferment the sugars, but they produce less by-products. This means the output from the process only requires sedimentation to remove the yeast before pasteurisation to kill any remaining yeast and extend shelf life. In comparison, he explained how ale yeasts (also used in the brewery) are rapid fermenters tolerating higher temperatures but produce many by-products including esters that give ales their fruity taste.
With the tour at an end, I had every intension of refusing my free pint of lager, but I was presented with an opportunity I couldn’t refuse. I could try the fresh, un-pasteurised lager. The microbiologist part of my brain was screaming “Cool!!! I have to try that!”. I proudly finished my live yeast culture with lager by-product.
Yeast hybridisation put to good use
After the tour, I had greater appreciation for yeast fermentation but I wanted to know more. What species had I encountered? Thankfully, the FEMS/SfAM conference included a talk by Jan Steensels on the beer yeast Saccharomyces cerevisiae. He started by naming products which can be made from S. cerevisiae fermentation including bread, beer, wine, chocolate and tea. I thought I had my answer to the species of yeast used in the Tennent’s brewery, but as Jan continued, he outlined a twist. His team sequenced 250 strains S. cerevisiae and around a quarter of the 250 sequenced strains showed hybridisation between 2 yeast species. The usefulness of this yeast means human industrial processes has driven its evolution allowing DNA sharing between species, thus creating new yeasts.
While his work was fascinating, my frustration grew - was I ever going to find out what species of yeast I had encountered? Finally, what I wanted to hear - Jan stated that all lagers are fermented by Saccharomyces pastorianus a hybrid between S. cerevisiae and Saccharomyces eubayanus. S. pastorianus was probably the yeast species I drunk in the Tennent’s brewery – I had my answer.
Yeasts of the future
Jan explained the evolution of Saccharomyces yeast was once common in breweries and created the array of flavours we have. However, this process is rare today with the industry focused on batch quality, so frozen cultures of stock bacteria are used for only a few batches before being killed and replaced. This has halted the evolution of Saccharomyces yeasts.
I began to wonder if new yeast strains would cease to be created in our factory processing. Again, Jan had the answer - by working with a traditional brewer they created some new hybridised yeasts. He added that some were good, but most were horrible. I joined him in laughing and reflected on how many ‘unwanted’ yeasts were probably created over the years but killed for producing ‘horrible’ flavours so the Saccharomyces yeasts we have today are probably only a small fraction.
I certainly learnt a lot more about Saccharomyces yeasts in lager fermentation, and have a new appreciation for them beyond their sugar fermentation ability. Next time I make bread I’m going to think about the complexity of the yeast doing all the work and wonder if it is a hybrid.