I just woke my sourdough starter up recently to bake some fresh loaves and while watching for “oven spring” to happen had some time to reflect on the activity of baking bread and brewing beer. While no longer necessarily life-sustaining activities, it is still a labor of love when humans engage in making fresh bread and good beer. Historically, both yeast-dependent processes happened in the household kitchen where “re-pitching” happened on a regular basis, and where the secret ingredient that made both bread and beer possible was kept alive and well by being regularly fed a steady supply of nutrients in the form of flour and malted grains.
(Photo from Vinepair)
Simply, re-pitching yeast is the process of taking live yeast that is present at the end of a fermentation cycle and using that to start fermenting the next batch of beer. So essentially, what happens is that live yeast of a strain that has been proven to produce a desirable end product if given the same conditions to ferment is just fed a constant supply of nutrients, in this case, wort, or the sugar-rich starting ingredient of beer.
The secret to a satisfying end product of brewing is ensuring that the yeast is alive and active.
Consequently, a good brewer is at least a conscientious yeast herder, and in exceptional cases, a yeast whisperer that understands the behaviour of specific strains of yeast so well that by manipulating the chemistry, temperature, and time around the brewing process and specifically fermentation, is able to produce exactly a beer with the desired aroma and flavour characteristics.
[long sentences make me thirsty…I’ll be back after a pint…or five]
In much the same way, bread was produced continuously as a quotidian ration. Using a sourdough starter in baking is pretty much the same thing as re-pitching yeast in beer. The baker is essentially taking live yeast that made the last batch of loaves in the creation of this and future batches. Similarly, good loaves are the product of live and active yeast.
The natural question in this journey of natural history is, “Are yeasts immortal?” The simple answer is, “Of course not!”
Yeasts, like all living things, get old, get cranky, and eventually die. What keeps the process going of course is the yeast reproducing itself repeatedly. There is evidence to suggest that yeast will become less effective and eventually fade in its fermenting ability with repeated re-pitching. Additionally, as generations of yeast are “born”, there are subtle changes in the characteristics of the yeast called mutations. These occur as a natural part of reproduction.
Some mutations may result in favourable characteristics like hardiness, alcoholic tolerance, or desirable fermentation ester profiles. In general, brewers want a yeast strain to be dependable and predictable because reproducibility is what brewers demand and consistency is what consumers desire. So especially in beer styles where unique products of fermentation are key distinguishing features, conscientious brewers will invest in the extra expense of time and space to maintain a yeast library. One such rare example is a gem of a brewery I had the good fortune of stumbling upon in Brooklyn, Transmitter Brewing. The batting average for all the Belgian beer styles that I tasted in my short time there was .400…on target on plate almost every time. When I discovered they maintained a yeast library, I wasn’t in the least bit surprised.
So the next time you enjoy a particularly satisfying quaff, remember the little individuals that made it possible. To the billions of yeasts in each pitch, we salute you!