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Beer yeast change over time due to new generations. It's biology. To my knowledge you can't keep the same generation of yeast alive. Which should mean the taste should change or evolve over time? At least that's how I understand how different beer yeasts are created.
Would the same brand of beer really taste and be equal over time, as in scientifically analysed and compared?
I think this homebrew post has a good explanation of how brewers keep yeast strains. Homebrew yeast discussion
White labs from San Diego also has a great slide about how they do it. White labs is the company just finished the study of the yeast domestication tree, where they analyzed the DNA of several yeast used in fermentation and determined how they are related. I've seen Chris White the founder speak at a homebrew event and it was very interesting. He has a Phd in microbiology from the university of california and sells to all sorts of professional brewers, distillers, and wine makers.
I'll summarize the information from the references here, but the bottom line is that they store samples at very cold temperatures so it's metabolism is almost shutdown and it won't evolve.
Brewers isolate there yeast in slants or plates. These are the pure strain.
The yeast that is isolated will be stored at a very cold temperature to remain dormant and unchanging. (-80 Celsius)
When they need to recharge because their yeast has drifted they remove a small amount from storage.
Using petri dishes they will grow a small size they can verify is still good.
With this small batch they will add to wort (what brewers call the sugary liquid that makes beer) and then make a large batch of yeast.
Macrobreweries such as Budweiser brew several gigantic vats at a time, and they blend the various vats' products to create a consistent product. They have a board of tasters at each factory to taste the batch and recommend what to blend to get the proper flavor. I know it's funny to think of Budweiser as having that kind of quality control, but they do have a specific flavor profile to protect and their drinkers are less likely to accept flavor variances than more adventurous beer drinkers and light pilsners have less of an ability to cover off flavors.
Sources: homebrewing experience and an Q&A with a Budweiser master brewer and taster.
There are really two questions here. 1) How do breweries keep the yeast they have from evolving over time 2) How to breweries keep a consistent flavor over time
I'll answer the second one first...
As to how do they keep the flavor the same all the time, year after year even though ingredients taste different from year to year? It's all about blending. They try to brew a consistent beer but it might be too hoppy or malty or strong or whatever but they will dilute, or add hop oil or blend in another beer that can adjust the flavor profile to exactly what they want. The same thing happens with Gallo and large wineries.
As to the answer to how they keep the same yeast the same year after year. You can essentially keep yeast forever by deep freezing it. I'm sure they have a vault filled with a library of yeast which they can go back to if they ever need to whip up a batch of new yeast from the original yeast.
I'll throw this in there too, that the taste of the major brands has evolved over the years. The Budweiser your great grandfather drank in the 1950s probably doesn't taste like the product you get now.
