One of the oldest and most successful human endeavors is the fermentation of cereal-based mush to create an ethanol-rich liquid. In other words, beer-making.

At the heart of this process is yeast, a single-celled fungus that does all the heavy lifting for brewers. It is so important that beer makers have learned to collect the yeast from the fermented liquid and reuse it in the next round. This trick, called “serial repitching,” saves both time and money. 

However, after several generations, the quality of the yeast begins to drop. This impairs fermentation and introduces off flavors that ruin the beer.

But why? The problem with serial repitching has beer makers and cell biologists scratching their heads. Clearly, something about the fermentation process is making new generations of yeast less able to ferment. But what?

Today, we get some interesting thoughts on this topic from Bianca Telini and colleagues at the Brewing Yeast Research Group at the Federal University of Rio Grande do Sul in Brazil. They say that ethanol itself may be responsible for the change, because in high concentrations it creates a kind of stress for yeast cells that changes the way their molecular machinery works. And they say a better understanding of this process could help make beer better for all of us.

First some background. Cell biologists have long known that fermentation dies down as ethanol concentration increases. That’s why fermentation cannot produce liquids with an alcohol content higher than about 20%.

Ethanol kills yeast in high enough concentrations. But beer making occurs at significantly lower concentrations, where ethanol merely stresses the cells, much in the way that elevated temperatures do.

One theory of why yeast becomes less effective over generations of serial repitching is that fermentation selects yeasts that are more tolerant to ethanol, and these are also somehow lower in quality.

The problem with this theory is that the evidence doesn’t support it. In 2007, researchers searched for genetic changes in ale yeast involved in serial repitching over an entire year. But after 98 generations, they found no evidence of genetic changes in the yeast at all. The yeast descendants were genetically identical to their ancestors.

So if the yeast isn’t “evolving,” something else must be responsible for the impaired fermentation. The question is what.

Over the millennia, organisms have evolved various complex biochemical mechanisms to cope with factors that stress them. For example, plant cells (and other cells) react to elevated temperatures with what’s known as heat shock response. The study of this response has become an area of intense research for scientists hoping to develop crops capable of flourishing in warmer climates.

What they have discovered is that heat shock response causes the cell to start making molecular machines called chaperones that help combat the effects of higher temperatures. Heat tends to change the shape of proteins and, therefore, their function. Chaperones mitigate this process by preventing or reversing protein misfolding.

Heat shock response is a complex process because cells are capable of making thousands of different proteins. But only a subset of these proteins are expressed at any one instant, and the subsets differ inside each organelle in the cell.

So maintaining the function of these proteins—proteostasis—requires a complex signaling mechanism that switches on the relevant genes in each organelle. This switching process must be coordinated across the cell, since organelles depend on each other.

The process of communication and coordination is called cross-organelle response, or CORE, and it is poorly understood. But this is an important emerging area of cell biology: biologists are beginning to realize that CORE plays a crucial role not just in heat shock response but in metabolism in general, and even in processes such as aging. 

The key idea that Telini and co explore is that ethanol induces a process similar to heat shock, and that this “ethanol shock response” must also be closely linked to CORE. They hypothesize that this somehow reduces the vitality of yeast cells during serial repitching. But exactly how isn’t yet clear.

One possibility is beginning to emerge. The molecular chaperones in yeast help ensure that proteins fold in the correct shape. But this can lead to structural changes in the genome itself. So a high ethanol concentration doesn’t change the genetic code or the phenotype of yeast, but it can change the shape of the genome and consequently the way it functions.

Yeast organisms that are recycled during serial repitching live in an alcoholic world—they are literally swimming in alcohol. In fact, they and their descendants live their entire lives under a level of alcoholic stress that triggers a constant shock-like response.

That means the cells are full of chaperones working overtime to maintain protein function. The new theory is that this inevitably lowers the vitality of the yeast. Indeed, researchers have found that yeast kept in high levels of ethanol in the lab eventually show changes in the structure of their genome. They have the same genetic code, but the DNA is packed in a different shape. In the lab, this becomes evident after more than 250 generations.

Exactly how this happens isn’t known. Neither is it clear why it should become significant after a large number of generations but not before. “The impact of this mechanism remains to be determined in beer fermentation,” say Telini and co.

But that raises important questions for cell biologists to get their teeth into. It might even give us a handle on other mysterious processes such as aging. Hence Telini and co’s interest: “A better understanding of the CORE network in the context of beer fermentation and/or ethanol stress will allow us to improve different aspects of brewing, which will ultimately improve beer yield and quality.”

And who doesn’t want that!

Ref:  arxiv.org/abs/1910.03927 : Does Inter-Organellar Proteostasis Impact Yeast Quality And Performance During Beer Fermentation?