A hearty welcome to Chris Powell on FOY! The description of his earlier work reminds me of a question regarding yeast stress.

Chris, I've briefly studied the literature on yeast stress re sponse. It appears that important stress response genes, such as those for heat shock proteins, are under the control of upstream Stress Response Elements (SREs) and are coordinately regulated. Studies seem to indicate that this results in cross-protection against a given type of physiological stress following exposure to a different type of stress. So, for example, heat shock may lead to increased resistance to osmotic stress.

The question is, can this phenomenon be exploited in brewing in preparing yeast for highly stressful fermentations? For instance, when pitching into a very high gravity ferment, such as a mead or barleywine (high osmotic stress), could the yeasts' performance be enhanced by a mild stress exposure during starter production or close to pitching? I stress (pun intended) that this would be a mild stress, just enough to start inducing the protective systems, but well below any level that would actually lower the yeasts' health/viability.

Looking forward to your response,
Alan Meeker, PhD

RESPONSE:

Alan, You are exactly correct in your statement regarding STRE, and I'll attempt to answer your question specifically in due course. However, it's worth discussing the subject briefly to appreciate the complexity involved. S. cerevisiae cells generally respond to stress in several ways which can be categorized as 'specific' and 'general' responses. Often 'specific' responses can aid in prevention against other stresses, but this is not their primary purpose. For a 'general' response yeast possess three positive transcriptional control elements (at least which have been identified so far). These are Heat Shock Elements (HSE's), stress response elements (STRE's) and AP-1 responsive elements (ARE's).

The reason I mention these elements is because the three types have overlapping but distinct functions. Of the three types, STRE has recently been the focus of most work due to the huge number of stresses which can cause activation of a STRE activated 'global stress response'. These range from temperature shock, osmotic shock, oxidative stress, starvation, nitrogen/glucose depletion, the list goes on. So far around 200 genes have been shown to be induced by stress purely through the STRE promoter sequence. The products of these genes obviously have a variety of functions, but many of them enable the cell to protect its organelles, membranes, produce specific proteins, lay down carbohydrate reserves and generally prepare for the bad times ahead. Importantly, once a stress factor is removed the cell must revert to its previous state in order to function efficiently. This again is more complicated than it sounds, as a cell must repair any damage sustained as well as removing the (now defunct) products of the stress response.

I wanted to express the complexity of the stress response in yeast before answering your question. Theoretically it is possible to prepare yeast cells for a stressful environment by applying a mild shock. However, even though viability may not be affected, there is a strong chance that the vitality of cells may be impaired in some way. Starvation for example may cause upregulation of pathways which may procure some form of osmotic protectant on the yeast cell, but simultaneously will also lead to depleted glycogen in the yeast, which could be disastrous for fermentation. To simplify, there is no benefit in increasing the levels of specific shock protectants in place of something that would be of more use in fermentation.

Another issue that is of importance is that the response of yeast to stress is commonly strain specific. Subjecting one strain to osmotic shock (for example) invariably does not produce exactly the same response in another strain, this can be seen in brewing yeast where one strain may ferment a high gravity wort (for example) particularly quickly, while another may not be able to cope at all. Consequently it is difficult to predict how a culture (or indeed cells within the culture) will respond to a particular stress. However, having said that many industries do employ mild stresses (most commonly heat) to prepare cells in some way. So to get back to the point, theoretically applying stress such as a mild heat shock, by raising temperature of the yeast to around 5 degrees C higher than 'normal' for a short period of time (1-2hours) may procure some benefits to the yeast, however, be warned - it could be a double edged sword !

Chris