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Study unlocks potential of adaptive evolution to produce hardy yeast
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Availability of commercial winemaking yeasts with an improved capacity to endure difficult conditions may be the eventual outcome of current research which applies the technique of adaptive (or directed) evolution.
Evolution has occurred naturally for living organisms over millions of years. Selective or stressful conditions allow only the fittest variants within the population to survive and eventually outgrow the weaker representatives.
The commonly used wine yeast Saccharomyces cerevisiae has an inherent liking for feeding on sugars and nutrients such as those found in grapes. Strains found in association with winemaking today, however, are undoubtedly different from those which might have been found at the advent of winemaking. Current strains have progressively adapted, or evolved, for the difficult environment of wine fermentation. Selective pressures of high ethanol content and nutritional deficiencies in particular have meant that only the most robust strains survive the fermentation and can go on to inoculate the next batch of juice.
A further selective pressure has been the winemaker, who discards fermentations that do not yield the desired flavours and aromas in finished wines.
University of Adelaide researcher Vladimir Jiranek (pictured) says selected strains with superior fermentation reliability have been useful winemaking tools. Their use, however, does not always mean stuck or sluggish ferments can be avoided and this makes the more resilient members of the yeast population an interesting focus.
He explains that optimised yeast strains, whether for fermentation performance, flavour enhancement or a combination of both, can be developed by several methods including hybridization and genetic engineering. An important feature of a yeast strain resulting from adaptive evolution, however, is that it involves no direct intervention to change the genetic structure of the organism. This feature will be important to some consumer groups, while for winemakers the enhanced capacity of yeast to withstand stress and complete fermentation will be welcome.
“Rather than screening candidate strains, whether isolated from nature or produced by hybridization or generic engineering, for improved fermentation properties we culture a population under the same stressful conditions we want it to thrive in and thereby force its adaptation or direct its evolution to these conditions,” Dr Jiranek said.
“In a sense this is what has happened to wine yeasts over the centuries but here we effect the process in a very precise and greatly compacted timeframe of several months.”
The experiments are conducted using a sequential batch approach. That is, by using the yeast from a completed fermentation as the inoculum for the next batch.
This is important to ensure that aside from any other improvements made to the population, the culture can still survive the varying composition of a grape juice fermentation – starting with high initial sugar and no ethanol and progressing to low sugar, high ethanol and low nutrients.
It is thought that as the fermentation progresses under these selective conditions the culture accumulates individuals with multiple beneficial natural mutations.
In the sequential process a 6 to 9 month adaptive evolution experiment can involve 350 generations. While this seems to be a long and intensive business it has been shown to be an effective means of enabling the hardiest members of the population to evolve to a dominant level.
The superior isolates compared with the parental strains appear from preliminary investigations to result in the same wine sensory properties. Further work in this area will no doubt be undertaken as the scale increases and evaluation is made under commercial conditions in Australia.
Although any new selections must be able to produce good wine and be suitable for the modern dehydration and rehydration method of supply and application, findings so far indicate the feasibility of the present approach to sequential fermentation technology for adaptive evolution. Importantly there is good potential to supply winemakers with yeast strains with greater fermentation reliability.
This project is supported by Australia’s grapegrowers and winemakers through their investment body the Grape and Wine Research and Development Corporation, with matching funds from the Australian Government.