What does the yeast get out of wine?

Idle thought ....

Yeast ferments grape juice, eating sugars and converting it to alcohol, which it will keep doing but when the alcohol gets too high the yeast dies...

What's in it for the yeast? What are the evolutionary advantages for the yeast?

The opportunity to spread their genes. When grape juice in inoculated with yeast, whether wild, from the grapes, or a commercially isolated strain, the first thing they do is reproduce. Only when a viable population exists will active fermentation commence, converting the sugars to alcohol and carbon dioxide. It will continue until either the alcohol content becomes so high it is toxic to the yeast cells and they die or all the sugars are used up, at which yeast cells will sporate, thus becoming inactive until more sugar is available. The latter is most notable in making sparkling wines, as when sporation takes place, the cells decompose and add their yeasty flavor to the wine. I would suspect that from an evolutionary point of view, the wild yeast will attach itself to various fruits, including grapes. When the fruit ripens and the skin becomes damaged, the yeast will start to ferment the opened fruit. When the sugars are used up, the yeast will then sporate, dry up and be spread by the wind or other means until they find a new home. My guess is that at some time in the past, yeast from wine making was knowingly or unknowingly used by ancients to start fermentations to make beer and bread.

What's in it for the yeast? What are the evolutionary advantages for the yeast?

The yeast doesn’t know it's going to die so it does it's best to live and reproduce which, as Howie said, is all it 'wants' to do.

Jon and Howie have it,there's no "what's in it for me" involved in natural selection, just which traits lead to more successful survival to reproduce.

I'm sure enology departments have done a lot of studies, I'd assume prevalent strains in the Rhone and Languedoc survive at higher alcohols than the prevalent strains in Alsace and Bordeaux?

Human beings breathe in oxygen and exhale carbon dioxide, which they will keep doing but when the CO2 level gets too high the humans die...

What's in it for the humans? What are the evolutionary advantages for the humans?
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The extraction of energy from glucose, which is at the root of life for us as well as for yeasts, takes place in two stages. The first stage rearranges the glucose molecule into two molecules of pyruvic acid, four hydrogen atoms (captured by molecules of nicotinamide adenine dinucleotide [NAD]), and some energy (captured by converting two molecules of adenosine diphosphate [ADP] into the triphosphate form [ATP]). The technical name is the E-M-P pathway, also known as anerobic fermentation.

If molecular oxygen is present, both people and yeasts further break down the pyruvate by converting it to CO2 and more NAD-captured hydrogen (the technical name is the Krebs cycle). They dispose of the hydrogen by using it to reduce the oxygen to water (in effect, "burning" the hydrogen) and some of the energy is captured as more ATP (this is respiration). This second stage produces many times the energy of the first stage.

Under anerobic conditions (no oxygen present), the second stage can't run and the problem is how to dispose of those hydrogen atoms from the first stage. Human tissue (muscle, in particular) shoves them back into the pyruvate to form lactic acid, which is then carried away in the bloodstream from the oxygen-deprived muscle tissue to the liver, where it's rebuilt to glucose. Yeasts do a similar thing to dispose of the excess hydrogens, but they knock off an additional CO2 to produce ethanol instead of lactate and they excrete the CO2 and the ethanol into the environment.

Aerobic metabolism produces far more energy than anaerobic metabolism, so both human tissue and yeasts prefer to go that route when they can. Aerate your grape must and all you'll get is a lot of CO2--no ethanol. You only get ethanol when you deprive the yeasts of oxygen. Similarly, human muscle tissue operates in high-energy aerobic fashion until you reach the point where consumption of oxygen exceeds the ability of the blood to supply it. Then you get a drop in energy output as metabolism switches over to anerobic. We call this muscle fatigue. Too much lactic acid buildup leads to cramping.

So yeast gets nothing out of wine (it gets food out of grape must). Wine is a waste product of yeast.

-Paul W.

They get to hang out in wine and procreate..very little downside in that

Human beings breathe in oxygen and exhale carbon dioxide, which they will keep doing but when the CO2 level gets too high the humans die...

What's in it for the humans? What are the evolutionary advantages for the humans?
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Its keeps humans living long enough to reproduce and bring up children.

But if yeast in wine gets the chance to reproduce,the entire colony dies at the end of fermentation.

Or do they? Do yeasts escape during fermentation? How?

They don't all die out. Like most fungi, yeasts can produce spores as well as more active yeast cells. The spores are essentially in suspended animation, and are able to survive harsh conditions. Active dry yeast sold in packets in stores consists mainly of yeast spores. Yeast cakes are also mainly spores, along with some active cells. When the spores are in favorable conditions, they activate again into a normal yeast cell and start feeding and reproducing. It's a good survival strategy for organisms that can't just move elsewhere to get away from unfavorable conditions. Most fungi and many bacteria employ the technique.

-Paul W.

They get to hang out in wine and procreate..very little downside in that

What a way to go!

Perhaps nature never intended grapes of potential alcohol exceeding 12 or 13% to be colonized and fermented by wild yeast.

They get to hang out in wine and procreate..very little downside in that

What a way to go!

But will they still respect each other the next day?

FWIW I could take some of the “used” pomace (solid matter) from one vat place it in another, (say a “second run wine”), and have a “new” alcoholic fermentation start up almost immediately.

Salute

Yes--the pomace is a combination of dead yeast cells and yeast spores. Press some of the moisture out, and you have yeast cakes. I'm not at all surprised that if you put it in a favorable environment again, the spores will activate almost immediately.

-Paul W.