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I am working on a story about a person who was raised on an earth-like planet and later moves to our earth. The planet he is from specializes in fruit juice and alcohol making, in specific, wine. When he gets to earth, he attempts to grow some of his grapes to make grape-juice to ferment. The problem is that once he makes this grape juice, it quickly ferments. This may not seem like a problem to us, but on his home planet, it takes much longer to ferment the juice past the first stage. (In most cases many years before it is ready to be aged)

So, my question is, what specifically does it take to slow down fermentation to such a degree? (Temperature, atmospheric content, greenhouse gasses)?

For this question, I'm assuming that the grapes he grew were the ones he brought from his home world, so they are the same and grow fine here. The yeast strain used is the same for both Earth and his home planet. So the causes for the slowing he was accustomed to are completely external.

I am definitely not an expert on wine, so some further clarification might be needed.

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Temperature

It's all you need to slow it down. The whole point of refrigeration is to slow microbial growth by cooling the item.

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    $\begingroup$ Yeast is a fungus, not a bacterium, but the same principle applies. $\endgroup$ – Patricia Shanahan Oct 21 at 23:37
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    $\begingroup$ I am downvoting this as factually incorrect. The OP says that the fermentation will have to take years. Even if you dropped the temperature to almost zero (and there will be no fermentation below zero, because the physiological processes in the yeast stop then) you can get a fermentation of normally 2 hours (as in dough) slowed down to about a day, so a factor of about 10x. With 3 to 5 days of primary wine fermentation, you're never getting to "years". $\endgroup$ – rumtscho Oct 23 at 12:37
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It is not actually the same yeast.

On his native world the yeast was less robust; more deliberate. Here on earth, he brings along his old yeast and adds that in. He assumes his yeast is what will do the fermenting; after all, he added it. But you do not need to add yeast to make wine. Yeast native to the grapes will gladly do that. Grapes are covered with yeast. That wild yeast is raring to go!

He is not accustomed to cooking the grape juice to kill any wild passenger yeasts. The native Earth yeasts living on his grapes are actually what is fermenting them. His alien yeasts are handily outcompeted by the earth native yeasts.

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    $\begingroup$ This is the best answer IMO. While the other answers are technically correct low temperature will slow fermentation, this fits the nature of yeast, and is seems like the most likely (and most interesting) cross-planet explanation. I bake bread with both conventional and wild yeast (i.e. sourdough), and they ferment at very different rates! $\endgroup$ – rsandler Oct 22 at 12:48
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    $\begingroup$ Our atmosphere is swarming with yeast, that's why grapes are covered with it. Maybe alien planet had nothing like that. $\endgroup$ – user28434 Oct 22 at 14:49
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@Thorne is absolutely correct. If you lower the temperature of the must (grape juice) to the minimum viable temperature for the selected yeast, you can extend the fermentation out greatly. This has the added benefit of improving the clarity and quality of the resulting wine, especially white wine varieties. If your selected yeast has a lower minimum temperature as common native bacteria and flora, the cold temperature can also be considered a hygienic improvement.

Also, fermentation is just the first stage of the wine making process. I have a batch of mead/honey wine which fermented to its target specific gravity in about three weeks, but has been conditioning in a dark temperature stable case for almost two years. If I am lucky, it will be ready to drink by Christmas time.

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  • $\begingroup$ "...improves the [...] quality of the resulting wine..." is a matter of opinion. A vintager could choose fast fermentation on purpose to achieve a different taste. I don't know if that's common or not, but if it matters, you can bet that there'll be different handling. $\endgroup$ – Gloweye Oct 22 at 10:23
  • $\begingroup$ @Gloweye, correct. The clarity improvement is definite because the lower yeast populations produce less contaminants over a longer period of time, allowing much of it to settle out while fermentation is still going on. But "quality" is a broad word. I probably should have said something like "truer to the yeast's normal taste profile" but that would have been difficult to explain in this context. Faster fermenting can definitely create different tastes, but these off flavors can vary from batch to batch and are hard to predict. Off batches are sometimes delicious but are hard to reproduce. $\endgroup$ – Henry Taylor Oct 22 at 13:47
  • $\begingroup$ Vintaging in general is more of an artform than a science in any case.. Perhaps a working like "improving clarity and altering taste" would be a more accurate wording ? $\endgroup$ – Gloweye Oct 22 at 13:49
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@Thorne provides the obvious solution, but let's go a bit more in detail.

Temperature

This works in both directions. Too cold and the fermentation does not proceed as the yeast hibernates or dies from shock. Too hot and the yeast starts dying from shock as well as producing substances toxic to other yeast (organic processes are generally quite temperature sensitive).

That said, you are more likely to get a quick fermentation if you run hot, and a slow (or non-starter) ferment if you are too cold.

Nutrients

Yeast requires various nutrients to survive. The most well known to brewers are organic nitrogen sources, as well as sugars. Yeast need simple carbohydrates that they can consume to breed. After the initial stage of reproduction, they require nitrogen sources, vitamins, and other micro-nutrients to survive.

It could be the case that the "grapes" that this brewer uses are mostly simple sugars, which would slow down fermentation as well as increase the incidence of off flavours (requiring longer ageing). This is observable on Earth with the production of mead. Mead is made from honey, which is 78% sugar, and basically nothing else. Mead made without the use of additional nutrients generally requires significant ageing, often on the order of months and years.

You mentioned that the brewer is still using the grapes from home, so unfortunately this doesn't work out for you. I've left it in for you reference if you need an additional reason.

Yeast Strain

Brewing yeast on Earth is bred to be reliable, produce a well-known ABV value, and withstand the stresses of a highly alcoholic environment. Throughout history, this was done via forced selection, where yeasts that made good alcohol would be re-used. In recent years, labs have bred yeast using various techniques to ensure that they perform well in making alcoholic beverages.

It's possible that the yeast on the brewer's home planet was never commercially cultivated, and the wild yeast did not deal well with the stresses of alcoholic fermentation.

This can be demonstrated by attempting to use baking yeast for brewing on Earth.

pH

Yeast requires a certain pH for their proton pump to keep functioning. The reason this is important can be summarised as "complex organic chemistry stuff", but suffice it to say this is how the yeast absorb nutrients and expels waste product. Generally, yeast like an acidic solution, but not too acidic. Somewhere between 4.5 and 5.5 is generally acceptable. Too low and the yeast can't absorb nutrients properly, too high and the yeast begins to decompose due to attack from hydroxide ions.

High pH is rarely a problem in practice though, as most musts are acidic (from malted barley or fruit acids, as the case may be), and will lower the pH of the water.

Too low pH is the more likely stalling condition, and is why meads have a habit of stalling (honey has an incredibly low pH of 3.5).

In our alien brewer's case, highly acidic grapes combined with a slightly acidic water supply could result in a stalled ferment back home, but on Earth, we buffer our water supply with minerals and try to keep the pH between 7 and 8, and this would fix the problem.

Other Concerns

Something to keep in mind is that if fermentation is stalled for too long, yeast cells will start digesting themselves. This is called autolysis. This can result in haze as well as odd tastes in the final product. These will air off eventually, but in general, this can range from "needs more ageing than usual" to "tastes like a tyre fire".

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Maybe life at this other planet had another chirality of nature? If life was hetero-chiral on the other planet, the yeast would be adapted to heterochiral sugar, and only receiving right-handed sugar. Thus it would ferment slowly and not fully. The wine would be weak and sweet, and the measurements a wine maker rely on (gravity, diffraction) would not give off any signals as to why this is. Maybe if the vinognier was of the clever type, and check the polarity of the light diffracted he would be alerted to the presence of "strange" sugar.

Life on earth makes exclusively left handed amino acids and right handed sugars. Left handed sugars taste sweet but can hardly be utilized by the living organisms on earth. Some synthetic left hand sugars are used as artificial sweeteners

In reality, yeast for grapemaking is usually grown on the grape by nature, they rarely add any different yeast than the grape actually "has". And if the yeast it has is not the best for that particular grape, wind and weather will replace it with a yeast that is. So, you might want to switch it around, the grapes still produce heterochiral, but the yeast is earth-like and consume right-handed ones only.

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Small amounts of sulfur can be used to dramatically slow the fermentation process. Such as is present in eggs.

Also, having too high a sugar content (not enough water) will slow the process. Among the ancients, it was not unheard of to hear the term "boiled wine", where the juice was boiled down to a syrup, and then reconstituted in water when desired.

As an aside, even when not boiled down, the reaction of the acids in the juice and the lead pots would leave behind a syrupy mixture in the bottom of the pot, which today we would call lead acetate, or "lead sugar". It was used as a sweetener for many years, with predictable results!

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There are actually quite a few factors. Here is my resource

  • Temperature as suggested by others
  • nutrient levels (not recommended to add extra nutriens, but possible)
  • yeast type / agressiveness
  • sugar levels
  • oxygen levels

In the comments: degassing can slow the fermentation down.

From my experience in bread making: maybe the strength of the yeast culture. If my yeast is too "slow", I did not feed it in the last couple of days, the fermentation is gonna be slower. The influencing factor is probably the quantity of the yeast and the composition of the different cultures.

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There are many great answers and all are scientifically accurate. I think that if your plot is not targeting an audience of brewmasters use can use simple adaptive radiation to explain that his fermentation on Planet A relied on a yeast strain which had evolved to consume sugars much more slowly. This evolution is because of all the scientific factors presented here: Planet A had a different temperature, Ph, solar index, and maybe even gravity. All these factors created Planet A yeast that simply works slower. His solution is to import the correct yeast, or go through massive efforts to adapt Earth yeast to act like Planet A yeast.

I'm going to add that if the yeast he brewed with on Planet A were identical to Earth yeast, that is actually more difficult to suspend disbelief. Silly rabbit, interstellar parallel evolution is for Star Trek.

Summary: Import yeast, leave the science back on Planet A so the plot can move forward.

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