Help me understand fermentation and yeast

I've come into possession of a bunch of 2 gallon carboys that I'm going to brew in. I'm planning on making a few small batches of EdWort's Apfelwein and trying varions in each carboy. For example, I'll ferment one with brown sugar and another with corn sugar. I'll try carbing some and not carbing others. I think it'll be fun and provide a good learning experience for a n00b like me to see what changing the recipe can do.

So, to the questions....

I guess that for a 2 gallon batch, I should probably use half of a dry yeast pack considering that I tend to use a full yeast pack for 5-6 gallons. Does that seem about right?

What happens if I use too much yeast?
What happens if I don't use enough yeast?

As I understand things, once fermentation starts, it continues until the sugars are used up. Is it possible for the yeast to run out before the sugars are used up?

Also, I have a couple older yeast packs from wine kits. They are about 1.5 years old. I was considering rehydrating these with warm water and a bit of sugar to see if the yeast is still alive. If it is still alive, is it ok for me to use this stuff for my apfelwein, or is it possible that the yeast looks alive when I rehydrate it but that it doesn't ferment my sugars due to its old age.

Thanks in advance, and I hope you guys aren't too hung over from new year's eve debauchery.... Sadly, I was the designated driver last night...

Brewer_Steve said:

I guess that for a 2 gallon batch, I should probably use half of a dry yeast pack considering that I tend to use a full yeast pack for 5-6 gallons. Does that seem about right?

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You could easily use a full one, but half one would be fine as well. The yeast should start fermentation quicker (aka the lag phase, where yeast reproduce should be shorter).

Brewer_Steve said:

What happens if I use too much yeast?

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Shorter lag phase. Possibly shorter fermentation (low nutrient environment).

Brewer_Steve said:

What happens if I don't use enough yeast?

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Longer lag phase. Most probably incomplete fermentation (residual sugars will be left) especially in low nutrient environments and high stress factor environments (aka apple juice's acidity and poor nutrient content). I would recommend always adding a little yeast nutrient to your apple juice must, especially with weaker yeast strains (Red Star Premier Cuvee and Lalvin EC-1118 are exceptionally good a tolerating low nutrient environments, where as Lalvin D-47 is known for needing nutrients) just follow the nutrient manufacturers instructions (normally about 1 tsp. per gallon of Diammonium Phosphate).

Brewer_Steve said:

As I understand things, once fermentation starts, it continues until the sugars are used up. Is it possible for the yeast to run out before the sugars are used up?

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Yes, I just gave you an example why it could happen. Other factors are temperature, osmotic shock (if you add sugar too much sugar before or during fermentation, the pressure of the dissolved sugars in the must will "make the little yeast cells implode") and too much alcohol in the must (or the alcohol tolerance of yeast).

Brewer_Steve said:

Also, I have a couple older yeast packs from wine kits. They are about 1.5 years old. I was considering rehydrating these with warm water and a bit of sugar to see if the yeast is still alive.

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Look at the expiration date on the pack. It is usually stamped on one of the seams. If it is passed the expiration date, you can try rehydrating the yeast, but I think it is more trouble than what it's worth (yeast packs are about $1-2). If it is close to the expiration date, you're better off doing the rehydration and starter, but if it looks active, it's fine.

Brewer_Steve said:

If it is still alive, is it ok for me to use this stuff for my apfelwein, or is it possible that the yeast looks alive when I rehydrate it but that it doesn't ferment my sugars due to its old age.

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Yes of course, and no, if it's active it should not quit.

Brewer_Steve said:

Thanks in advance, and I hope you guys aren't too hung over from new year's eve debauchery.... Sadly, I was the designated driver last night...

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My pleasure to help you, hopefully, you will understand the work of the wonderful yeasties a little more.

Great information. Thanks MrFocus.

Looking closer at my yeast, I have 2 packs of Levocell which have expired May 2008.
I may try rehydrating them to see if they are in fact still alive. If they are, maybe I'll try to use them.

I also have a brand new pack of Lalvin EC-1118 which I can use if the others are dead.

Thanks again!

Well, it turns out that the expired Levocell was dead.
I pitched the EC-1118 into my Apfelwein with an OG of 1061. Isn't that a little high? I made it using brown sugar instead of corn sugar...

Brewer_Steve said:

I pitched the EC-1118 into my Apfelwein with an OG of 1061. Isn't that a little high? I made it using brown sugar instead of corn sugar...

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no problem, it will just have a little more alcohol (assuming it ferments to the same FG as the corn sugar batch).

ifishsum said:

no problem, it will just have a little more alcohol (assuming it ferments to the same FG as the corn sugar batch).

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Yes, that was my concern.... I've noticed several posts cursing the EdWort Apfelwein hangover, and it looks like mine might become even more potent than the average batch of Apfelwein.

mrfocus said:

Yes, I just gave you an example why it could happen. Other factors are temperature, osmotic shock (if you add sugar too much sugar before or during fermentation, the pressure of the dissolved sugars in the must will "make the little yeast cells implode") and too much alcohol in the must (or the alcohol tolerance of yeast).

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A follow up question.

So, if the yeast is alive, and it has enough nutrients, and is kept and an appropriate temperature, and there aren't too many sugars, etc, will the yeast just keep growing and growing and never run out??

If I add just a very small amount of yeast to a carboy under appropriate conditions, will it ferment as per usual, but maybe take a bit longer and have a longer lag phase??