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Second-time brewer (to be).

When I did my first kit brew, I didn't want to mess anything up, and the instructions I was following told me to avoid the yeast sachet taped to the top of the LME can and get a reasonable, brand-name yeast that you can be confident is going to work well (it did); I got an 11g sachet of Safale S-04.

However, I only have a 11L carboy, so I only needed about half the sachet for my first brew. Next, I'm going to do a 9L brew, so I only need about the same amount of yeast again.

So, the question is -- does it make a difference at all if the yeast packet is opened? After I opened it, I folded it over and it's been sitting on my (cool) kitchen counter for the past 6 weeks (doh, I should have put it in the fridge, yes, I know).

Should I just pitch it in the trash, or pitch it in the beer?

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up vote 4 down vote accepted

Dry yeast will be fine at room temperature for a short period. However, I'd be concerned about pitching rates and contamination.

Did you weigh out the yeast before, so that you pitched 5-6g? If you only eyeballed it, then it might we way off, since pouring a quantity from the sachet is difficult - the sachet isn't full to begin with. If you weighed it, or can weigh out this addition, then is best to get in the ballpark for suitable pitching rates.

The contents of the sachet are close to sterile before being opened. After being opened, airbourne bacteria will make their way in, and also some moisture will be absorbed, facilitating bacteria growth.

If you want to take the risk, the go ahead and proof the yeast in 90F water, and verify that you get foaming after 20-30 minutes. If the yeast are healthy then you'll probably be ok, but it's not something you really want to make a habit of.

In future I would look for a small sanitary airtight container you can store opened yeast sachets in. Or, better still would be to pitch any unused into some cooled boiled wort (e.g. some made up LME or DME, around 1.030 SG), and store that in the fridge until you need it. (Don't screw down any caps tightly - CO2 will be produced by the yeast which needs to escape to avoid explosions.)

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Good answer. I'll just add that a packet of S-04 is $3.49. Isn't peace of mind worth less than the cost of a venti mocha frappuccino? –  JoeFish Feb 23 '12 at 14:27
    
@mdma - Actually, I did weigh out the yeast (we have a nice digital scale sensitive to 1g), so I am sure about that part. Since I have a "taped to the can" fresh yeast packet with the kit I just got, I'll use that this time as my back-up -- and will try the suggestion of proofing the yeast. Next time I order a kit will get a few extra sachets! Lesson learned! –  makdad Mar 1 '12 at 11:28
    
Good that you measured it out - I know many people that wouldn't! If you have some spare wort, you could try fermenting it with your opened sachet as an experiment to see if it would go bad or not. If the wort ends up tasting fine, then you can pour off the wort and pitch that yeast in your next brew. While most would agree that the low cost of a new sachet doesn't justify taking the risk of using an opened one, it would be nice to know exactly how much risk there is of a 6 week old open sachet going bad. –  mdma Mar 1 '12 at 16:52
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