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After my last brew(beginning of may)I harvested the yeast cake(slurry) to 3 glass jars, put them in the fridge to store them for my next brew(11. June 2016). I'm about to make a starter to get the yeast count up, and hopefully use it in my brew.

A couple of hours before I started to boil the DME wort, I took one of the yeast jars out of the fridge to get the temperature of the yeast up(to not shock the yeast). After a 1/2 hour I noticed the jar was under severe pressure, and the lid was about to pop. When I slowly opened the jar, the yeast was percolate out.

I'm wondering if my yeast has gone bad or if harvested yeast normally builds pressure when in exposed to warmer temperatures(room temp.)? I know the "old" Whitelabs yeast tubes had a little pressure when they were warmed(room temp) up, but noting like the yeast I harvested last month.¨

Thanks!

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It sounds like you are doing nothing wrong.

Most likely the yeast had been very slowly fermenting/respirating in the fridge, I always crack open the lid every few days on any yeast I am storing. Cooling the yeast doesn't fully stop their metabolism, but slows it greatly unless you are dropping them into a -80C lab freezer or Liquid Nitrogen -170C.

after 4 weeks your yeast is likely fine, it would appreciate being given some food, and a couple of days to get it self sorted before starting again, keeping yeast in a fridge created a lot of Heat Shock Protein in the cells, this is to protect yeast fro high or low temps, basically any temperature stress. It need time to clear out this protein and rebuild its energy reserves.

Best to take it out 48 hours before you want to pitch it, allow it to warm over 6 hours, decant off any excess liquid. Then add the yeast slurry to at least double the volume of 1040SG DME solution, this will give it resources to build itself back up. If you have a stir plate all the better, but if not shake it often to drive off CO2. Keep a loose cover on the vessel you are using I use a bit of tin foil.

This is a basically a starter, but you are not necessarily aiming to double/treble the amount of yeast you have, just allow it to clean itself up and build up some energy stores. This should lead to a cleaner fermentation with fewer off flavours.

But, if time is precious, you can dump it in and it will make you beer.

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Most likely the yeast had been very slowly fermenting/respirating in the fridge, I always crack open the lid every few days on any yeast I am storing. Cooling the yeast doesn't fully stop their metabolism, but slows it greatly unless you are dropping them into a -80C lab freezer or Liquid Nitrogen -170C.

After 4 weeks your yeast is likely fine, it would appreciate being given some food, and a couple of days to get it self sorted before starting again, keeping yeast in a fridge created a lot of Heat Shock Protein in the cells, this is to protect yeast fro high or low temps, basically any temperature stress. It need time to clear out this protein and rebuild its energy reserves.

Best to take it out 48 hours before you want to pitch it, allow it to warm over 6 hours, decant off any excess liquid. Then add the yeast slurry to at least double the volume of 1040SG DME solution, this will give it resources to build itself back up. If you have a stir plate all the better, but if not shake it often to drive off CO2. Keep a loose cover on the vessel you are using I use a bit of tin foil.

This is a starter, but you are not necessarily aiming to double/treble the amount of yeast you have, it sounds like you have plenty, just allow it to clean itself up and build up some energy stores. This should lead to a cleaner fermentation with fewer off flavours.

But, if time is precious, you can dump it in and it will make you beer.

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The gas is formed by regular anaerobic yeast fermentation. When you put a lot of yeast with a small amount of food (the residual sugars in your beer) the yeast cells are better able to consume that food than they would be otherwise. In addition to the danger of the jar exploding, excess pressure is also bad for the yeast. I use canning jars that have a flat lid and a separate screw band to hold the lid onto the jar. Once the yeast is in the jar I tighten the screw band and then back it off a quarter turn. That way the jar is usually closed but any pressure will push the lid up and vent the gas.

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I'd wager your yeast is just fine after only one month under refrigeration. That's still relatively fresh. Here is an example of an experiment that involved pitching a five-month-old slurry without even making a starter. The tasters were unable to distinguish that beer from a beer made with a fresh pitch of yeast.

Speaking from personal experience, I've pitched month-old slurry directly without making a starter multiple times with good results.

The pressure in the jar might not have built while it was warming to room temperature but instead after it was moved to the jar from the fermenter. Again, from personal experience, I once harvested yeast into a pint jar and sealed it tightly before refrigerating it. When I opened it a few weeks later while it was still cold, it actually scared me how much pressure was in that jar. The sound of the lid popping open was like a champagne cork. The yeast produced a fine beer so I doubt it was some other microbial activity that created that pressure.

Unfortunately, I can't find a reference to explain exactly what the yeast are doing to create that pressure. I assume it's just regular fermentation activity. I strongly recommend you cover the jar but keep the lid loose so the gas can escape.

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