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When making a beer kit some people say to sprinkle the yeast on top of the wort and other people say to stir in the yeast to incorporate it into the wort.

I know that there is the other method of rehydrating the yeast but this is another topic all together so if possible can we keep it to sprinkling on the surface against stirring into the wort as the kit says it is not necessary to rehydrate.

Normally I gently stir in the yeast. Are there any benefits of stirring or sprinkling the yeast?

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Rehydrating is so easy. I start the brew day by boiling a cup of water, turning off heat, putting on a lid, and letting it sit. Meanwhile, I add a tiny cup to the sanitizer. By the time I am chilling wort, the water is room temp, so I add 10 ml per gram of dry yeast, and sprinkle the yeast. When the wort is chilled, I stir the yeast with my sanitizer thermometer. When my wort is aerated, I pitch, using a splash of my boiled/chilled water to rinse any remaining yeast into the fermenter. –  Chino Brews Dec 22 '13 at 2:29
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3 Answers

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In my experience, I haven't see any significant difference between stirring it vs just sprinkling it in. I've done it both ways and neither seem better than the other.

Here is my thinking on why it shouldn't matter: Dry yeast is dormant and it takes time for it to rehydrate and become active. The amount of time it takes for it to rehydrate and become active from absorbing wort by just sitting on top of in from sprinkling it in vs the amount of time it takes it to rehydrate and become active by you stirring it up is negligible. All your really doing by stirring it up is breaking the surface tension created by the yeast flakes (or whatever you wanna call them) and allowing them to sink to the bottom.

Fermentis recommends (if not rehydrating) sprinkling the yeast onto the wort, waiting 30 minutes for it to rehydrate, then stirring it up ether by aerating or adding the rest of your wort. (via their website)

When I use dry yeast, I usually chill my wort to pitching temperature, sprinkle the yeast in, then shake the heck out of the carboy to aerate. I don't wait the recommended 30 minutes and I've never had a problem.

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I suggest being careful if you simply mix yeast into high-gravity wort. There is research on wine and beer yeast strains that suggests yeast subjected to osmotic stress, will initially spend their resources on adjusting their cell walls, instead of preparing to multiply. In the longer term, the yeast adjust their cell size downward. If you are pitching adequate or excess amounts of yeast into a medium- or low-gravity wort, it probably doesn't make a difference, but if you are making a bigger beer, why not just rehydrate, and increase your chances of success? It requires minimal effort. –  Chino Brews Dec 22 '13 at 2:24
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To answer your question directly, I'd recommend a gentle stir. It will introduce a bit of oxygen to your wort. Also, if you aren't pre-hydrating, stirring will help hydrate your yeast and get them active.

Yeast have 2 phases: aerobic and anaerobic. During the aerobic phase (while oxygen is present) yeast cells will consume the oxygen and multiply into more yeast cells. During anaerobic phase (no more oxygen), they are digesting sugars and making alcohol and CO2.

Not enough oxygen can lead to too few yeast cells. Too few yeast cells will cause a slow start where other anaerobic critters (bacteria, wild yeast) can get a foothold. You want to be sure your yeast are warm and ready and itching to ferment when you pitch!

An over abundance of oxygen, on the other hand, can lead to too many yeast cells. When the yeast are done, they will settle out and eventually start to break down. You also don't want to ever introduce oxygen to fermented beer, as this can lead to 'cardboard' off flavors. I wouldn't worry about over-oxygenating with a gentle stir.

Personally, I brew 10 gallon batches that I almost always kick off with a 1.5qt starter. I don't do any stirring; all the siphoning into 2 6gal carboys plus just dumping in the yeast, I believe, adds plenty of oxygen. But if I was sprinkling dry yeast on top, I'd want to give it the best shot to get a good strong start!

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Can you explain why you recommend stirring? Does it relate to the discussion of O2? The answer doesn't explain the connection. –  mdma Oct 31 '13 at 15:10
    
There's ample evidence (start with Chris White's "Yeast" book from Brewers Publications) that dissolving oxygen into unfermented wort is beneficial to yeast growth. For the clarity of this answer, you should specify what you mean by "oxygen bubblers" and the duration of their use, and the amount of oxygen to add, as your opening paragraph regarding the aerobic phase is nearly contradictory to your later recommendation to avoid this practice. –  Galapagos Jim Nov 1 '13 at 23:16
    
Edited. Hopefully more direct now. Thanks for the feedback! –  djs Nov 2 '13 at 1:57
    
Stirring won't introduce much oxygen and certainly not the 8ppm max you can get from air, so a separate airation step should be used. –  mdma Nov 2 '13 at 15:51
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Charlie Papazian recommends rehydrating the yeast in the "bible" of home brewing, "The Complete Joy Of Homebrewing, 3rd Edition" (page 80 - 81). Following his instructions:

To do this, boil 1.5 cups of water for 10 minutes, then cover with foil. Add the dried yeast at about 100 degrees Fahrenheit and add the yeast and wait for 30 minutes before pitching.

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