Yeast Rehydration

What can be easier and more straightforward than yeast rehydration, right? Apparently not…

As I’m sure all of you are aware, there is an ever ongoing debate in the homebrewing community in regard to dry yeast rehydration. Some say dry yeast must be rehydrated before pitching while others just drop them straight into the wort. Personally I’ve always belonged to the latter school of thought and indeed have never rehydrated yeast in my life. My main problem with this approach is the idea of letting the yeast just sit in a cup of water for 10-30 or so minutes. Knowing the microbiological prowess of majority of homebrewers this step just screams “contamination” to me. I’ve even seen people say they just use cold water with some fructose in it because “bacteria can’t be active in cold water so there is no need for sanitation”. Another problem is that a lot of new brewers just boil their yeast when they do it by not letting the water cool enough before pouring the yeast into it and then freaking out that there is still no fermentation after 3 days.

Recently this topic caught my attention again and I decided to try rehydrating and show how I’d do it so that perhaps some new brewers would avoid making silly mistakes. As always if there is an error with my approach, I’d be happy to discuss. First of all there is a debate on water vs wort rehydration with evidence seemingly pointing in the direction of “rehydrating in wort kills a lot of the yeast while doing it in water does not”. Names like Jamil Zainesheff and Chris White are often mentioned and their experience and expertise sited. An interesting study HERE addresses this issue and though I find reports utilizing methylene blue vital staining dubious, I’m going to choose to believe this one because it seems credible, well composed and with good references. Perhaps some day I’ll repeat this and check with other stains (been aching to try Neutral Red and Phenol Red as vital stains lately) when I have a little more time and energy. Thus we will continue this little project under the assumption that rehydrating in water is better than in wort and its temperature in terms of RT vs 80⁰F has no significant effect on yeast viability. I think the reason why they die in wort is osmotic pressure and cell wall fragility. These guys are just waking up, their cell membranes are literally like a mesh so they are unable to control what goes in and out, and suddenly they are dropped into this sugar-rich and hop-rich environment and some cells are just too weak and couldn’t rebuild their membranes yet and get killed, i.e. they are unable to retain the water inside and it’s being sucked out by the relatively high sugar gradient outside, sugars rush in, hop acids and oils rush in and so on and so forth and basically poison or physically rupture the cells.

Bottles after cleaning

It is fortuitous that I recently came by a whole box of 30 autoclavable 125mL glass cell culture bottles along with some other glassware (they just kind of dropped into my lap as me and some guys were on our way out to celebrate passing our qualifying exams and advancing to candidacy, which also means that I was wondering the night NYC subway drunk and with a box of glassware later that night). For the next couple of weeks there was no obvious application for them until this dry yeast idea came into my mind. So, after washing them in boiling water and soaking in ethanol to disinfect just in case there was anything there, I filled them up with around 100mL water and sterilized in the pressure cooker at 15 psi for 30 minutes. Both Danstar  and Fermentis recommend rehydrating in sterile water or wort that is 10 times the weight of yeast so 100 mL for an 11g packet should be good. After that their directions differ in that Fermentis recommends stirring/shaking while Danstar advises not to do it until later and also recommends feeding in a little wort at later stages to get it to the same temperature as the wort prior to pitching. What I decided to do is just rehydrate in room temperature sterile water as per Danstar instructions since I’m using their Windsor yeast and then pitch without the wort additions. After sterilizing the bottles with water in them they were allowed to cool to room temperature and then stored in the fridge until needed.

Sterilized bottles in the fridge waiting to be used

On the brew day one bottle was taken out and allowed to warm up as the day went on. During wort chilling the yeast packet as well as scissors used to open it were rubbed with alcohol to sanitize and minimize possible contamination and stored under UV light (yes, I finally installed UV light in my hood) until use.

Warming up for use. Could those be Brett starters in the background?

Contents of the packet was poured into the bottle and allowed to incubate for 15 minutes. After 15 minutes the yeast turned into a paste-like mass. It was then gently swirled to resuspend and pitched into the fresh wort. Fermentation started much sooner than the normal 12-16 hours I usually see when just pouring the dry yeast into the wort. This would suggest that rehydration prior to pitching really is better for yeast survival. All in all this procedure barely adds any more time or effort to my brewday, especially if there are already bottles with sterile water available so I’ll continue rehydrating before pitching in future.

Let’s recap the protocol:

1. Fill up culture bottles with ~100mL water and sterilize in the pressure cooker for 30 minutes at 15 psi. If such bottles are unavailable, one could use a small jar.
2. Allow the water to cool enough to use or store until later use.
3. Sanitize the yeast packet and scissors used to cut it with alcohol.
4. Pour the yeast into your bottle/jar with sterile water and let sit on our table for 15-20 minutes gently swirling it once in a while.
5. When the yeast reach a paste-like consistency gently swirl the container until homogeneity is achieved.
6. Pitch into the wort.
7. Let ferment.
8. Enjoy your homebrew!

UV lamps and hoods are unnecessary and were only used since I have them. Just do it quickly and carefully at your kitchen table and you’ll be perfectly fine.

Hopefully this was an interesting and helpful read for some of you guys.

Cheers!

Dry yeast poured into the bottle

Same bottle 15 minutes later ready for pitching