Water for Yeast Rehydration
The increasing popularity and availability of dry yeast for brewing makes the issue of rehydration important. There are now several studies that show that yeast viability is increased when dry yeast is first rehydrated in water, instead of wort. While you can get by with sprinkling dry yeast into your wort, rehydration with water will mean that you'll pitch more LIVE yeast cells into wort. It also turns out that what's in your water matters too!
Research by (Rodriguez et al, International Journal of Food Microbiology, 2008) presented in an article: "Vitality enhancement of the rehydrated active dry wine yeast", tested a variety of rehydration water additives and found that Magnesium has a profound effect on improving yeast viability. That research evaluated a number of additives and ions and found that including a substantial concentration of magnesium in the distilled water produced the best improvement in viability. They assessed 5, 10, and 50 millimolar magnesium solutions and found that both 5 and 10 millimolar solutions outperformed all other treatments (50 was too much!). 5 and 10 millimolar equals 120 and 240 ppm concentrations. Producing those concentrations using Epsom Salt (magnesium sulfate heptahydrate) is easy. 1 to 2 grams of Epsom salt per QUART of distilled water will do it. Since the lower concentration was almost as good as the 10 millimolar concentration, I recommend that brewers target the 1 gram/quart Epsom Salt dose to be safe and not overdo it.
This enhanced effect of magnesium on yeast is not a surprise since there is substantial research showing that magnesium is more beneficial to yeast than calcium is. My articles on Calcium and Magnesium requirements for Yeast that were published in Zymurgy and The New Brewer get more in depth on that phenomena.
These results also mean that straight distilled water is NOT ideal for yeast rehydration. The water needs some ionic content in order to improve viability. Starting with tap water or distilled/RO water is OK and adding Epsom Salt is recommended.
Water temperature matters too. Research by (Jenkins et al, Journal of the Institute of Brewing, 2011) showed that rehydrating dry yeast in water that is too warm will decrease viability. While some dry yeast instructions say to rehydrate in warm (around 100F) water, this research proved that room-temperture water produces the highest viability. So IGNORE any instructions telling you to rehydrate in warm water. I like to boil my rehydration water before pitching to help assure sterility, but I allow that water to cool below body temperature before adding the yeast.
I hope this research will help you dry yeast users to improve your brewing. Enjoy!
