Some advise on freezing yeast please. at the moment I store yeast on slants and re-propagate about every four to six months. I believe you can store frozen for twelve months using glycerine and some other ingredients. Can I do this on slants In a home freezer. Please advise on temps and techniques.

-Neville

RESPONSE:

Freezing yeast is very tricky. At the temperature normally associated with a household freezer, ice crystals begin to form within the cell and often times grow large enough to rupture the cell, even in the presence of glycerol. To prevent the crystal formation we begin by harvesting the yeast in the exponential phase and mix 50:50 with glycerol and divided into very small vials. This is instantly frozen and stored at -80C.

Our technique gives us assurance that a large % of the population will survive the frozen state many years and revive in a healthy condition. Any technique short of this offers no guarantee.

A 50:50 mixture of yeast (about 12-18 hours into a fermentation) and glycerol, divided into very small aliquots, and quick frozen and stored in a household freezer might allow a small % of the yeast cells to survive over a year or two. It only take a few surviving viable yeast cells to start up a new starter culture.

Your present technique of re propagating and preparing new slants is the safest. If you seal the tubes airtight with wax and store as close to 35F as possible, you should be able to re-slant on a less frequent bases, perhaps a year. If you try the 50:50 yeast/glycerol in your freezer, let me know the results. Use the methylene blue technique to determine viability.

Dr. Clayton Cone