Many HBDers like to store yeast slurries. Storage under finished beer or deionized water at refrigeration temperatures is common. Some homebrewers reuse these stored slurries after as much as 60 days of refrigeration ! I have very grave concerns about the vitality and viability after such long storage and think that 30 days storage is pushing the envelope.

1 - What is an acceptable figure for pitched yeast viability (say by a haemocytometer + Methyl Violet stain count). Is the commercial target of 90% viability too stringent for the homebrewing environment?

2 - What is the viability of properly rehydrated dried yeast like Lallemand's ?

3 - Please comment on the expected vitality and viability of a wet homebrew slurry stored at refrigeration temperatures for various periods. Obvious there is no definite answer to such a general question, but perhaps you can suggest how long is 'probably OK’? How long is 'likely too long’?

4 - Some yeast vendors’ package wet yeast in refrigerated 'pitchable' tubes. Is there any means that they could employ to improve the yeast storage properties beyond that of a homebrewer's refrigerated slurry? For example, does anaerobic handling or phosphate buffers, etc improve 'fridge shelf life of a wet slurry by a great amount?.

5 - Does the iodine test (iodine into slurry sample to detect glycogen) have any comparative value to homebrewers as a quick and dirty test of slurry vitality? [Some sources state brief exposure to O2 will rapidly deplete yeast glycogen.]

6 - What are the consequences of pitching low viability slurries given that sufficient viable cells are pitched? If I pitched one unit of 100% viable slurry versus 3 units of 33% viable slurry what would be the expected difference in the beer? Autolysis of non-viable cells? Other? Does racking the beer off trub to a secondary prevent this harm ?

On a different topic ... Many HBers build up large yeast starters, but they do not wish to dilute their 5 gallon batch of XYZ-style wort with several quarts of pedestrian starter wort. They wish to separate the yeast from the starter wort before pitching. Many will allow the starter fermentation to nearly complete then refrigerate it and allow the yeast to sediment before decanting and pitching .

7 - Does the process hold potential harm for the yeast ? For example is the yeast likely to be less vital than if pitched directly from a high kreusen starter ?

Yet another direction ... 8 - Is there any significant advantage to *very* short 'lag times' (time between pitching to first sign of fermentation - usually CO2 outgassing) ? Many HBers seem obsessed in reducing lag times from 8 hours to

5. Personally I doubt it makes much difference so long as the lag period is reasonably brief.

-Steve Alexander


RESPONSE:

Ad 1) the viability for liquid culture (crop yeast) is also an indication for the physiological state or vitality of the yeast. If the viability is relatively low then there was something wrong (stress, nutrient deficiency...) during the previous fermentation and the "viable yeast" is probably not in very good condition as well. Therefore a viability of more than 90 % would be good standard to maintain.

Ad 2) For dry yeast it is a bit different. Here some yeast will die during the drying process and even more during the rehydration process. But this is an unavoidable process related loss and does not say anything about the physiological state of the viable cells. You will always have higher numbers of dead cells in dry yeast. The vitality of each live cell will be great. Usually the viability will be greater than 85%. The viability will be above 90% after a few multiplication cycles.

Ad 3) It depends on how you store your yeast. There are reports that you can store yeast up to 1 year in distilled water if all sugars are removed. We have a little program running to test this and after one month the yeast is still fermenting well. But it is critical that all sugars are removed. A lot of breweries keep their yeast for up to a month under water (removing the wort/sugar residuals) without any problems.

Ad 4) to my knowledge they are no special tricks to improve shelf life of commercial liquid cultures. These commercial cultures are propagated in nutrient rich media under optimum conditions means the yeast is very healthy when harvested and can be stored longer than crop yeast from a fermentation.

Ad 5) For the iodine test you need a microscope. Other wise you don't know if the starch/glycogen you are detecting is inside living yeast or if it is in the medium coming from starch residuals in the wort or released from dead yeast. Oxygen will deplete glycogen in yeast.

Ad 6) see ad 1. 100 % viable pitching yeast is very vital (healthy) compared to the 30 % viable yeast. So even if you compensate with a higher pitching rate you will have problems in the fermentation. Autolysis of non-viable cells is definitely an issue.

Ad 8) You are probably right!! I don't believe there is a significant advantage in-between 5 hours and 8 hours lag time. It is when the lag phase begins to extend well beyond this time period that you would begin to suspect a weakening of the pitching yeast. The HBers that try to keep the lag phase below 8 hours and moving down toward 5 hours are to be complimented on their efforts. It means they are doing all they can to keep a healthy yeast for pitching.

Dr. Clayton Cone and Dr. Tobias Fischborn