postal_penguin

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I am putting together a plan to start freezing yeast stocks first in my home freezer and eventually at work in our -80C freezers. My plan is to use a 10% skim milk solution rather than the glycerol solution. You basically use powdered skim milk and add 10g of powder to make 100mL of solution then autoclave.

My question is: has anyone tried using a 10% skim milk solution for freezing yeast?

I ask because at work we use this solution to freeze bacterial strains and a professor had bacteria survive Hurricane Katrina and the 10 day power outages while bacteria in glycerol stocks were all dead. The milk in the freezing medium seemed to provide better freeze protection and when temperatures rose, it provided nutrients for them to grow/survive while also eliminating the toxicity of glycerol at warmer temps. We have bacterial strains >3 years old that are readily recovered from the freezer with this solution.

This to me seems like it might provide a better solution to homebrewers with frost-free freezers. As the freezer defrosts, temperatures rise and glycerol becomes toxic to the cells, a 10% skim milk solution would not have this problem and would allow the yeast to survive the brief trip above freezing.

Anyone tried this with yeast before? A google search just returns freeze drying yeast with skim milk.

SenorPepe

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This sounds very interesting. Would it be a viable method without autoclave/pressure?

postal_penguin

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You need someway of sterilizing the skim milk solution. You could try standard boiling but I believe the milk sugars present will start to caramelize if boiled too long. I am not sure of the sterility of powdered milk either and whether you could get away with just adding the powder to sterile water.

CCBrewer

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This is what I am reading


http://histemb.medicine.ankara.edu.tr/Cryopreservation%20and%20Freeze-Drying%20Protocols.pdf



Page ~99 is on freeze drying yeast protocols and step by step with formulas and suppliers of chemicals

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Hermit

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Not sure about the nutrient part as brewers yeast don't seem to eat lactose.

ScoRas

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You could potentially sterile filter it.

Maybe the protein matrix in the milk helps stabilize the cells during freeze/thaw.

I ran across this publication today: http://www.ncbi.nlm.nih.gov/pubmed/1571943

In short, heat shocking the yeast at 42C (as we do in the lab for transforming plasmids), induces a heat-shock response by upregulating chaperone protein expression. These chaperone proteins make up a huge fraction of the cytosol and protect from protein misfolding.

JohnMc

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IIRC, skim milk is used with freeze-drying or other types of dessication. That would definitely out last glycerol stocks if a freezer died. The main reason glycerol stocks are preferred is that you just dig into them (with sterile technique) and then plate the stuff and the stock itself is OK.
Freeze dried stuff generally requires reviving the whole stock, again, IIRC.