The Fresh Loaf

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How does a SD starter survive drying / freezing?

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kolobezka's picture
kolobezka

How does a SD starter survive drying / freezing?

We were discussing this question with a friend but our knowledge in microbiology is not so deep :(

The "wild" yeast and lactobacilli do not form spores, is that right?

So how come that the starter survives drying / freezing for longer storage?

Any idea what percentage of the SD microflora usually withstands the unfavorable conditions?

And is it better to dry / freeze the starter when it is at its maximum (just when I would use it or feed it) or rather when it gets more acid, when it falls back? Why? (the info I found on internet are not very consistent :(

Thanks!

zdenka

dabrownman's picture
dabrownman

drying works better than freezing and it will last longer if wrapped in plastic and an air tight container, that is stored in a cold dry place - like the freezer!

You want to  dry & freeze it when the labs and yeast are at their peak so you have as many of hem as possible in the sample - not after it has fallen.

Many folks don't realize that the very best way to dry something, with the least amount of damage, is to freeze dry it.  Cold vs hot will do the best job at drying just about anything.  Sadly, we don't really have a good way to do this at home except in the freezer.  But if you live in northern climates where the humidity is low in the winter and schmear some starter on parchment and put it outside to freeze dry like the Indians did with their fish. Otherwise, do it in your freezer:-)

Alton Brown did a whole show on how to freeze dry food at home.  Off topic, he also recommends making a simple syrup to soak fruits, that are at their peak, so that when frozen for storage the moisture won't be pulled out of the fruit as much.

Syd-a's picture
Syd-a

So if we feed he starter and around 4 hours after we dry it and then freeze we are looking at optimal preservation? I am going to also take some dried starter with me on my journey and hope for the best.

Thanks again dabrownman, you are invaluable here

Andy

dabrownman's picture
dabrownman

starter and time of year or temperature to be more precise.  In the winter, since I store my 80 g of stiff starter in the fridge and don't feed it or refresh it till there is 10 g left,  if the the starter has been in the fridge for 3 weeks and it is cold in the kitchen say 65 F,  my starter needs (3) full 4 hour builds, 12 hours total, to get up to freeze drying  and bread making speed.  In the summer at 80 F in the kitchen, it takes half as long to get the 3 builds to double in the last 2 hours of 6 hours total.

After working in the food business for many years and meeting all kinds of folks that know stuff about food, it wears off on you, but thank you for your kind words Andy. Two years ago i didn't know much about bread and nearly caught the house on fire with my first loaf after joining TFL - not many can say that! 

I do work hard at the food craft thing that has me totally addicted.    All you have to do is be willing to say 'yes, I will do that.'   As Yoda said, (the smartest and most talented fictional creature ever created by a human),  paraphrased, 'Try or try not... makes no difference - you must do instead!'

The more you do - the more you learn.  The more you learn the more often people will see how much you know and the more they will be willing to pay for it   Shows you how stupid people really are since the very best things in life can't be bought and the things you can buy aren't worth owning :-)

Generosity is the greatest of all the many character attributes required for success.  Giving away for free, what may have cost you dearly, without wanting compensation is the definition of generosity.  Teaching others what you know when you are old and actually know something worth knowing,  is what makes your life worth living.  Not teaching what you know before...... you move on to another existence,  would be a travesty of the highest order, something, vile, despicable and selfish,  It would make your life and death itself - worthless in my book.  Yoda would call it the Evil of the Dark Side :-)

There is no telling what you might learn around here -  some of it might actually be useful! I'm also sure your dried starter will be just fine once you re hydrate it when ever and where ever you end up.  It might out live us all.....

Happy baking Andy

kolobezka's picture
kolobezka

Thank you "dabrownman" for your suggestion. I'll certainly have a look how freete drying can be done at home.

However my question was more about in what form the yeas and bacteria survive.

I also read the it is better to dry / freeze the starter after it has fallen. Either because higher acidity is better for later revival, or that more yeast and bacteria get into the form / regime to better survive the long storage.

Of course drying / freezeing is not necessary in most cases. But some people prefer to take a preventive measure before a longer journey (a month or more) or of they have previous experience with spoiled starter.

phaz's picture
phaz

not sure about the labs, but yeast is a fungi, and 1 of the characteristics of fungi is they survive and spread as spores. think commercial dried yeast - the yeast forms a protective outer layer when stressed, or dried in this case. water dissolves the layer and the yeast starts reproducing again if food is available. please correct if mistaken - I'm curious now!

kamamav's picture
kamamav

I love coming here! I currently have a dried starter in my freezer, for that "just in case". I am understanding that after re-hydrating, it has to be fed 3 or 4 times to regain its regular yeasty activity. I have wondered if it was possible to just freeze the starter myself. I have placed both of my starters in the fridge, about 36 hours now. I keep a WW and an AP going. Hopefully the fridge thing works, last time I ended up having to throw out my starter because it went bad. I have had them both going for a couple months now.  I have read and learned the same as phaz; that the yeast gets the protective outer layer. The heat/cold can damage that layer? I am no scientist, but this SD science is fascinating!

hanseata's picture
hanseata

In my experience, even though it survives, a lot of yeast cells die off, and it needs much longer to recover. Drying is the better way of keeping your starter alive, when you have no chance to feed it.

Karin

phaz's picture
phaz

I read the same thing - viability drops over time in the freezer. granted, time mentioned was a year or 2, but I'd imagine those who do freeze would like longer storage.

dabrownman's picture
dabrownman

in the fridge for.... decades.  White wet ones do not do as well as stiff whole grain ones ones and the stiff ones don't require weekly maintenance and have no waste.  For someone who only bakes one loaf of bread a week, stiff is the only way to go.

Before i merged my 1973 SFSD starter with a rye and whole wheat one at the end of last year i dried and froze two of the original for posterity and to continue the line.  Still I have some it in my multigrain starter I keep today,

 

hanseata's picture
hanseata

my Rose Hip starter 100% (white AP) starter. It had been fed about 4 weeks ago, and had hooch on top. It smelled still nice and fruity, and came to live just fine. 

A few days ago I had used it as mother for Ken Forkish's Wheat Walnut Bread, it performed great and the bread tasted wonderful. So much for the overanxious starter treatment.

Karin

 

 

kolobezka's picture
kolobezka

Karin: your Rose Hip Levain bread looks great!

However I'm afraid my initial question was misunderstood. I have no doubts that a starter can survive in the fridge several weeks / months without feeding. I've tried that several times.

I was mainly interested in what happens in the starter during refridgerating / freezing / drying from the microbiological, biological and / or biochemical view. Whether wild yeast form spores or not. To what extent the yeast and bacteria survive freezing and drying and what happens to them in such conditions. Etc....

kolobezka's picture
kolobezka

Fot those interested I got some notes to the freezing / drying problem from Debra Wink:

These are very good questions. Freezing microorganisms is tricky. The few times I have tried freezing sourdough, it failed to rise after thawing. So, clearly the yeast didn't fare well, and some species of sourdough lactobacilli are too fragile for freezing as well. I'm not a big fan of drying because of its spotty success rate, especially if kept for long periods of time. Maybe the combination of drying and freezing would increase the odds, but I haven't tried it, so can't say with any degree of confidence. You are right that these bacteria do not form spores, which are specialized cells. Yeasts in the genus Candida are non-spore-forming as well, andSaccharomyces tends to lose the ability in traditional sourdough. But microorganisms have other means to protect themselves. I can't say that I know the mechanisms, but they seem to survive desiccation for periods of time if conditions aren't too severe for them.

In the lab, we kept our reference cultures frozen, but we took special measures to protect their viability, and they weren't kept in the frozen state for more than a few months at a time before they were thawed, refreshed a time or two, put back into cryoprotective medium and refrozen. We used a -70ºC freezer and small aliquots so they were frozen solid in less than a minute. The faster you can get them frozen, the smaller the ice crystals will be. Ice crystals are like tiny daggers to cell membranes. The modern frost-free freezer is not a great alternative due to the temperature flucuations of the frost-free cycling. Below are a few links to give you a good idea what the physical effects are on the cells.

Storing Bacterial Samples for Optimal Viability ­ Thermo Scientific

A Guide to Bacteria Preservation 

Glycerol - Wikipedia, the free encyclopedia


The cryoprotective ingredient we used in the lab was glycerol, and if I were experimenting at home, that's what I would try. Glycerol and glycerin are the same thing, and here in the US glycerin is available through candy-making suppliers and pharmacies. Hopefully you have a source where you are as well. Most sources generally recommend 15% glycerol, which is a straight percentage (not the baker's math kind). Example: 15g glycerol + 85g water (15 in 100 total). What I would do is factor in the water already in the starter to determine how much glycerin and water to add to achieve the 15% for all the water present. Unfortunately, I couldn't say when the best time is to freeze, or what hydration and inoculation rate to use. Probably during the log phase of growth, but if this interests you, you'll just have to experiment with it. Try this: feed, let it get to log phase, mix in the glycerin well, and freeze as quickly as possible.

Sorry I don't have better answers to your questions.

All the best,
dw

Debra Wink's picture
Debra Wink

Oops. Should be 15 ml glycerol + 85 ml water in my example above---it's 15% by volume. So used to measuring in grams for bread. Not sure how the weight of glycerol compares with water. Sorry about that.