Friday, February 27, 2009

Freeze Your Yogurt Starter?

This started as a question from a user of MakeYourOwnYogurt.com, and when I didn't know the answer, it turned into an experiment. If you are familiar with my homemade yogurt recipe, you know that it requires 2-3 Tbs of plain yogurt as a starter. If it is your first batch, I have you start with a quality yogurt with lots of active cultures (like Dannon), and for all batches thereafter, you can use your own. But this user was getting to the end of her current batch of yogurt, and was not going to be in town the following week to warrant making more. So she asked, can I freeze the 2-3 Tbs of yogurt, and will it work as a starter in the future?

It seemed perfectly logical, since you can buy freeze-dried yogurt cultures online and at health food stores, but I could not say for sure. So I took enough fresh yogurt to make a starter, and froze it in a 2 oz. plastic container. When I went to make my next batch of yogurt, I removed it from the freezer and put it into a slightly larger container filled with warm water (a water jacket). As I went about the rest of my preparation (sterilizing equipment, heating the milk, and then cooling the milk), it was defrosting to room temperature. Once my milk was at 110°F, I pitched my yogurt starter in, and mixed it up well. After waiting seven hours with it on the heating pad, I returned to a perfect pot of homemade yogurt!

So freezing some starter from your last batch is a great way to make sure you always have some on hand, and ensure you will never have to buy store-bought again. I imagine it will keep in the fridge for a few months, so make sure you always have a little, and you can make yogurt any time you like. Thanks again to my reader for this question. I had fun answering it, and learned a valuable, money-saving tip.

11 comments:

  1. excellent, thanks so much - im new to this but it seemed like you can freeze anything with relative success these days and you have done the ground for me on this one!

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  2. Hmm, for some reason we tried this today and it didn't work. :( Over half a gallon of milk wasted... Not sure what exactly went wrong! Do you think there's a time limit on how long the starter can be frozen for? It's probably been in our freezer a month or two.

    Such a bummer! We were gone for a while and were so much looking forward to homemade yogurt again. Now we'll have to wait until I can go to the store.

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  3. There will definitely be a limit on how long you can store a culture in a household freezer. The problem is twofold, first the temperature isn't low enough and second the defrost cycle.

    Both of these factors cause ice crystals to grow and migrate. When this happens they start cutting through the cell walls of the bacteria, killing them. With a short freeze time enough bacteria survive to make the next culture. The longer it is frozen, the more bacteria are killed.

    In the laboratory, cells (and bacteria) are stored in freezers at -125 degrees F and there is no defrost cycle, or in a bath of liquid nitrogen. This greatly improves the survivability of cells. Also, additives like glycerol are used to help limit ice crystal formation.

    You can probably get a culture to last during your summer vacation in the freezer, but don't count on having a "permanent" back-up in the freezer.

    A couple things might help longer survival of your culture in the freezer. Freeze it fast (promotes smaller ice crystals). Pre-chill the culture in the coldest part of your refrigerator before moving it to the freezer. Have as much surface area as possible, also to promote fast freezing. Also, culture in a high fat medium, like cream. The fat will help promote smaller ice crystals.

    It would be worth a few experiments to see how long it is possible to store your culture under optimal conditions. It will probably vary with each individual freezer, culture media and bacterial strain.

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  4. Libby,

    I am sorry it did not work for you. I have never tested the limits on how long you can freeze it. Like I said in my post above, I imagine it would be a few months, but you result seems to suggest otherwise. My experiment with it was using culture that had been frozen a few weeks.

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  5. Interesting thought that long-term freezing can destroy bacteria. I'm used to thinking that it has no effect on them - because that's what you're told from a food-safety angle. Of course, food such as meat would probably have dried out and lost its flavour long before it was effectively sterilised!

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  6. libby: I doubt very much that having your starter frozen for a month or two was the problem here. as you stated "Not sure what exactly went wrong". Good point. Most likely it was not the amount of time your starter was frozen.

    Regarding the "wasted" milk, not sure that was necessary either. I make yogurt a gallon of skim milk at a time, and one batch did not turn out. After 6 hours incubating, it clearly was not yogurt.

    So I simply added fresh starter from another source, let it incubate another 6 hours, and it turned out just fine.

    I was relieved my milk was not wasted.

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  7. I can back up the claim that frozen culture works. We've refreshing our yogurt strains with the same batch of Trader Joe's 6 culture plain organic yogurt divided into little baggies of 2 Tbsp each. I just pulled out the last baggie last week, after 5 solid months of being frozen, and it worked beautifully.

    We sterilize everything when making the yogurt (the milk, our 1 qt canning jars, the ladle) in hopes that the culture remains a bit more pure. We can usually make about 8 batches sequentially before the yogurt starts to get runny. Then we just pull out a frozen baggie and start fresh. It's been a great system.

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  8. Excellent! I just experimented with frozen starter from store-bought yogurt last night. I still haven't checked on my yogurt, two more hours!

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  9. This is great information! I have an heirloom Bulgarian yogurt that I culture weekly. I was worried that my coming 2 week vacation might mean the end of my yogurt. I have successfully dried and frozen sourdough starter for over a year and was hoping I would also be able to freeze some yogurt starter.

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  10. I came across your blog on google, great information thanks. I also came across this site which offered instructions for keeping your starter for periods longer than 4 weeks.

    http://www.culturesforhealth.com/storing-taking-break-yogurt

    Not sure if it works, I haven't tried it.

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    Replies
    1. I haven't tried that yet either. I never go more than 4 weeks without homemade yogurt!

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