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Does freezing microorganisms such as probiotics kill them? If not, what is the process that allows them to "come back to life" after the temperatures are increased?

As an example, lets say you freeze a yogurt (whether it be Greek, Australian, Turkish, Indian, Russian, European, or Western) and eat it while frozen. Would the probiotics in the yogurt "come back to life" and be beneficial?

Specifically, I am referring to freezing refrigerated yogurts, and not products that are sold as "frozen yogurts".

What's the biological basis for this?

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    $\begingroup$ I can't say anything about the bacteria in yogurts (usually Lactobacillus) but labs routinely freeze E. coli to -80 degrees C without killing them. $\endgroup$ – March Ho Apr 16 '15 at 21:36
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    $\begingroup$ @MarchHo Are those glycerol stocks? If so, the glycerol probably acts as a cryoprotectant. I don't know if anything in the yogurt would have similar enough properties to protect the cells. $\endgroup$ – user137 Apr 16 '15 at 21:52
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    $\begingroup$ This question inspired me to freeze some vanilla yogurt with added chunks of chocolate. No word on the bacterial content, but 100% of survey respondents agree that the results are delicious $\endgroup$ – Luigi Apr 19 '15 at 0:03
  • $\begingroup$ @Luigi Yep! For an even better culinary experience, stick a popsicle stick through the seal or lid before freezing it! In a few hours, you'll have a delicious frozen treat on a stick! $\endgroup$ – RockPaperLizard Apr 19 '15 at 0:08
  • $\begingroup$ I work for a probiotic manufacturer. We use a cryoprotectant in the process. We are able to freeze and thaw bacteria many times with little to no loss in cell count. Odds are most of the bacteria in your yogurt are still alive after you freeze it. If there is loss it is probably not much. Don't have any exact numbers, but know from experience. $\endgroup$ – Alex Aug 2 '18 at 16:43
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Yes, they do. For a look at survival of Lactobacillus and other bacterial species after multiple freeze/thaw cycles, check out Harrison 1955 (below).

The awesome hand-drawn graphs show that many bacteria survive after being frozen for 11 weeks. There's also a figure on the following page showing that many bacteria also survive after multiple freeze-thaw cycles, but I won't include that page here for brevity.

Prof. Don Schaffner of Rutgers also supports this conclusion:

So it's possible, even likely, that frozen and thawed yogurt will contain living lactic acid bacteria, although it may only be 10% or 1% of the total number of bacteria that were there before the yogurt was frozen.

I couldn't find a great deal of official resources on the topic, hence the paper from 1955 and random quote from a food science professor. However, in a lab setting, scientists regularly freeze bacteria and even eukaryotic human cell lines. Usually scientists use a cryoprotectant like glycerol, but many foods (like yogurt and ice cream) may contain an FDA-approved cryoprotectant (Wikipedia):

One recent, successful business endeavor has been the introduction of [anti-freeze proteins] AFPs into ice cream and yogurt products. This ingredient, labelled ice-structuring protein, has been approved by the Food and Drug Administration.

So, yes, at least some of the bacteria in your frozen yogurt are still alive, and will have whatever beneficial/harmful/lack of effect when you ingest them.

journal page

Judging from figures 1-3 above, bacterial survival depends on how long and how many times you freeze the bacteria. My rough estimates of the graph above say 40%-10% after a single freeze, and after multiple freeze-thaws getting into the 1% range. Also, it doesn't matter whether you thaw it first or eat it frozen as far as bacterial content is concerned, the bacteria will be unfrozen either way when they reach your digestive tract.

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    $\begingroup$ Thank you for your answer. After reading your answer, I realized that my question needed clarification. After you contributed your answer, I updated my question to clarify that I am speaking about freezing standard yogurts, and not products sold as "frozen yogurts". It's a little unclear if your answer needs to be updated accordingly. I'm sorry that my original question needed to be clarified. $\endgroup$ – RockPaperLizard Apr 16 '15 at 22:34
  • $\begingroup$ Thanks! Now the big question: how much survives? $\endgroup$ – RockPaperLizard Apr 17 '15 at 3:42
  • $\begingroup$ Judging from figures 1-3 above, it depends on how long and how many times you freeze the bacteria. My rough estimates of the graph above say 40%-10% after a single freeze, and after multiple freeze-thaws getting into the 1% range. $\endgroup$ – Luigi Apr 17 '15 at 14:25
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    $\begingroup$ Note that even 1% of bacteria left is still a lot of bacteria $\endgroup$ – Luigi Apr 17 '15 at 14:26
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    $\begingroup$ When thawed, wouldn't the remaining (live) Lactobacillus grow and populate in the yogurt? $\endgroup$ – Yan King Yin Feb 16 '18 at 4:25
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usually the growth of bacteria is slowed down as they are in freezing temperature and they almost carry out no metabolic reaction at this stage.but another problem occurs that is ice crystals formation which disrupts or pierce the bacterial cell wall causing the death of bacterial cells,this problem can be solved by adding glycerol or any other cryoprotectant into it which will make the concentration equal,inside and out side the bacteria,thus bacteria will not be harmed. in case of lactobacillus,they are psychrophile and also facultatively anaerobic so yes,they can survive in freezing condition http://www.ncbi.nlm.nih.gov/pubmed/10843056

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