I'd brought along a bit of kefir today to mix into a salad dressing but it ended up frozen in the fridge. I nuked it in the microwave to get it back into liquid form. Still cold, but I know probiotics don't stand up to heating. If, however, they are brought from solid to liquid form and still cold, would the probiotics survive?
And as a follow up - are frozen probiotics (fro yo, kefir, etc.) as potent as room temp or the fridge?
Yes, microwaving can kill bacteria. But it is the heat of the microwave, not the microwave's themselves that kill the bacteria.
So most likely, the vast majority of probiotics were not harmed, although some likely were sacrificed, especially any occupying the surface.
However, this sounds like a really cool science experiment. It would be easy to grow harmless bacteria, and then with a control (left on the counter), one in the freezer then microwave, one on the counter and microwave, compare the results. Try it out and post back your results.
As for freezing probiotics. It is very likely that freezing greatly reduces the bacteria, but does not kill off all of it. This study (done by a student who needs to learn how to label his graphs and table better (but I digress...)) suggests that the traditional method of producing kefir is more resilient than commercial products to freezing. http://etd.lsu.edu/docs/available/etd-04252012-155457/unrestricted/obrien-thesis.pdf
I use to make my own yoghurt with probiotic cultures (a mixture of Lactobacillus acidophilus LA1 and Bifidobacterium animalis ssp. lactis BB12). From my experience, it doesn't make any difference whether I microwave the milk to ca. 40°C inoculation temperature before or after adding the freeze dried starter culture, so I conclude that the radiation from an ordinary microwave oven doesn't hurt probiotic bacteria unless due to the heat it generates.
Freezing won't harm the probiotics but it will slow them down--kind of like a movie in slow motion. Heat can kill them, but whether or not the heat from the microwave was enough or not, I don't know. Microwaves "excite" the molecules, that's what generates the heat, so I would expect that some of the probiotics did get harmed, but probably not all.
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