MadSci Network: General Biology

Re: Will freezing yogurt for two weeks kill the probiotic/yeast cultures?

Date: Wed Oct 20 20:11:42 2004
Posted By: Don Schaffner, Faculty, Food Science, Rutgers University
Area of science: General Biology
ID: 1098166154.Gb

A few clarification and a few answers...

Yogurt should NOT contain yeast.  Yeast are different than bacteria.  They 
are actually more closely related to you and me than they are to bacteria!

By "lactic ferments" I think you mean lactic acid bacteria.

Freezing does kill bacteria, but not 100%.  This is why frozen chicken 
still needs to be cooked.  It may still contain Salmonella bacteria.

Depending on many factors (age of yogurt, speed of freezing, etc.) frozen 
yogurt may still contain viable (i.e. alive) bacteria.

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.

Since the yogurt is already quite acidic, and the yogurt would 
not "develop new cultures" upon thawing, however if those thawed bacteria 
were placed into fresh milk, at warm temperatures you might be able to 
make more yogurt out of that milk, but it might not be as good as the 
original yogurt (because of contaminating bacteria from the milk or your 
kitchen, or because the ratios of the two or three different yogurt 
bacteria have changed after freezing).

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