Sour cream and yogurt.

[Sue_Duhnym]

Once again…a boring question.

In this thread:

http://boards.straightdope.com/sdmb/showthread.php?threadid=46096

I asked:

How is it that when you open a container of sour cream or yogurt it seems almost solid, yet when you stir it, it becomes creamy and NEVER goes back to it’s original semi-solid state? And it never seems to leech juice (whey?) after it’s been stirred! What’s up with that?

Rocket88 answered:

Yoghurt is what’s known as a non-newtonian fluid; in other words, the yoghurt’s apparent viscosity (thickness) is dependent on temperature, shear rate (velocity of flow) and time. Yoghurt exhibits the specific property of thixtropy, IOW, it appears to become less viscous under flow; this in turn is related to the molecular bonds in the compounds that make up the yoghurt.

Water, OTOH, is a newtonian fluid; the apparent viscosity does not change appreciably with changes in flow.

Yet, while he’s obviously a genius, I don’t believe he answered my question fully. (What IS that liquid it leeches?!)

Thanks in advance.

[lesa]

I think the simple explaination is you don’t give it time to seperate. Yogurt may be in the carton for a few days before you buy it. This gives it a lot of time to settle and seperate. Many recipes have you leave the yogurt straining through a coffee filter overnight to make “yogurt cheese.” So the liquid will strain back out, and the yogurt will become more solid, if it has enough time.

If you want to try it, put the yogurt in a coffee filter. Put this in a sieve over a bowl. Leave this in the fridge overnight. The “yogurt cheese” can be used like a soft cream cheese. Or so they say.

Want to see what yogurt looks like under the microscope?
http://users.vr9.com/milos/yogurt.htm

This article seems to say that the watery liquid is…water. If I understand, the more milk solids involved, the denser the yogurt, leading to better water retention. Since most yogurt today is low fat or no fat I’m guessing that’s why it gets watery.

[Sue_Duhnym]

Thank you, Ma’am.