Curdling

[buddyrv]

What is it with cream and lemmon juice that causes curdling in some cases and not in others?

Is it the temperature being too hot that breaks the cream into a curdle?

Thanks

[THINKVAIO]

Well maybe it is because the cream is a dairly and it curds...maybe not....

[phatch]

There's probably lots of factors in this issue. Cream is rather different through out the year, winter cream is fattier than summer cream on average. Cartons don't tell you what you get of course, but the time of the year should be sufficient.

There has been a trend to less fatty creams anyway, meaning you're getting more protein in the cream. Protien is what curdles. Pasteurization methods could also play some tricks. Whether they used the high heat or low heat pasteurizaion. UHT Cream is even more different in flavor and texture, but I bet it wouldn't curdle much.

Again, Lemons vary in acidity too depending on growing conditions.

There are so many variables outside of your control there is no simple answer.

Phil

[Pete]

I have also noticed that it seems that older cream (closer to its expiration date) curdles more easily than fresher cream. Anyone else notice this also??

[kokopuffs]

It's a fact taught in general chemistry class (I took it 25 years ago). As I faintly remember milk (as well as cream) is either a suspension, a colloid, or a colloidal suspension. And somehow the addition of either an acid or ionic substance promotes precipitation aka separation.

That's how cottage cheese is obtained from milk - by the addition of lemon juice which causes separation.

For more chemistry info pertaining to colloids, visit this site:
http://www.ifm.liu.se/~boser/surfacemodes/L18.pdf

[phoebe]

According to McGee, (On Food and Cooking: The Science and Lore of the Kitchen ) milk has lots of different kinds of protein, but the one concerned with here is casein. It comes in "bundles" of components and the bundles are called micelles, 1/10 micron across. Curdling can be caused when acid is added because it lowers the ph of milk from about 6.5 to about 5.3. The acid causes the micelles to lose the negative charge that normally keeps them repelling each other. When the ph is lowered, the charge is lost and they clump, forming curds

Aren't you glad you asked?

[shroomgirl]

and yet I've made lemon pot de creme....go figure.

[phoebe]

OK, I think we're quickly coming to the end of my abilities to follow all the chemistry involved , BUT. The acid of the lemon also lowers the ph of the egg (I'm assuming eggs are involved?) the same as it does for milk/cream. And in the same way the protein molecules then carry a lower charge (which normally repels) and thus bond together. I think the key is the difference between curdling and coagulation (maybe ) which has to do with temperature. I'm taking this from McGee again (regarding custards): "The lower the rate of heating, the greater the safety margin between setting and curdling." And I guess with a lemon-type custard, temp and time would be even more crucial?

[Nicko]

Shroom I would guess that when you are making lemon creme de pot you are cooking the lemon juice which breaks down the acid which will curdle the cream.

Pete I to have also noticed that older cream does curdle quicker.