Reader Helen writes with a very interesting problem:
I’ve had a pastry cream disaster that remains a mystery to me. I’ve been making it with no problems for a long time. Just made 4 batches 2 weeks ago. Today, I tried it 3 times and every time it curdled as soon as it came to a simmer. I’ve never had this problem before and usually simmer it for 1-2 minutes to make sure to kill the enzyme in the yolks that thins out the starch as the cream sits. The only thing I did differently today was use all new ingredients (milk, cream, and eggs). I tasted milk and cream and they didn’t taste spoiled. I noticed that my corn starch expired 2 years ago, but it worked fine 2 weeks ago, so I can’t imagine it went bad all of a sudden. Here is my recipe and procedure: http://www.beyondsalmon.com/2014/07/pastry-cream.html
It’s pretty standard stuff. When the cream curdled, it looked like broken mayo with fat oozing out. The only thing I can think of as a bit unusual is that my eggs were very fresh from a local farm. I know that would be terrible for hard boiled purposes, but I’ve never heard of the age of the egg effecting pastry cream. Any ideas where I might have gone wrong?
Very interesting problem you have there, Helen. Any time there’s curdling of any kind I always think about heat first. Could the heat have been higher than normal? If so it’s possible that the cream on the bottom or sides of the pan might have been hotter than normal even though the whole mass hadn’t simmered yet. Whisking might not have helped much in that case. That’s the simplest explanation.
Still, from your description it sounds more like a broken emulsion than a curdled custard (cooked egg proteins usually form small grains). You see this with buttercreams when there’s a temperature disparity between the butter and the meringue. I see from your formula that you like to add butter to finish your pastry cream. Had you added it by this point? And if so might you have added faster than normal? Or might it have been colder than normal? That’s another possibility.
Barring those two possible causes the eggs are the next most likely suspect. Custard, like hollandaise and mayonnaise, relies on the emulsifiers found in egg yolks to remain stable. The main emulsifier in egg yolks is lecithin, a fatty substance (a so-called phospholipid) that’s responsible for keeping the oil-in-water custard emulsion smooth and even. The thing is, lecithin levels aren’t necessarily constant from egg to egg. They can fluctuate depending on the diet of the chicken. Some very interesting recent research from Israel (where researchers have been working to develop a low-cholesterol egg) has demonstrated that dramatic changes in egg fat levels can be achieved with relatively minor adjustments to chicken feed. Those studies were focused specifically on cholesterol, though it wouldn’t surprise me a bit if a similar principle applied to a fatty compound like lecithin. That said, my suggestion is to go back to grocery store eggs which come from chickens that have a more consistent diet.
Those are my best ideas, Helen. Anyone else have any theories?