How does cornstarch prevent curdling?

Love that question, Susanna! The lemon meringue pie recipe below has cornstarch in both the filling and the meringue, to serve as both a thickener as well as a stabilizer in the event the egg proteins get too hot. But how exactly does that work? Well you may remember me talking about “the clenching fist” in the past, a metaphor that describes what happens when intertwined egg proteins get too hot and curl up…squeezing out water as they tighten into little clumps. That’s curdling. It can’t ultimately be prevented but it can be forestalled by the addition of starch to the mixture. Starch molecules do a couple of things in a custard. First, they absorb some of the heat energy, thus protecting the more delicate proteins. Second, they get in between the proteins, making it harder for them to curl up tightly. Eventually they will of course, if the heat remains too high for too long, but starch can perform amazing feats as an insulator and curdle inhibitor. Thanks, Susanna!

4 thoughts on “How does cornstarch prevent curdling?”

  1. What about gums? Like when used to stabilize an ice cream base? Do they have similar anti-curdling powers? I assume the quantities are too low to make a dent in the heat energy, but they’re pretty good at inhibiting motion within a suspension …

    1. Hey Paul!

      Gums are starches so yes, they would work well too! Great thinking, Paul!


      – Joe

  2. Hi Joe,

    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:

    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?

    Thanks so much for your help!


    1. Hi Helen! Very interesting. I’ll put my answer in a post, some readers might have additional ideas for you as well!

      – Joe

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