Monday, December 20, 2010

How did my students blow up churros in a fryer? - from Mrs. Jamison's cooking class in East Dubuque, IL

Here is the situation:
"Once again my students in cooking class have given me a reason to contact you again.  Two young men were making churros, they boiled the water, flour and sugar and were then to beat in eggs, one at a time, beating well after each.  In their rush to beat the girls and get to the fryer faster, they put the eggs in at once and mixed.  They ended up with pieces of cooked egg in the mixture.  I suggested they try to work in some of the egg to make it smoother and so they kneaded it - a bad idea.  Fifteen - thirty seconds after they placed the churros into the hot oil, some churros exploded out of the hot grease, up to the ceiling and onto the floor.  After a brief pause a few more flew out.  I eventually go to the fryer to pull the plug and fortunately the boys had walked away from the fryer after putting their churros into the oil.  NO injuries, just a mess.
What reaction did they cause when they kneaded the dough?"
I am starting to love this blog. Thanks for the great question! 
I don't think the problem came from kneading.  What you're describing sounds like a steam explosion!
Since the boys did not blend the eggs properly into the hot liquid, and then were able to knead the dough (so the dough was really thick), the density of this churro batter was probably way off. Density is a measure of weight per volume - so a block of cheese is really dense, but a an equally sized marshmallow is less dense. Churros are supposed to be an airy, lighter dough that frys up nice - kind-of like funnel cakes - sounds like these boys had a super dense dough. When they put the churros into the fryer, the outside of the churro rapidly cooked, sealing the moisture inside. Then the water inside the churros heated rapidly, turned into steam, which expands - and voila!  Steam explosion!  Steam explosions occur when trapped water almost instantly turns into steam and rapidly expands. Your students who made the dough correctly didn't have steam explosions because the steam was able to escape bit by bit. 
Steam explosions are very common - you might even say that water is a very, very dangerous substance. The ingredients for a steam explosion are: trapped or sealed water + very rapid heating = steam explosion.
I am reluctant to give other examples, as your students might get ideas.
Cooking is chemistry!     


  1. Too water put less water and never blowup

  2. I made the churro dough with just water, flour and salt and they were blowing up in the oil. Do you know why?