The chemistry of custard

What’s happening at the molecular level when custard gets cooked?
19 November 2019

Interview with 

Ljiljana Fruk, University of Cambridge


Ljiljana Fruk and Phil Sansom sampling a Croatian custard recipe


Now that we know the how of making custard, it’s time to look at the why. What’s happening at the molecular level when custard gets cooked? Speaking with Chris Smith and Phil Sansom, chemist - and chef - Ljiljana Fruk...

Chris - Now Ljiljana, we've heard from Tristan about egg and egg proteins. He says they're critical to his original custard recipe, so let's look at that first. So what is the egg yolk doing to the mixture to make that happen?

Ljiljana - So egg yolk is full of proteins and fats and basically if you would centrifuge it, you would divide the egg yolk, you will see that there is a little bit more of the liquid part and a little bit more of grain part in the eggs. This is composed of fats and proteins mixed together. So when you basically heat up your egg, when you are in the process of making the custard, you are basically unfolding the protein structures. Proteins usually have globular structures.

Chris - Like a ball of wool?

Ljiljana - Yes. So they are like three dimensional and when you heat them up you unwind them. So they become a little bit elongated - longer. You do this because the heat is used as a catalyst to destroy a particular molecular bonding, by which of these proteins are held together.

Chris - Do we know what that is?

Ljiljana - So this is disulphide linkage. The linkage between these two thiols and this thiol molecules. And these thiol molecules they actually sometimes are very smelly.

Chris - And if you break those bonds, the protein strings out into long chains. But why does that make the custard then?

Ljiljana - This long chain now have the ability to interact with each other, a little bit easier. So what happens, you are creating almost a gel like structure and the beauty here is that you do need to have some minerals, so you need to add a little bit of salt and you need to add sugar. Sugar forms a tiny little coating on top of this molecular structures, which are now unwound so they don't kind of assemble very quickly. They don't form a very rigid structure and they don't turn hard.

Chris - So rather than a series of balls sitting next to each other, once you allow them to chain out into strings you can imagine that they're going to basically link the molecule - this raft of proteins or other things like the sugar - all together so you get this jelly forming, and that's what it's doing?

Ljiljana - Exactly; and so this process needs to be slowed, because if you heat up suddenly.

Chris - That's my next question! Why does overheating - and 'overegging' [excuse the pun] - why does that make a difference?

Ljiljana - Because you are then removing this layer of sugar that protects a really strong binding and strong kind of unregulated process. You are just destroying it if you heat slowly dispersing it, it has happened slowly or slowly interlocking all of these structures and you are creating nice gel.

Chris - So that's how egg works. What about the more - for the want of a better phrase - poor man's custard; the stuff that comes out of a packet that - we learned from Tristan - does not have eggs, it's based on starch molecules. How does that work then?

Ljiljana - Starch molecules are also involved in thickening information of this similar gel as the egg is forming. It is just the process is a tiny bit different.

Chris - So what actually is starch?

Ljiljana - Starch is the polymer made of glucose. That means that there are many glucose sugars which are bound together in the long chains.

Chris - So that's why it's similar to what the egg does: when you unchain the egg by heating and breaking those sulphur linkages, you get long chains that are sticky. You do the same thing with the starch.

Ljiljana - Yes, starch is made of glucose that doesn't have any thiols. So the principle is a little bit different, because the structure of starch is different so it doesn't contain any thiols. So, instead, if you heated starch in water, the granules are formed, which are embedding lots of water, they swell. As long as you keep on heating, the viscosity of the whole solution will change because now you have these huge granules, which are formed and at a certain temperature, that will burst; and as they burst, they release tiny pieces of molecules, which then act to cross link these long linear pieces. So, basically, what you are now creating is a gel, which is a combination of different molecules and molecular pieces, made of glucose, which previously have been part of the starch.

Chris - So it amounts to the same outcome. It just uses a different chemical root and different ingredients to get it. But the basic premise is that you end up with long chains of molecules that are also cross linked together. That forms this challenge.

Ljiljana - That forms this beautiful texture that we all like.

Phil - Liliana you've brought in some custard for us to try. But it's not a custard that I've seen before, what's the story behind that?

Ljiljana - This is the best selling Croatian custard and it actually it is a poor man's custard because it doesn't have any eggs, but instead it uses starch and a little bit of sugar and it can be done with water, it doesn't necessarily require milk but it's much tastier if there is a full fat milk that is used for making this. Actually this was the kind of a tiny social revolution when this starch based custard came and became available to the masses. Eggs are expensive and they have been expensive and so they were considered a luxury product and anything that was made with eggs was a luxury product. So having a possibility to have a wonderful custard, made without eggs gave the opportunity to everybody to enjoy a beautiful dessert.

Phil - Equal opportunity custard!

Ljiljana - Equal opportunity custard. I need to tell you that in Croatia, we have this luxury version custard which we all really adore and it's made with liquor which is made of rose petals!

Chris - Oh wow. Well next time you come, you can bring that because you're going to be opening a restaurant every time you come on this programme. We were talking about your restaurant you're going to open in Zagreb, a very beautiful city! Hello to my Australian friends at the Australian embassy in Zagreb. Libby Petrovich is the Australian ambassador, who made me very welcome couple of years ago. I'm looking forward to coming back and seeing them all. I'll I'll drop in and see your restaurant - you're going to serve this?

Ljiljana - You should. Oh we will definitely serve the rose petal version!

Phil - I'm excited. I want to go see it!


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