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At once light and flaky, multi-layered and complex, they challenge the conventions of both science and baking. Rarely do flour, butter and yeast come together in such a flavorful feat of engineering.

Chef Jürgen David, ICE’s Director of Pastry Research & Development, talks us through the science behind croissants, explaining the ingredient nuances and technical factors that make this iconic French pastry possible.

Croissants 101: Understanding Leavening

To fully understand croissants is to understand the multi-stage rising process that happens when you make them.

“Traditionally, bread is raised by using yeast and letting it ferment,” says Chef Jürgen.

In what is considered biological or organic leavening, yeast feeds on the sugars and starches in the dough, breaking them down and releasing carbon dioxide in the process. “That carbon dioxide gas will be trapped by the gluten that you developed in the dough, and that's how bread becomes puffy.”

There is also chemical leavening, such as what happens with baking soda or baking powder, and mechanical leavening, wherein air is whipped into the dough via egg whites or cream.

Croissants are made by applying a highly technical version of the latter method — going “a step beyond,” says Chef Jürgen — known as lamination.

“If you are creating layers of butter and dough, then you get lamination,” says Chef Jürgen. Steam is created when the layers of butter react with heat, which then lift the layers of dough.

In the case of croissants, an intricate folding process, outlined below, creates a multitude of layers for this process to happen on a many-tiered scale. According to Chef Jürgen: “The biological leavening combined with lamination is what makes croissants soft, flaky, and light.”

Croissants 201: Creating Lamination

Lamination comes down to a complex process of folding, rolling and turning to create a finished product that, before baking, has dozens of distinct layers of butter and dough stacked on top of one another.

For recreational bakers, one of the most surprising aspects of croissant making (speaking from experience) is that butter — just like dough — can be rolled flat and remain firm while you manipulate it. That’s because it’s a fat whose melting point is above room temperature. This means that, so long as you aren’t working in overly hot conditions, butter will stay solid during the croissant formation process.

To laminate your croissant dough, follow the below steps.

1. Make a "package." Form a flat, rectangular layer of butter, and encase it within a larger rectangular layer of yeasted dough to create a “package.”
2. Begin your book folds or letter folds. “You then have this one butter layer in the middle, and [you roll] it into a rectangle from there,” says Chef Jürgen. “Then you fold it into either a book fold, or a letter fold.” (A book fold takes the dough and butter package and creates four layers; a letter fold creates three.)
3. Fold and fold and fold some more. Roll dough into another rectangle and fold again — then roll again and fold again several times over. Depending on the type of fold used (book or letter), each roll and fold will triple or quadruple your layers of butter and dough.
4. Cut, roll, crescent! Cut and roll dough into a crescent shape.

Ingredient Considerations: Butter and Flour

Given its complexity, croissant making requires quality ingredients to ensure that the techniques involved can achieve the desired results: a tender dough with distinctive layers that will create air in the pastry while it bakes.

“What type of butter you have matters,” says Chef Jürgen. “The less water content you have in the butter, the more bendy it will be,” he says, making it easier to roll and coax into the necessary shape for use in croissants.

“The more water content you have,” he adds, “the more likely it will shatter in the process.”

American-made butter must contain 80% butterfat in order to be called “butter.” However, in the E.U. butter must be between 82 and 90% butterfat, making it a better choice for croissants.

As for flour, the type you use is important. That’s because both mechanical leavening (lamination) and biological leavening (yeast) are required for croissant making — and gluten is required for both. (Recall that gluten is necessary for trapping the carbon dioxide given off by the yeast.)

“You want a high gluten flour,” says Chef Jürgen, “so not cake flour.” All-purpose flour typically has a high enough gluten content to be put to use in croissants, and many croissant recipes call for a mixture of both AP flour and bread flour.

Mixing, Resting, Chilling, and Proofing

The mixing and shaping of croissants comes together in several stages, as described above, each of which requires considerable attention to detail.

Mixing the dough

Like any dough, the intended outcome governs how the dough should be handled. Crusty breads have a very different process than delicate cakes and pastries. In the case of croissants, “you want to make sure that the dough is combined and there are no dry spots, but you don't really need to develop it very much,” says Chef Jürgen, “because you are going to work it quite a lot more by rolling.”

Proofing the dough

One of the most challenging parts of croissant construction is the proofing stage: The temp has to be warm enough for the yeast to activate, but not so warm as to adversely affect the butter. “You have to make sure that wherever you put the dough does not get too hot, because then you will melt the butter out,” says Chef Jürgen. “It will make a big puddle.”

Resting the dough

Resting is an integral part of croissant science, and it happens at several points in the croissant creation process. That’s because croissant’s dough — owing to the extensive rolling and manipulation required to layer it — gets more of a workout than most breads and pastry. “Resting helps the gluten relax,” says Chef Jürgen.

Chilling the dough

Chilling the dough at several spots during croissant formation is key to keeping the butter temperature low. While butter stays solid at room temperature, there’s risk of it becoming warmer during the multi-stage rolling process, so taking turns in the fridge is important.

Avoiding Croissant Pitfalls: Temperature

Croissants are challenging to make, and require patience and attention to detail. One final note on the science behind croissants: Be aware of temperature throughout the process. Both too-warm butter and too-cold ovens are common pitfalls.