Friday physics fun: baking cookies. Earlier today I made some Amaretti cookies. What is going on here?
There is a lot of cool physics and chemistry in baking. See "A Thermomechanical Material Point Method for Baking and Cooking" by Ding et al. 2019 simulating baking viscoelasticity, chemistry, heat diffusion and much more. math.ucla.edu/~myding/
My Amaretti recipe is simple: 2.5 cups almond flour, 1.25 cups sugar, 3 egg whites, 1/2 tsp vanilla extract, 1 tsp almond extract. This is in many ways the minimal cookie, far simpler than the baking of Ding et al.
Most cookies are flour based: wheat contains starch and the protein gluten. When wetted & worked the gluten proteins unfold and cross-link forming a stretchy network that can trap air bubbles (from leavening). Little gluten: crumbly, like shortbread. Lots: chewy chocolate chip.
Many cookies have fat (e.g. butter): this melts early on, making them spread out. There is a lot of interplay between gluten, sugar and fat in determining the kind of cookie texture and shape. doi.org/10.1016/j.jfoo…
Almond flour has no gluten, so the egg whites are the sole binder. As they are heated the protein chains unfold (denaturation), and then stick together into a network (coagulation). This holds the almond grains together. Since I had no fat, the cookies did not change shape much.
Heating also makes air bubbles expand, making the cookie lighter. If one whips the egg whites well beforehand there will already be a lot of bubbles, and the mechanical stress causes some denaturation and coagulation that stabilizes and binds them into the network.
The sugar in this case mostly melts and recrystalizes afterwards, making the cookies snap. This takes a while: today the cookies were chewy, but tomorrow they will be more snappy. Leaving them on the oven tray to cool helps the drying process and promotes snap.
My cookies were heated to 150C: high enough that the Maillard browning reaction happens. Amino acids react with sugars, producing a vast number of flavourful compounds. It can be boosted by making cookies more alkaline, e.g. add bicarbonate or egg white. en.wikipedia.org/wiki/Maillard_…
Had I heated them to 160C caramelization would have started. Sugars break down into lots of flavourful compounds with the caramel taste spectrum. en.wikipedia.org/wiki/Carameliz… Beyond that there is pyrolysis that reduces the cookie to bitter coal.
In the oven the water from the eggs (and alcohol from the extracts) will start to evaporate. That keeps the temperature at 100C until it is mostly gone: before that, no browning. There is diffusion of heat inward, and diffusion of water outward (in bread, making crust).
There is convective heat transfer via air, conductive transfer from tray underneath, and a pinch of radiative transfer. The parchment paper is mostly for non-stick, but it also allows a bit of downwards diffusion (my Al foil made the cores more chewy). prezi.com/9o385cdjwesq/p…
My friend @LenFisherScienc gained fame and fortune for analysing proper cookie dunking. The key is to judge the time (using the Washburn equation) until capillary flow soaks through the cookie, destroying cohesion. nature.com/articles/17203 lenfisherscience.com/92-the-art-and…
