Why cookies don't soak linearly in function of temperature?

Question

I like to soak cookies in large coffee mug for breakfast. As a direct result of this, sometimes I try to soak the cookie in too quickly. The whole story is this:

• When I try to soak the cookie in and the coffee is at a lower temperature (around 40-60 C I assume) the soaking happens in a couple of seconds and it's nice and soft. Like pudding. I can eat it straight after that.

• So, I would expect that when I soak the cookie too quickly and the temperature is very high (80 or 90 I think), the cookie will dissolve in a split second. But that's not what happens! After the same soaking time (2s), the cookie results soaked only at the borders, but not inside. So it's kind of unpleasant to eat and I have to eat a half soft and half hard cookie.

Bonus question: Why is it easier to soak the cookie in vertical position and more difficult when it's horizontally placed?

The cookie in question is a Grancereale cookie, an Italian brand

Answer 1

A cookie is roughly starch granules held together by sugar and sometimes fat. At higher temperatures this matrix dissolves faster, and this makes it fall apart quickly. However, at higher temperature the starch likely absorbs water too, and this will make it expand.

Len Fisher observed that biscuit dunking is accurately modelled by the Washburn equation. That is, the distance the liquid reaches is $\sqrt{\gamma R t/2\eta}$ where $\gamma$ is surface tension, $R$ pore size, $t$ time and $\eta$ viscosity. He also noted the starch swelling has a significant effect on pore size. So if the starch swells fast enough $R$ goes down a lot, and the diffusion is inhibited.

So my theory is that the fine grains of milled flour on the surface of this type of cookie expand quickly and block diffusion at high temperature, while at lower temperature the water gets further.

(Water viscosity also decreases with temperature, but I assume this is a smaller effect than the pore size change.)

Addendum: I asked Len about the issue, and he responded:

From my earlier work (unpublished) starch granules have a very sharp swelling transition, somewhere in the range 60C - 65C (potato starch is 62C). Below that they are rock-hard; above it they take up water in impressive amounts, swelling (in the case of potato starch; others are similar) by some 70x. So you are quite right that this would block liquid flow through the packed granules, but in biscuit dunking hot tea or coffee probably doesn't reach the swelling temperature. It would have been interesting to test the effects of small temperature changes in this range, but life's too short.

80-90C drink is usually beyond normal tea and coffee, but seem to be hot enough to cause major swelling.