Eggshell Calcium Carbonate Leavening Part 2

If youve been following this blog lately, you know that were engaged in a multi-week battle of wits with a pile of eggshells.  Specifically, were trying to figure out a way to isolate calcium carbonate from eggshells to use as a leavening agent.  The calcium carbonate is bound up in a matrix of protein that makes it less accessible for leavening action, so for maximum leavening effect, we have to either dissolve away the protein or dissolve away the calcium carbonate and then regenerate it.  Last week, we tried boiling ground-up eggshells in lye to dissolve away the protein.  (It didnt work very well, but at least the biscuits were tasty.) Today, we take a look at the other option--dissolving the calcium carbonate and regenerating it.

The first thought we had was that the CaCO₃ in the eggshells can be dissolved by the acetic acid in vinegar to make calcium acetate (Ca(Ac)₂), which can be decomposed to CaCO₃ around 400 °C.

Unfortunately, some of the eggshell proteins are also apparently soluble in vinegar, and when we made calcium acetate by dissolving eggshells in vinegar and evaporating all the liquid, we ended up with a light-brown colored solid, which yielded a gray powder after a clean cycle in the oven (which gets close to 500 °C).  We got a similar looking powder when we put ground whole eggshells through the oven clean cycle.

The product from calcining eggshells in the rocket silo was actually a little darker colored.  As a point of reference, were looking for CaCO₃ as a fine, white powder.

This is actually a problem thats bothered us since we wrote about grinding up eggshells way back when this blog was just an infant.  While its usually possible to burn organic matter (e.g., proteins) off of inorganic residue (e.g., wood ash, glass, stainless steel) at 400-500 °C (750-930 °F), eggshells hold on to the organic matter from their protein until 900 °C (1650 °F).  Unfortunately, at that temperature, our desired CaCO₃ has transformed into lime (calcium oxide, CaO).  Thus, its no surprise that when we put a pile of eggshells in our oven and set it to the clean cycle, our pile came back grayish-colored instead of the white color of pure CaCO₃. (Although, we were surprised at the time since we hadnt done much reading on the topic!)

So, were 0-for-2 on getting our pure CaCO₃ out of the eggshells at this point, but its worth noting two things.  First, while we havent been able to get pure CaCO₃ from eggshells, the gray powders from either the decomposed eggshells or the decomposed calcium acetate react much more vigorously with vinegar than the raw eggshells.  Still not as vigorously as baking soda as the video below shows, but bubbles abound nonetheless.  So, maybe the gray powders are worth trying as leavening. 

Second, can we approximate a best-case scenario for obtaining pure CaCO₃ from eggshells?  Yes! We can get a bag of pure CaCO₃ for a couple bucks at the local homebrew store.  So while our blog post declaring victory on purifying CaCO₃ from eggshells will have to wait until another day, we can still see what a best-case scenario for eggshell-based leavening would look like. Biscuit baking time!

Same recipe as last time, but only four sets this time: no leavening, gray CaCO₃ from eggshells, white CaCO₃ from the homebrew store, and NaHCO₃ (baking soda).  Very similar results as last time, too.  The gray CaCO₃ biscuits are definitely more risen than the no leavening control, and similar to the biscuits we baked last week from raw and lye-boiled eggshells.  The white CaCO₃ biscuits were noticeably more risen than the gray CaCO₃ biscuits, but still couldnt hold a candle to the baking soda biscuits.

The textures of both sets of CaCO₃ biscuits were similar to last weeks results, too. Not completely cooked through at the 20 min mark, while the baking soda biscuits were definitely done. 

The effect is more pronounced for banana bread.  Can you guess which loaf used gray CaCO₃ from eggshells as leavening? (Hint: its not the one on the right--that one had baking soda.) The grand conclusion from all these experiments?  Even though the CaCO₃ releases carbon dioxide gas when mixed with an acid (same action as baking soda), the slower reaction kinetics mean that eggshell-based leavening cant get the job done.

Have you ever baked with eggshells or tried to isolate CaCO₃ from them?  How did it turn out?