Monday, February 29, 2016

Eggshell (Calcium Carbonate) Leavening, Part 2

If you've been following this blog lately, you know that we're engaged in a multi-week battle of wits with a pile of eggshells.  Specifically, we're 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 didn't 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.

Hypothetical route from eggshells to calcium carbonate; doesn't work in real life
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.

Calcium acetate calcined at ~500 °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.

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

This is actually a problem that's bothered us since we wrote about grinding up eggshells way back when this blog was just an infant.  While it's 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, it's 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 hadn't done much reading on the topic!)

So, we're 0-for-2 on getting our pure CaCO3 out of the eggshells at this point, but it's worth noting two things.  First, while we haven't 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!

Biscuit leavening comparison: no leavening, calcium carbonate, and baking soda
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 couldn't hold a candle to the baking soda biscuits.

Biscuit texture comparison: no leavening, calcium carbonate, and baking soda
The textures of both sets of CaCO3 biscuits were similar to last week's results, too. Not completely cooked through at the 20 min mark, while the baking soda biscuits were definitely done. 

Banana bread leavening comparison: calcium carbonate and baking soda
The effect is more pronounced for banana bread.  Can you guess which loaf used gray CaCO3 from eggshells as leavening? (Hint: it's 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 can't get the job done.

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

EDIT: After going through our wood ash leavening experiments and realizing that we could get good leavening effect by incorporating more of the leavening agent, we came back and baked another set of biscuits with 1 tsp of the finely-ground eggshells (i.e., with quadruple the amount of eggshells of our recipe).  The biscuits were definitely still not as light and fluffy as the baking soda-leavened biscuits, but were better than anything we had achieved so far, and actually, not too bad on texture.  So, if you do any experimenting on your own, start with at least four times the volume of ground eggshells as the recipe calls for in baking soda (e.g., if the recipe calls for 1 tsp baking soda, use at least 4 tsp ground eggshells).


  1. Not sure I would want to try eating the grey ones...just saying! But the other ones look worth a try. Did you try eating any of them? Did they taste different?

    1. We ate all of them. :-) They tasted pretty similar, except the baking soda ones tasted more...baked, or, more toasted, I guess. The other three tasted more like the dough/batter.

  2. It seems to me that trying to extract the CaCO3 either way is a fairly big undertaking, although for the sake of the experiment, well worth it.

    Just to clarify the edit - you used eggshell powder like you made in the original eggshell post (Feb. 2013)? Was it pretty much a true powder? Any grittiness? What I'm wondering is if it was heavier than, say, baking soda and if that would effect the rise.

    1. It's definitely turned out to be a bigger undertaking than we expected! The main motivation is if we wanted to use it for something else where we wouldn't really want the rest of the impurities hanging around, like toothpaste or scouring powder. Even for that, it's probably more trouble than it's worth!

      But yep...same jar of ground eggshells from 2013 actually! There wasn't any grittiness, even with the coarser-ground stuff. I think it was actually reacting pretty much all the way with the vinegar during baking, just not fast enough to do good leavening.

      To your question about the powder size and heaviness, you're right that it would have been better to compare by mass instead of volume. But unfortunately, our kitchen scale isn't that sensitive! :-)

      But we can calculate what ratio we should have used, at least for the store-bought calcium carbonate. For example, this chart gives the bulk density ratio of baking soda:calcium carbonate as about 1.13:1 (actually, 801:700). Then, you could say that baking soda is about 52% CO2, while calcium carbonate is about 44% CO2. Combining those numbers, we should use about 35% more calcium carbonate to get the same leavening effect.

      We did actually try it with 4x the calcium carbonate loading (recently added a note at the end), and it did help. But still not close to the baking soda biscuit. :-)

    2. Thanks Jake, good information.

      Have to agree the baking soda biscuits are best. So while I'm glad to have an alternative if needed, I'm stocking up on baking soda nonetheless. :)