|Calcium carbonate (CaCO3) reacts with acetic acid in vinegar to make calcium acetate, carbon dioxide gas (CO2), and water (H2O).|
First, some eggshell chemistry. Eggshells are about 95% CaCO3, but the CaCO3 is bound in a matrix of protein, with a proteinaceous membrane also attached. Thus, one might expect that eggshells would make a better leavening agent if the CaCO3 could be isolated from the protein (and/or ground very finely) so that it would be more accessible to the acid during baking. The question is, how to get rid of the protein? We'll have to either dissolve the protein away from the CaCO3 or dissolve the CaCO3 away from the protein and then regenerate it somehow. Today we'll try the former.
There's quite a bit of precedent for dissolving away the eggshell protein (or at least, most of it) with a strong base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), but specific recipes are hard to come by. Several articles refer to this original gem, the most useful being this one, which allows us to deduce that those guys boiled their eggshells in a 2.5 wt% NaOH solution for 5 minutes, which easily removed the membrane and part of the protein matrix. They then increased the lye concentration to 10 wt% and boiled for a long time, finding that all the protein that could be removed was gone by about 7 hours. They didn't give a lye-to-eggshell ratio, though. Additionally, this patent references another patent (we couldn't track down the original) claiming that boiling eggshells in 3 wt% NaOH would reduce the protein content of the shells to < 0.1%, although the boiling time and lye-to-eggshell ratio wasn't specified.
A protein content of < 0.1 wt% sounds good enough to us, so we decided to follow that route most closely. Having to guess on the time and lye-to-eggshell ratio, we decided that if we had to boil for more than half an hour and use more than a 1:1 ratio, that it wouldn't be worth our trouble. (In that case, we'd just use Leigh's ash-based leavening instead!) Alright, experiment planned; let's do this!
|Here's our recipe: 15 g NaOH, dissolved in 500 g tap water, with 15 g coarse-ground eggshells (1-2 mm particles) added. Boiled for 30 min. Wear safety glasses and gloves until everything is neutralized later on (see below).|
|As the mixture simmered, the lye water turned a cloudy yellow. A good sign that we're dissolving protein.|
|The filtrate is still highly caustic, so be careful with it! We wanted to neutralize it before doing anything else, so we added a couple tablespoons of our good ol' red cabbage pH indicator, causing the filtrate to go from yellow to slightly-darker-yellow. Note if you're following along at home--dumping the filtrate down the drain without neutralizing might kill some of your friendly septic system bugs, so please neutralize!|
|Drying the lye-boiled eggshells makes them easier to work with. In the oven at 300 °F for 15-20 min ought to do the trick!|
But we're not done with these experiments yet! Stay tuned for Part 2, where we dissolve and regenerate the calcium carbonate part, and see how that works as leavening!