I've been making stock from vegetable scraps I've been keeping in the freezer (mainly carrots, celery, onion, tomato, and parsley). I read elsewhere that one could include egg shells in stock. Is this a good idea? How many should I include?
I've not heard of egg shells being used that way. I'm not sure what they would add.
The classic way of clarifying a meat stock to make it crystal clear (ie: for a consommé) is to whisk egg white (and I know at least one chef who adds crushed up egg shell to this mix) and finely ground meat into the cold stock and then gradually heat it. As the added ingredients cook they rise through the stock trapping all the bits that make it cloudy and the gunky 'raft' can be skimmed off the top.
All the ends and bits of vegetables, including onion skins, etc., are the most nutritious parts of the vegetable and lend flavor to the stock. It's great if you can save them and add them in.
As for eggshells, it's some of the most bio-available form of calcium (add a little vinegar when boiling), very similar to our own composition. I haven't tried it yet, but will be saving my shells from now on. It always felt a little weird to throw them out, and am excited now that I can stick it in the freezer collection bag.
A classic way to clarify stock is to stir in some beaten eggs and then bring the stock to a simmer. The egg proteins coagulate, rise to the top, and form a sort of strainer that filters out the bits and pieces that would otherwise make the stock cloudy. I've seen some recipes that instruct to you to break up the egg shells and mix them into the eggs before adding to the stock. I always assumed that the shells just add bulk and structure to the egg raft, helping it to hold together and form a better filter.
If you watch the Travel Channel episode on The God of Ramen -- renown ramen soup shop in Japan -- you will see that the chef there uses the egg shells to clarify the broth. That is all that is said about it. You will see the egg shells floating atop the soup, which also has lots of ground pork and other ingredients simmering along with them.
When they serve the soup, they ladle it into a strainer first, so that the broth is clear.
Like many other posters, I think egg shells will not change any culinary property of your stock significantly. However I do have heard about egg shells being used by people who want to increase the nutritional value (egg shells are rich in calcium)
However I never use them since I rather go with more a pure and traditional taste. Something for example that seems being forgotten (specially at homes) but still enhance the stock are chicken feet. If I were making chicken stock, I would make sure that above all things, the feet are not missing.
Eggshells will absolutely not harm you. I have a friend that I used to work with at his catering Company. He is a chef from Scotland. Where he went to culinary arts school. He was also the head chef at HCA. Very good at what he does. He always used eggshells in his stock. And like I said he is the best.