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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?

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They will have no culinary benefit. I guess that adding shells in a slightly acidic stock will leach calcium from them, so there could be a nutritional reason for adding them. But then, we can't give you a good answer, because this site isn't for nutritionists. My unqualified opinion is that adding a calcium-reach vegetable like broccoli is less hassle. On a side note, stock is very susceptible to the "garbage in, garbage out" principle, you only get a mediocre stock by using "scraps". –  rumtscho Nov 9 '11 at 11:53
    
Should I throw this stuff out instead of bothering, then? –  Michael Hoffman Nov 9 '11 at 17:24
    
I don't know what you mean by "this stuff". For scraps: If you just freeze good quality veggies while still fresh, and then use them at once in stock, it's OK. If you cut off the almost-bad parts and freeze, or freeze veggies after they have withered, and then make stock, stop that. For egg shells: I'd say throw them out, but again, I don't know how much calcium you'll get by cooking them, or how much your body needs that additional calcium, or if you can absorb eggshell-leached calcium easier or harder than calcium from broccoli, cheese or pills. –  rumtscho Nov 9 '11 at 17:51
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My rule is that if I wouldn't eat it as it is, it doesn't belong in a stock. Carrot peels and celery ends are removed because they are too dry to be good, and onion skins are practically inedible. The whole point of a stock is to make a flavor concentrate, and recycling the things you wouldn't eat means you concentrate flavor which is missing, wrong, or gone bad. –  rumtscho Nov 9 '11 at 19:03
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Good on you for keeping your scraps and making stock yourself. However as long as you are freezing the items chances are you will always get a cloudier stock than if you had used fresh. The freezing breaks down the cell membranes and will cause smaller particles to float about. Of course you also get more flavour so it's a give take thing. Egg shells are just old school "I refuse to throw anything out!" French thinking. As you could use them in a raft as stated from others, and the stock should only simmer (90-100F) that means salmonella issues from the shells. Salmonella dies at 140F+ –  Chef Flambe Feb 21 '12 at 8:44
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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.

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Adding egg shells to a brown stock is a great way of clarifying the base. It creates a 'raft' which helps absorbs the impurity's which would otherwise spoil your stock.

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Egg shells? I have heard of egg whites used this way, but never the shells. How do they absorb, and don't you risk ending up with crunchy pieces of shell distributed through the stock? –  rumtscho Jun 17 '12 at 11:17
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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.

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The use of a raft (as correctly described by @Caleb) is for the production of consomme. If you do this, be VERY careful afterward (when separating the consomme from the raft and stock materials). If the raft breaks, all the 'crap' will come out and cloud your product. –  mrwienerdog Jun 19 '12 at 17:25
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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.

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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.

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