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Are there other dishes where this is done to increase the bio-availablity of calcium in a dish, such as pork knuckle vinegar stew, which is considered a post-partum dish in Cantonese style cooking.

https://www.thefooddictator.com/the-hirshon-chinese-black-vinegar-pork-trotter-post-partum-stew-%E7%8C%AA%E8%84%9A%E9%86%8B/

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    Uh.... this is borderline at best. We will not answer dietary questions, so the first part is off topic. And asking for suitable dishes is opinion-based, which is also off-topic here. So what remains is the part for which vinegar will work best. Could you please edit your post? – Stephie May 28 '20 at 17:46
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    yes, you can do that if you wish, but I wouldn't count on the calcium being bio-available. – dandavis May 28 '20 at 19:40
  • And the story of how (the majority of) the stew is shared with friends and family won’t help the mother recuperate, so your link seems to have contradicting claims. – Stephie May 28 '20 at 20:41
  • Both calcium acetate and calcium hydroxide are pretty soluble in water. It should go into the stomach just fine. en.wikipedia.org/wiki/Calcium_acetate – Wayfaring Stranger May 29 '20 at 1:59
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    @cbeleitesunhappywithSX of course bone growth also requires phosphorous and plenty of protein (as does bone healing which is why I've about this, having broken a few last year). So the interaction with phosphate uptake gets even more complex. – Chris H Jun 3 '20 at 15:07
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Addressing the food chemistry aspect:

Vinegar (acetic acid) reacts with calcium carbonate in the eggshells to make calcium acetate (Wikipedia), as in the naked egg experiment. Calcium acetate can be used, among other things, to gel alcohol; in food it can be used to coagulate tofu as well as having a stabilising effect. There's a chance it will act as a thickener depending on quantity depending on what else is present in the sauce; the chemistry is quite intersting.

This chemical reaction means you won't have as much acidity left in the vinegar as when you started but the additional flavour of all but the most refined vinegars should survive, so you can choose one that matches the other ingredients. For a Chinese-inspired dish, rice vinegar would be an obvious choice. It's not clear to me whether the calcium acetate itself would have a flavour, pleasant or otherwise

I can't comment on the nutritional aspect, but will just add a note of caution from a food safety point of view (actually 2):

Calcium acetate has a medicinal use and making and consuming medicinal compounds at home can't be assumed to be safe. In this case it treats cases of excess blood phosphate by preventing the absorption of phosphate in food; how much you'd consume, and how this relates to the levels seen in foods/medicines are not things we can help with.

If you wished to make a vinegar-preserved food (rather different to the example you've added since I started writing this) you'd have to take into account the fact that you've neutralised much of the acid in the vinegar, and therefore much of it's preserving power.

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    The question changed while I was answering it, but I think I've managed to make the answer on topic, even if the question is borderline – Chris H May 28 '20 at 20:33
  • Thank you Chris, for the food chemistry and safety aspect. – meractus May 29 '20 at 3:25
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    Of course, substances with medicinal properties cannot automatically be assumed to be safe. Calcium acetate however is quite harmless - the exception being persons who should avoid calcium-rich food, but the question basically precludes that. Calcium acetate is food additive E 263 and as such does not have a specific restriction on the quantity added (de.wikipedia.org/wiki/Calciumacetat). The bioavailability of the calcium (and in consequence also phosphate) is also no different from eating the egg shell as finely ground powder (other than this will lead to burping). – cbeleites unhappy with SX May 29 '20 at 11:02
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    @cbeleites English Wikipedia lacked the note about no upper limit, but I deliberately didn't look for an authoritative source as I felt I'd then need get into a discussion about dose and phosphate uptake. Your comment is helpful though (and I can just about make sense of the German Wikipedia article you linked to) – Chris H May 29 '20 at 12:57

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