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I am interested in knowing how I can increase the solubility of gelatin, marrow, and minerals in my stock.


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6 Answers 6

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In terms of commercial food processing, there are more efficient ways to extract gelatin than slow-simmering the bones, generally by treating the organic matter with a strong acid prior to boiling, then using commercial evaporation and filtering equipment which is far more efficient than anything a home or even restaurant cook has access to.

According to Gelatin Food Science (see the "Gelatin Manufacture" section) it can also be first treated with a strong base solution before acidifying it, which lowers the isoionic point. It's kind of difficult to explain exactly what that is if you don't have a background in organic chemistry; technically speaking it's a relationship between pH and electrical charge - but applied to extraction it refers to the pH at which the solubility of a protein (such as gelatin) is the lowest. Lowering this is a good thing in extraction, because it means that the gelatin will be easier to filter out in an acidic solution.

Thus I have to point out the question is actually a bit contradictory; if your goal is the extraction of gelatin then you want to decrease the solubility.

But I think this is all going to be beside the point anyway, because none of this applies to stock making; the goal in food processing is to extract the pure gelatin, not to get a flavourful stock. When making a stock you definitely don't want to use an acid solution, it's going to ruin the flavour.

Realistically, when it comes to stock-making, especially at home, the only way you're going to be able to extract more gelatin is to simmer it longer. That's it. When the bones break without any resistance, that means you've denatured all the collagen and you've got all the gelatin you're going to get. I wouldn't worry about solubility because typically in stock-making you're already using more than enough water to dissolve all the gelatin that you could possibly hope to extract.

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@Aaronut I don't understand why I would want to decrease solubility? –  David Dec 8 '10 at 1:42
@David: Because extracting the gelatin means you want to get it out of the water, which you can only do if it isn't dissolved. You want more gelatin in the water, which is the opposite of extraction. –  Aaronut Dec 8 '10 at 1:47
@Aaronut thanks for the clarification. I was intending to say 'extract from the bones' –  David Dec 8 '10 at 3:41
@aronut what about using vinegar? Or exchanging the water half way through? –  David Dec 8 '10 at 5:57
@David: I guess what I am trying to say (and obviously didn't communicate well) is that the solubility really isn't pertinent because you already have 100% of the gelatin dissolved at the end, and if you've simmered a stock overnight (or really about 6 hours or so, long enough to break the bones as I mentioned earlier) then you've also converted very close to 100% of the collagen to gelatin. If you want to significantly increase or accelerate the yield then you need a very low pH, near zero; that's concentrated hydrochloric acid we're talking about, not a splash of vinegar. –  Aaronut Dec 8 '10 at 6:13
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Starting your stock with cold water and 1/4 cup of vinegar should help you get the most goodness out of your bones.

If you want a more gelatinous stock, try adding in extra wings/feet* for poultry, or veal knuckle bones for beef. For a pork stock, pork neck bones work quite well. It's also helpful to break or crack the bones to expose the marrow.

*Chicken feet are awesome. They make a very gelatinous stock, but don't impart any chickeny flavor. They also look creepy. If you get feet that still have skin and nails on, you can just peel the skin off like a glove. The nails have layers that will pop right off. That's the best way to clean them.

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Cold water can help a little with fat skimming but for the most part it's a myth. Hervé This documented his experiments with it in his Molecular Gastronomy book; under controlled conditions the results are identical regardless of starting temperature. Separately, white vinegar is at most 5% acetic acid, so a tablespoon in 8 quarts of water comes out to a pH of about 3.8 at the lowest. I'm not saying that won't have any effect, but commercial hydrolysis is usually done at a pH near zero; the effect of a single tablespoon of vinegar is going to be barely noticeable. –  Aaronut Dec 8 '10 at 17:16
Oops- you're right. One tablespoon isn't the right amount. It should be 1/4 cup. Cold water isn't about fat skimming- it's about flavor extraction. –  Mrs. Garden Dec 8 '10 at 17:30
Cold water isn't about flavour extraction, that's what I'm saying. It does not help flavour extraction, that is a myth that was debunked by Hervé This, one of the most prominent experts in the food science field. And 1/4 cup is just 4 tbsp; that won't push the pH any lower than about 3.5. Sorry to go all science wonk on ya, but the information in the first paragraph of this answer is wrong; the second two paragraphs are good information however. –  Aaronut Dec 8 '10 at 18:36
Yeah, I'm not a scientist. I just know what's worked for me and others. –  Mrs. Garden Dec 8 '10 at 18:41
Does he talk about it in Kitchen Mysteries? That's all my library has... Maybe I'll try WorldCat. –  Mrs. Garden Dec 8 '10 at 18:50
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If you have a pressure cooker, try using that. You only need to cook it under pressure for about an hour, plus 20-30 minutes for it to reach pressure in the first place.

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Yeah, I use a pressure cooker too. It doesn't make the most refined stock -- I don't aspire to a perfectly clear stock -- but the results are perfectly good for making a rustic soup. –  slim Jan 12 '11 at 13:10
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Purely anecdotal, but I find that boiling turkey bones for hours gives me as much gelatin as I can handle. My stock turns into jello when I refrigerate it.

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If you actually mean boiling, as in at or near 100° C, then you'll be able to get a much clearer stock by lowering that to a gradual simmer; boiling disperses and eventually breaks down the fat which ultimately isn't what most people want. –  Aaronut Dec 8 '10 at 2:27
@Aaronut, thanks for the advice, I'll try that next time. –  Mark Ransom Dec 8 '10 at 3:41
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Long, slow simmering of the meat and bones - I usually do it overnight - extracts (into the stock) all of the gelatin available, but if you put the aromatic veg in at the beginning of this process, you will lose a lot of the flavor in the long cooking. According to the CIA cookbook, you should put the aromatics in for the last hour.

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It's probably worth pointing out that "simmering for hours" for me means at least (or about) 11 or 12 for turkey, and I don't even start getting flavor out of chicken bones until 4 hours in, and that too I keep going for 12.

Some recipes seem to suggest that you can simmer for just 4-8, and that just doesn't work the same. So, yeah, "simmer longer." And if you're simmering for that long already, then I don't know.

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