There is heated debate about bone broth ‘scum’, the bits and foam that float to the top in simmering. What is this disgusting stuff? And if consumed, how quickly will it kill you? It’s mostly coagulated protein and, bad reputation aside, it’s probably of no great concern (Curiosities: When boiling meat, what causes foam on liquid?), even as it gets referred to it as ‘impurities’. Some are religious in skimming it off because there must be something bad about it. Others don’t see the point.
The original purpose for this practice might have been because, back in the good ol’ days, meat often had insects and larvae on it which floated to the top when added to a heated liquid. But at this point, it’s just a tradition passed down the generations. People do it because their momma did it that way and she did it because of her own momma before her. Then people come up with rationalizations for why they continue doing so long after the original reason was forgotten.
As for the coagulated protein, it doesn’t have a lot of flavor and so shouldn’t affect that aspect of the bone broth, but some will swear by the ‘scum’ adding a bad taste. So, if you don’t like the flavor, then by all means skim it off or otherwise prevent it from forming. And there are many ways to deal with it (Do You Really Need to Skim Off Scum on the Surface of Your Bone Broth?). Simply roasting the bones beforehand supposedly will reduce the amount of flotsam.
That said, it’s theoretically possible that toxins from fat could be in the scum if using feedlot bones, but if that is your concern all of the fat should be thrown away along with the scum. The body primarily stores toxins in the fat, although heavy metals can get into the bones. There are studies that show some lead in bone broth, but small amounts of lead are found in all kinds of things, including the water you drink.
Dr. Catherine Shanahan argues that the level of lead in bone broth is typically low, although it is a reason to rely on pasture-raised animal foods in general (Broth: Hidden Dangers in a Healing Food?). For other reasons of oxidation, she’d still recommend removing the fat from broth and not using it (Bone Broth Risks: Skim the Fat!). As long as it is simmered at low temperature, the ‘scum’ won’t emulsify and so will remain at the top where it can later on be removed along with the fat cap.
But if it’s good quality animal parts, you don’t necessarily want to waste that nutrient-dense fat. Megan Stevens explains that you should make an initial batch of gelatinous meat broth in the first 2-3 hours (or 30 minutes in a pressure cooker) because the fat won’t have gone rancid yet (How to Make Bone Broth and Avoid Rancid Fat — A Complete Broth Guide). Only after the removal of the meat broth and fat would you want to do a long simmering bone broth, in one or two further batches, which gets more of the collagen and amino acids.
Not skimming fat will seal in heat and so increase the temperature. This might turn a simmer into a boil. If cooked too high, amino acids, minerals, and fat will emulsify into the broth. This would affect clarity and might affect flavor. High heat will also break down gelatin and so the bone broth won’t gel as well. For a thick soup or gravy, emulsification is a good thing and, in that case, boiling is recommended. So, this depends on personal taste and purpose.
There is a related issue with long cooking periods. Proteases will break down proteins into free amino acids and proten fragments, some of which taste bitter (What To Do With Bitter Broth?). If enough of these bitter-tasting molecules get emulsified, it could negatively affect the flavor. While simmering, skimming the coagulated proteins (i.e., ‘scum’) from the surface will help to prevent this. This will solve multiple problems by removing fat and proteins, whether or not one wants to save the fat for other purposes.
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I’m convinced that cooks who insist that a stock must be skimmed of excess fat and scum religiously are really only saying that so they have an excuse to stand by the pot and inhale.
Scum will rise to the surface. This is a different kind of colloid, one in which larger molecules–impurities, alkaloids, large proteins called lectins–are distributed through a liquid. One of the basic principles of the culinary art is that this effluvium should be carefully removed with a spoon. Otherwise the broth will be ruined by strange flavors. Besides, the stuff looks terrible. “Always Skim” is the first commandment of good cooks.
A lot of people will tell you to skim the froth that forms at the surface of a stock as it cooks, but it’s harmless. Skimming the foam or “scum” as it’s sometimes called, is simply a matter of culinary preference and is done to create a clear broth or stock. If you don’t mind the way it looks, leave it and all the goodness that it might contain.
Myth 8: You must skim the scum from the bone broth before the impurities in the scum pollute the flavor of your bone broth.
Poor scum! I feel so sorry for it at times. Another rule from the days of crystal clear broth bites the dust. The fluffy white foam that sometimes collects at the surface of your broth is just protein from the meat and bones in your broth. The heat changes the outer surface of proteins, and causes them to change shape or denature. That’s what happens to all proteins exposed to heat during cooking – unless you just eat your food raw! Scum isn’t anything awful. It’s not blood, toxins, fat, or anything awful. Just proteins. Even the reddish juice you sometimes see on bones isn’t blood (hemoglobin). It’s myoglobin. I am cooking bones to get at the proteins, nutrients, and healthy fats. The so-called scum – it’s just protein. I don’t skim it out of the pot. After an hour or two, fat rises to the surface of the broth and those misunderstood proteins become a source of flavor and color. Our minds are taught that scum is to be avoided. We think we can taste the evil in our pots, and if you truly can taste it and it bugs you, by all means remove it. If you have clean, high quality bones, why throw away the outer layer? Did it look at you funny? No, I didn’t think so.
The scum has some amino acids and impurities, which could include toxins. Chefs and traditional cooks often teach to skim the scum off with a fine mesh strainer, so that the impurities are removed.
However, when I polled some of my friends who are traditional cooks, none of them skimmed the scum. When I went to some of my favorite GAPS diet websites, I found that there was a mix of responses to people who skimmed and people who did not. I must confess that I have taken the easy way out and did not skim, but I wanted to find out whether it was a good practice to have.
Many cooks have an intutitive feeling that pressure cooking stocks is a bad idea. Their reasoning isn’t related to the previous discussion and isn’t born-out by our tests. Here are the reasons they usually give (and my responses):
- Pressure cooking will make the stock cloudy. That is incorrect. The boiling in a pressure cooker is no more violent than in a pot, so stocks don’t get any cloudier. We have done many side-by sides to prove this.
- Pressure cooking extracts bitter components. No one has detected bitterness in pressure cooked stock we’ve made.
- Not being able to skim the stock will introduce off-flavors. We have not noticed this in any of our tests.
The final stocks were remarkably similar. If anything, the not-skimmed stock was a tiny bit clearer than the skimmed one, which definitely contradicted my expectations.
I don’t have a great explanation for this, but here’s one theory I’ve come up with: A lot of the scum that initially floats to the surface of a stock is protein from some of the meat’s fluids. When you’re making consommé, which is concentrated, crystal-clear broth, one of the classic techniques for clarifying the liquid is with a protein raft on the surface, often made from egg whites. Perhaps, at a gentle enough simmer, the protein blobs that come to the surface of the stock end up working like a consommé’s protein raft, trapping particles in the broth and clarifying it in the process. If the stock is simmered and handled gently enough, those impurities won’t be distributed back into the broth and can be fine-strained out.
Either way, this test suggests that as long as you keep the heat low and have a fine-mesh strainer, you’re safe letting the stock be without skimming it. As for the fat that accumulates on the surface, I find it easiest to remove once the stock has chilled and the fat has congealed on the surface.*
* It is worth mentioning, though, that I tested these stocks in smaller batch sizes. It’s possible that larger batches could generate a deeper layer of grease on the surface, which, in turn, could affect the stock’s flavor and clarity in a different way.
The scum is not just protein. The scum floats; protein is more dense than water and would sink. The floating stuff is coagulated (denatured) lipoprotein, the same “L” that is in the terms HDL and LDL (from your cholesterol workup).
Protein combined with lipid (fat) is less dense than water (the “D” in HDL and LDL refers to the density). When boiled, these lipoproteins coagulate and float. The scum tastes just fine to me, it’s laughable to call it “impurities.” All cells have lipoproteins in their cell membranes.
However, leaving them in will cause the stock to be irreparably cloudy in the end. If the goal is a stew, then who cares if the stock is cloudy. If the goal is consomme, then skim away.
I think there is a “Chinese cream stock” in which the items (pork, duck, chicken) are deliberately cooked at a rolling boil so as to incorporate all the flavors into the liquid. The result is quite creamy-looking. In this method the lipoproteins are physically forced into a colloidal suspension.
Skimming is for aesthetic purposes.The scum is denatured protein, mostly comprising the same proteins that make up egg whites. It is harmless and flavorless, but visually unappealing. Eventually, the foam will break up into microscopic particles and disperse into your stock, leaving it grayish and cloudy. The more vigorously your stock bubbles, the faster this process will occur.If the grayness or cloudiness bothers you but skimming is not an option for some reason, you can always remove the micro-particulates later through the clarification process used to make consomme.
Removing the scum makes it easier to control the temperature of the stock so you can maintain a constant simmer. If you don’t skim it off, the scum aggregates in a foamy layer on the surface, which acts as insulation. It traps more heat in the stock and can cause your stock to boil when it would otherwise be simmering. Also, since stock often sits unattended on the stove while simmering, un-skimmed stock presents a risk of boil-over.
Firstly, I agree that’s for aesthetic purposes, many Cantonese stews are very clear when served.
Secondly, some people think it influences the flavor. I think it might be related to the slaughter method. For Halal meat, almost all the blood is drained, so it doesn’t influence the taste. But usually, it’s not completely drained.
And I think if the myoglobin is not boiled, like the juice in medium steak, it’s very juicy. But if it’s boiled for a long time, it tastes less tasty.I think for chicken and beef, the difference is very small, especially when you use a slow cooker and your chicken is grass-fed. But for pork, some people think the odor of pork is stronger, maybe because of boar taint, hence you will see them skim pork ribs when they make rib stew.
Lastly, you can scoop the fat.Update: I found a thesis trying to explain this:
Cause and Prevention of Liver Off-flavor in Five Beef Chuck Muscles
It said “residual blood hemoglobin is known to contribute to liver off-flavor development”.So I guess some people are sensitive to this smell.
There are two answers:
- If you are boiling meat, the scum is most likely animal fat. If you leave the scum in and just mix it together, it will add to the flavor. Though there are reasons to still remove the scum. One is that you might be trying to make a leaner more meaty flavored stock. Another reason is that pesticides in the animal’s food collect in the fat cells. You probably won’t taste it, but if you’re trying to go organic, you might want to dispose of this rather than consuming it yourself.
- If you are boiling vegetables, the scum will include potassium hydroxide leaching out from the vegetable matter. Potassium hydroxide, or lye, is a basic solution that will taste bitter, though won’t harm you in such minute doses. A typical westerner raised on a western diet has a dulled sense of taste and probably won’t notice the bitter, though a person from a different food culture will and as such might have a custom of skimming the scum even from boiled vegetable stocks and soups.
The foamy scum the forms is a result of blood and other proteins cooking and floating to the top of the liquid. If you’ve roasted your bones, you will not get much if any scum.
When I’ve boiled raw bones for 10 minutes (per Pho recipe), emptied water, washed bones and pan and started again, I have gotten minimal to no scum as well.
I’m not a scientist but I don’t buy this “impurity” nonsense. How is it that the “impure” stuff (whatever that even means) just happens to get foamy while everything else isn’t? I never skim my stock and it always tastes fine.
Agreed — I think “impurity” is the wrong word here. The scuzz at the top won’t render your broth inedible. It may add a slight bitterness, but the real issue is cloudiness. It’s more of an aesthetic issue if you’re making a clear soup or aspic; not an issue if you’re using the broth in a stew or gravy or blended soup. Washing the bones will help. Roasting the bones will help. Starting the broth with a cold water soak and adding vinegar helps. Skimming helps. But it’s not an essential step.
Yes. Skimming the stuff off is important for several reasons. Presentation for sure, but it also affects flavor and texture. It is not toxic to consume the blood and everything, but it does have its own taste. (I would be hard press that anyone thinks it has no flavor on its own)”You can try just tasting the floating stuff and it does have its own taste. Needless to say, when you mix it with the rest of the stock, it will alter the overall flavor which many people dislike.
I have seen it, I have tasted it, but never have I noticed any negative effects to the stock in taste, only clarity. Nor do I think there are any “impurities” in the foam that we should avoid eating.
I’d like to see some kind of source on that. I know everyone *believes* it to be true, but I’ve never seen any kind of taste test that proved it to be true. The only tests I’ve seen all indicate that skimming really isn’t worth while.
Meats and meat products have not always been as clean as they are today. Open air hanging, no refrigeration, etc led to flies and other critters leaving things behind them.The original “impurities” were bug eggs, dust, bone fragments, and whatever else you can imagine.Much of this loosened in an initial vigorous boil and floated to the top with the foam.With today’s much cleaner food handling in many countries, this isn’t much of a problem anymore. The continued use of the term “impurities” is most likely a carryover from the past, and come to mean the blood and liquid fat.
It has nothing to do with celebrities or magazines, you’re totally off base- there is nothing new about calling it bone broth. There’s a lot of info out there if you’re genuinely interested (I’m sure your Google works as well as any one else’s.) There is a lot of interesting research that was done on gelatin up until the 50s when food companies figured out how to chemically synthesize natural flavors. Long cooked bone broths contain glycosaminoglycans like hyaluronic acid and chondroitin sulfate, proline and glycine, glucosamine, land animal bones are rich in calcium, magnesium, potassium, and phosphorus- fish are also rich in iodine. The bones literally crumble when a batch of bone broth is done.