Cover the bones in cold water, bring it up slowly, and let the scum rise. Pour the whole lot away, wash the bones under the tap, and wash the pot as well. Then refill with fresh water and cook the soup at a bare simmer. That is the whole method, and the difference between a soup you can see through and one that looks like dishwater.
The usual problem. You skim carefully for ten minutes, the soup still comes out grey and cloudy, and there is a fine grit at the bottom of the bowl. Then you eat at a coffee shop and the soup is clear enough to read a newspaper through. They are not using better bones than you. They are throwing the first pot away.
What the scum actually is
The grey-brown foam that piles up in the first few minutes is coagulated blood protein and soluble muscle protein, plus loose marrow and bits of fat. It is not dirt and it is not dangerous. It is protein that has come out of the bone and set solid in the water. Leave it in and it breaks into fine particles that never settle, and those particles make a broth cloudy and give it a faintly metallic edge.
You cannot cook it away. Once dispersed, it stays dispersed. So the trick is to get all of it out in one controlled stage at the start, before you commit your real water and your aromatics to the pot.
Why the cold water start matters
Put the bones in the pot and cover them with cold water from the tap. Never drop bones into water that is already boiling.
Bones dropped into boiling water seize on the outside instantly. The surface protein sets in place and seals everything in, so the blood and marrow stay locked inside and leak out slowly over the next two hours, straight into the soup you intend to eat. A cold start does the opposite. The temperature climbs through the range where those proteins loosen and float free, and they all come up together as scum you can pour away in one go.
Frozen bones go in straight from the freezer. No need to thaw them, since cold water and frozen bones are the same starting point. The pot only takes longer to come up.
The blanch, step by step
- Rinse the bones under running water, rubbing off bone dust and splinters left by the saw.
- Put them in a pot and cover with cold water by about 5cm.
- Bring it to the boil over medium heat, not high. You want the temperature to climb steadily, and you want to be at the pot when it arrives.
- Hold it at a rolling boil for 2 to 3 minutes. Grey foam will pile up. Let it.
- Kill the heat and pour the pot through a colander into the sink. All the water goes. Do not save any of it.
- Rinse every bone under the tap, and mean it. Run your thumb into the crevices and around the marrow. Anything slippery or curd-like comes off.
- Scrub the pot, refill with fresh cold water, and start the actual soup.
Wash the pot, not just the bones
This is the step almost everyone skips. Scum sticks to the sides of the pot at the water line as a grey-brown ring, and the moment you refill, it slides back into your clean water. You end up with spotless bones in a dirty pot, wondering why the soup is still hazy.
Thirty seconds with a sponge fixes it, and do the colander too. If you are making Teochew bak kut teh, where the clarity of the soup is half the dish, that ring is the only thing standing between you and it.
Clear or milky is mostly about heat
Once the bones are blanched properly, whether the soup finishes clear or white is almost entirely a question of how hard you boil it.
A bare simmer, where a bubble breaks the surface every second or two, keeps the fat and the gelatin as separate things. The fat floats and you spoon it off. What you get is a clear soup the colour of weak tea: Teochew bak kut teh, sliced fish soup, lotus root soup.
A hard rolling boil does something else. The agitation smashes the fat into tiny droplets and beats them into the gelatin coming out of the bones, and the two form an emulsion that scatters light. The soup turns opaque white and tastes rounder and richer. That is tonkotsu, and it is also what happens to your rib soup by accident when the flame is too high and nobody is watching.
Neither is wrong. But pick one and hold the heat there. A pot that boils hard for twenty minutes then simmers for two hours comes out murky rather than properly clear or properly milky, which is the worst of both.
Which bones, and how much water
Roughly 500g of bones to 1.5 litres of water gives body without turning heavy. Prime ribs and spare ribs give you meat to eat as well as flavour. Leg and neck bones cost less and give more gelatin. Soft bone is a different animal altogether and belongs in a braise rather than a long boil.
Which bone suits which soup is worth settling before you start, because it changes the character of the finished pot more than any amount of technique does. ABC soup, lotus root soup and bak kut teh each want something different from the bone.
What blanching will not fix
Blanching removes scum. It does not remove fat. If the finished soup is greasy, that is a separate job: chill it overnight and lift the set fat off the top in one sheet, or drag a sheet of kitchen paper across the surface while it is hot.
It also does not make tired bones taste fresh. Bones that have sat too long in the freezer go flat and slightly stale, and no blanch brings that back. Keep them at minus 18C, use them within a few months, and the blanch is left doing the one job it can do.
And it does cost you something. A little flavour goes down the sink with that first pot. It is a real cost and a small one. What you get back is a soup that looks the way it tastes.
Bones worth the pot
Frozen pork bones cut for soup, straight from the freezer into cold water. One blanch and they are ready.