The past few winters, some friends came to my house in December to do a big yak meat drying process so they could store meat for summer at their off-grid house. This yellow fat you see in the photos is just yak tallow, and I guess it's colored by the yaks being wildly free-range and eating high altitude desert plants. I made big loads of broth and tallow as a by-product of drying meat. The tallow I cleaned up well lasted in jars on the shelf for a year without getting any off-smell (we are not there now, so later I'll find out if a second year works). Yak fat is quite mild (unlike, say, mutton tallow that I find to smell sheepy).
We receive the meat as large pieces, like an upper leg, a lower leg, a quarter of a ribcage, and we store them in another friend's deep freezer. Then we take one out and partially thaw in a chilly part of the house for a couple days (the north side stairs in my passive solar-heated house stays at fridge temps and is ideal). When it's still lightly frozen but soft enough to cut, we slice off the meat and string it up to freeze-dry in a screened cage on the roof (in the high desert). Pieces of gristle and chunks of fat go to the broth pot. We cut up the bones with an axe, and I start the broth and fat process.
1) Simmer the bones overnight and ideally 18 to 24 hours. The first round, I use plain water, lightly salted, and no onions or spices. Drain them into a colander OVER A POT!!! and pick out the bits of meat for use. Put the broth in the chilly place in a straight-sided vessel.
2) Put the bones back in the pot with any remaining fat chunks or odd bits, this time with any other alliums or spices desired for broth, and simmer another 24 hours. Again drain over a pot and put the broth to chill in a straight-sided vessel.
3) Optionally simmer the bones a third time for another 24 hours, and this time add just a little acid such as vinegar or lemon juice, to draw more broth out of the bones. At the end of this process, the bones are mostly soft and go right in the compost, where they will disappear.
4) Meanwhile, when a broth pot has cooled, loosen the sides with a knife and lift out the disk of hardened fat. Scrape the bottom off, back into the broth. There will be little specks in the bottom of the fat, so scrape those off. (For storage of the broth, heat it, pour into containers, and freeze).
5) Combine the various batches of fat, melt them in clean water, stirring the fat into the water, and then chill again in a straight-sided container. For good long storage at room temperature, repeat this step again (chill, lift off the disk, wipe or scrape any last bits off).
6) Finally, take the chilled hard disk of fat, turn it upside-down to expose the wet side to the air, and let it dry in the chilly place for a couple of days. Melt it in a dry pot, and as it gets hot make sure there are no no moisture bubbles left inside. In theory there's a risk of it catching fire if it sits on the stove getting too hot, so I like to keep an eye on it and pay attention. You want any moisture to come out as bubbles of steam, but you don't want the fat itself to volatilize and come up as bubbles.
7) Pour into clean, dry, heat-resistant containers and store. I reuse glass jars that commercial food came in. The fat doesn't touch the lids so you can reuse jars with plastic lids, but if your storage will be unattended and risks rat attention, then stick to metal lids.
