I had a purple cabbage head that was left over from a church function. It had been used to hold appetizers, so wasn't deemed usable by others, but I immediately saved it from the fate of the garbage can. It had science project written all over it! I forgot to weigh the cabbage before I chopped it up in the food processor, but it all worked out. After it was a finely chopped mass, I cooked it in plain water to extract the color I needed, then processed two skeins of yarn in the cabbage water. I used my standard mordants of alum and cream of tarter, which are both on the acidic end of the pH scale. After the yarn was done, I took the yarn out out, added baking soda (which is a base, aka other end of the scale) to the cabbage water solution, and then after the foaming went down, I put one skein back in. I was quite pleased with the immediate color change of the yarn!
Each of the skeins were 50 grams/137 yards, and I have more of the same yarn, so I think I need to go get some more purple cabbage!