Yellow fat can indicate a lot of things, most of them not good: 1) The animal was old. 2) Worst case: the animal was losing weight when it died. This will mak

Yellow fat can indicate a lot of things, most of them not good: 1) The animal was old. 2) Worst case: the animal was losing weight when it died. This will make the meat taste sour (as best I can describe). 3) Best case: it's grass fed and not grain finished. This fat will usually be hard. If you're in the UK (guessing because you said "mum"), that's probably what you got. 4) It's dry aged beef. The outer layer will turn a bit yellow, but otherwise the fat is going to be whatever color it was before aging. Temperature and duration of aging has a lot more to do with how tender the cut is going to be. Really, it's the spongy feeling stuff you want to avoid. If you're in the US, and it's not marketed as grass fed, it's almost certainly from an old dairy cow or an animal which was sick when it was slaughtered (and losing weight).