Suet is beef fat, though not just any beef fat. It’s taken from the kidney region of the steer, so in that way it’s analogous to leaf lard from a pig (normal subcutaneous beef fat is simply known as “tallow”). What’s special about suet is that it’s mild tasting and extremely firm, in fact it’s the hardest of all the fats humans commonly eat. Whoever can tell me what it is that makes suet the firmest of the fats gets a A for this course. Anyone? Anyone? Yes you at the back with the heart condition. That’s right: it’s the highest in saturated fats. Saturation = firmness, that’s rule of fat.
It’s that very firmness that makes suet valuable in the pastry kitchen, notably in the realm of English puddings. Being so rich in saturated triglycerides it has a very broad melting point, also a very high one, about 120 degrees Fahrenheit. That helps create a more open crumb and lighter texture in a pudding, as the starch and egg proteins around the suet shreds has time to gelate and firm before the fat melts entirely. That creates lots and lots of little holes, which keep the pudding from settling into a brick, as many traditional puddings contain no leavening.