Leaf Fat and Suet
Also known as leaf lard, flare fat, kidney fat, suet, Flomen, panne, sugna cruda, manteca de pella
The soft crumbly fat from around the kidneys, quite different from back fat. It renders to the cleanest, most neutral lard there is, which is why it is the fat of pastry and of the sugna seal, and why it is useless for a salami dice.
Leaf fat is the internal fat that sits around the kidneys and along the loin cavity, and it comes away in soft crumbly sheets rather than the firm slab of back fat. In beef and mutton the same anatomical fat is called suet. The distinction from back fat is not a subtlety: they are different materials with different uses, and a recipe that names one will not work with the other.
What makes leaf fat valuable is purity. It carries almost no muscle, almost no connective tissue and very little of the flavour compounds that give other pork fat its taste, so rendered gently it gives a white, mild, almost neutral lard with a clean flavour and a fine crystal structure. That is the lard of pastry, where its crystals give the short, flaky texture that vegetable shortening imitates; it is the frying fat of half the world's traditional kitchens; and it is the base of the sugna paste that seals the exposed face of a drying ham.
Rendering is where the craft lies, and the rule is low and slow. Leaf fat cut small and held at a gentle heat gives up clear fat and leaves crisp cracklings behind. Pushed hard, it browns, and browned lard carries a roasted flavour that is delicious in the wrong place and ruins a pastry or a delicate confit. For the cleanest result some renderers add a little water at the start, which holds the temperature down until the water boils off.
What leaf fat cannot do is hold a shape. It is soft, it has a lower melting point than back fat, and it smears rather than dicing, so it has no place in a salami where the mosaic depends on discrete cubes. Where a formula calls for fat in a dry sausage it means back fat, and substituting leaf fat gives a greasy, blurred, weeping product.
Beef suet behaves similarly but is harder and higher-melting, which is why it is the traditional fat of steamed puddings, mincemeat and dumplings, where solid grated fat is meant to melt slowly during a long cook. It is also considerably more prone to a tallowy flavour than pork fat, so it belongs where that is wanted.
Both oxidise, and rendered lard oxidises faster than the intact fat did, so it wants a cool dark place, a sealed container and reasonable turnover.