You can put aluminium foil to many uses in the kitchen. Its heatproof and non-flammable properties make it suitable for the highest of cooking temperatures, and the non-reactive material can be used with even the most acidic ingredients. It protects cooking surfaces, provides an airtight seal and slows excessive browning, among other virtues. Many of these aid the preparation of a pork loin.
The extra tender pork loin shines in almost any preparation except slow cooking. It is very lean, with most of the fat on this lean cut is confined to a protective covering at the top of the loin, and thinner layers between the muscles at the loin's sirloin end. Because it's so lean, pork loin can easily become dry and tough if you overcook it.
When you're roasting a pork loin, you can use aluminium foil to speed the after-dinner cleanup. Lining the roasting pan with aluminium foil to contain the pan juices prevents them from scorching onto the pan, so you can clean it in seconds without scrubbing or soaking. When the roast finishes cooking, you can pour the pan juices from the foil into a saucepan and make a sauce or gravy.
Most butchers trim pork loins with at least 1/4 inch of fat cap to protect the meat from drying out during cooking. This is generally adequate, but depending on the cooking method, you may need to provide additional protection. Covering the roasting pan with foil traps moisture inside, keeping the roast moister as it cooks. You can remove the foil near the end of cooking to let the roast brown. Alternatively, you can brown the roast first, then cover it with foil to finish cooking.
Resting a roasted loin under a loose cover of aluminium foil is an important step often overlooked by home cooks. The proteins in a piece of meat contract when they're heated, driving juices out of the muscle cells. The juices get forced to the middle of the roast, and drain out of the meat if you carve it immediately. Resting it under foil keeps the roast warm, but gives the juices time to evenly redistribute themselves through the roast. When you carve it after resting, the slices are uniformly juicier as a result.