The K-ration was a United States military ration consisting of three separately boxed meal units: breakfast, dinner, and supper. It was originally intended as an individually packaged daily ration for issue to airborne troops, tank crews, motorcycle couriers, and other mobile forces for short durations. The K-ration differs from other American alphabetized rations such as the A-ration, consisting of fresh food; B-ration, consisting of packaged, unprepared food; C-ration, consisting of prepared, canned food; D-ration, consisting of military chocolate; and emergency rations, intended for emergencies when other food or rations are unavailable. In 1941, Ancel Keys, a University of Minnesota physiologist, was assigned by the U.S. War Department to design a non-perishable, ready-to-eat meal that could fit in a soldier's pocket as a short-duration, individual ration. Keys went to a local supermarket to choose foods that would be inexpensive, but still be enough to provide energy. He purchased hard biscuits, dry sausages, hard candy, and chocolate bars. He then tested his , meals on six soldiers in a nearby U.S. Army base. The meals only gained "palatable" and "better than nothing" ratings from the soldiers, but were successful in relieving hunger and providing sufficient energy. The new rations were initially intended as individual rations suitable for short durations only, to be used for a maximum of fifteen meals before supplementation or replacement with 'A-ration' or 'B-ration' field rations. They were soon called the "paratrooper ration", since paratroopers were the first to be issued the ration on an experimental basis. The actual prototype of the K-ration was a pocket ration for paratroopers developed by the Subsistence Research Laboratory (SRL) at the request of the U.S. Army Air Forces (USAAF) early in the war. Two original samples (one version used pemmican biscuits, a peanut bar, raisins, and bouillon paste; the other used pemmican biscuits, a small D ration bar, canned processed meat, and lemon beverage powder) evolved into the one-package breakfast-dinner-supper combination later adopted as standard.