Sealing excluding air from food, is a method of preservation used since early times. The Romans covered apples in clay to make them keep through the winter. The first systematic use of sealing was in medieval Europe, where it was discovered by trial and error that perishable foods would keep better if enclosed in a pie. After the pie was cooked, hot fat was poured in through a hole in the crust. This solidified and sealed the contents from the air in the space under the crust. As long as the melted fat and the pie were more or less at boiling point when this was done, the contents would remain sterile and might last for several weeks.
