Oil rises above pickles during storage because oil is less dense than the brine solution (water, salt, and vinegar) in which the pickles are submerged, causing it to float to the top. This is a natural separation process based on density.
The phenomenon of oil rising above pickles is a direct result of differences in density. Density is defined as mass per unit volume. The brine solution used to preserve pickles is primarily composed of water, salt, and vinegar. This solution has a higher density than the oil used in the pickling process, which is typically a vegetable oil.
When two liquids of different densities are mixed and allowed to settle, the less dense liquid will naturally rise to the top, while the denser liquid will sink to the bottom. In the case of pickles, the oil, being less dense, migrates upwards through the brine solution until it forms a layer on the surface. This separation is governed by gravity and the inherent properties of the liquids involved. The oil doesn't dissolve or mix evenly with the brine because of their different chemical compositions and polarities. Water is a polar molecule, while oil is non-polar, further contributing to their separation.
To minimize oil separation, store your pickles in a cool, dark place. Warmer temperatures can accelerate the separation process. Gently shake or stir the jar before serving to redistribute the oil, although it will eventually separate again.