Mass transfer is an important part of many food processing operations, including transfer of water to air during drying and humidification, extraction of solutes during separation processes and leaching, provision of dissolved oxygen during aerobic fermentation, and delivery of solutes during crystallization. It involves the migration of a component of a mixture or food product due to chemical potential differences that are typically produced by nonuniform concentration. The presence of a concentration gradient within a food material leads to molecular diffusion of the component from areas of high concentration to those with low concentration. For example, if you put a cucumber in a salt solution to pickle, the salt will diffuse into the cucumber because the concentration of salt in the liquid phase of the cucumber is lower than the concentration of salt in the solution. This unsteady-state mass transfer will continue until the salt concentration everywhere inside the cucumber is in equilibrium with the salt concentration in the solution outside the cucumber. An example of steadystate mass transfer is the transfer of water vapor released during respiration through packaging films covering fresh fruits and vegetables. With liquids and gases, an enhancement of molecular diffusion, called convective mass transfer, occurs when there is flow or agitation that speeds up the rate of mass transfer, which is analogous to what occurs during convective heat transfer.