Mass transfer

The movement of matter from place to place, considered with respect to a defined boundary, is termed mass transfer — water migrating within or out of a wet product during drying is the classic example. It sets the pace of drying, extraction (coffee, oilseeds), absorption, distillation, crystallisation and membrane separations; engineers describe it with Fick’s laws of diffusion and convective mass-transfer coefficients to design equipment, predict drying times and scale processes from pilot to production. Salt diffusing into cheese during brining and oxygen permeating packaging film are everyday food instances; managing mass transfer through temperature, airflow, agitation and packaging permeability is central to hitting target moisture, texture, flavour and shelf life.