Critical moisture content is the threshold level of water in a food, above which the product becomes unsafe or unmarketable and below which it stays stable. It is expressed as a percentage and marks the point where enough free water becomes available for microbes to grow, enzymes to act, or texture to collapse — the moment dried or intermediate-moisture foods lose their shelf life. Food scientists set it by mapping water activity and moisture sorption isotherms for each product, since the critical point differs between, say, milk powder, crackers, and dried fruit. In manufacturing it governs drying targets, packaging barrier requirements, and moisture-proofing of the whole supply chain. Staying safely below the critical moisture content is one of the most basic rules of preservation for dry foods.