Separating suspended solids from a liquid by passing it through a permeable medium that lets the liquid through readily. A fundamental separation in food processing, used to clarify juices, wines, beers and oils, sterilise liquids, and recover valuable solids. Crossflow membrane systems — microfiltration, ultrafiltration — clarify juices and wines while concentrating valuable proteins and polysaccharides, and they sterilise without heat. Filter fouling by pectins and proteins is the main operational challenge, managed by enzyme pretreatment, backflushing and clean-in-place regimes.