🔄 Filtration

Milk filtration is the removal of anything that should not be in the milk: on the farm, hairs and bits of bedding and feed; at the dairy plant, fine impurities; and with membranes, even bacteria. It is one of the first steps on milk’s journey: after milking, it is usually filtered before it goes into the cooling tank (Tetra Pak).

With hand milking, the milk is poured from buckets through a strainer into a 30–50 litre churn, and the churns are cooled to 4 °C. In a milking system with a pipeline, the filter sits right in the line. It is worth remembering that an ordinary filter removes only visible impurities, not microbes; on the farm, cleanliness and rapid cooling are what help against those.

At the plant, milk from the tanker is pumped through a mesh filter and a flow meter into raw milk silos. Next a centrifugal cleaner, the clarifier, separates out solid particles, and bactofugation removes spores and some of the bacteria; these processes are mostly run at 40–50 °C. Membrane methods differ by pore size: microfiltration holds back bacteria, ultrafiltration holds back proteins, nanofiltration partly demineralises, and reverse osmosis removes only water.

Filtration or sieving? A farm strainer is the simple end: it holds back hairs, bedding and feed, but not microbes. At the dairy plant the picture widens: a mesh filter first, then a centrifugal clarifier for solids, and bactofugation for spores and some bacteria, mostly at 40–50 °C. Membranes sort by pore size: microfiltration holds back bacteria, ultrafiltration proteins, nanofiltration partly demineralises, and reverse osmosis removes only water. Straining curd through cloth is a different, ancient method: it separates whey and gives labneh, chhena or shrikhand.

Milk filtration is invisible on a pack, but its results are named on labels. Look for words such as “microfiltered” or “extended shelf life” (ESL), which usually point to pasteurised milk that keeps 2–4 weeks thanks to microfiltration. Ultrafiltration stands behind Greek-style yoghurt and lactose-free milk, so check the ingredient list and the product name. Find them in the chilled dairy section of a supermarket, and keep them cold. Remember that a farm filter alone is not a safety mark.

Microfiltration with 1.4 µm pores retains up to 99.99% of bacteria and spores, and with 0.8 µm pores up to 99.9999%; this allows pasteurised milk to keep for 2–4 weeks. Ultrafiltration is used for Greek yoghurt, lactose-free milk and standardising the protein in cheese milk. A separate, ancient method is straining curd through cloth: this separates the whey and gives labneh, chhena or shrikhand.