🥛 UHT milk

Ultra-pasteurised (UHT) milk is milk that is heated in a continuous flow to 135–150 °C for a few seconds and filled into sterile packaging that keeps microbes out. According to FAO, the typical holding time is 2 to 10 seconds. Unopened packaging can be stored for months without a refrigerator, and once opened the milk is kept cold.

Heating can be direct — with steam mixed into the milk — or indirect, through the wall of a heat exchanger. According to Tetra Pak, direct systems treat the milk more gently, while indirect ones are cheaper but give a higher heat load. The sterile product is poured into sterile cartons under aseptic conditions, and the cardboard between layers of polyethylene has to protect against light and oxygen.

Shelf life is determined not by microbes multiplying but by quality: cream separation, gelling, off-flavours. Tetra Pak gives examples of about 3 months for local sales and about 9 for export. According to FAO, losses of riboflavin, niacin, pantothenic acid and biotin are under 10%, but in one study the denaturation of whey proteins after UHT was 56% versus 0.4% after pasteurisation; a "boiled" flavour appears. UHT milk was at first mostly cow's milk, and today goat's and buffalo milk are also sold in some countries.

UHT or fresh milk? What many shoppers call long-life milk is UHT milk: heated in a continuous flow to 135–150 °C for 2–10 seconds and filled into sterile packaging, whereas pasteurised milk gets a milder treatment. The trade-off is flavour, because denatured whey proteins give a “boiled” note. UHT is also not the same as sterilised milk, which is heated inside its package for longer, at 110–125 °C for 3–40 minutes, and loses more lysine. According to FAO, losses of riboflavin, niacin, pantothenic acid and biotin stay under 10%.

On the pack look for “UHT” or “ultra-heat treated”; “long-life” is the everyday name, and “sterilised” on a label means a different product. Unopened cartons sit unrefrigerated on ordinary supermarket shelves or in a corner shop, in whole, semi-skimmed or skimmed versions. The producer sets the date; keep unopened packs in a cool place and put an opened pack in the fridge. In some countries goat’s and buffalo milk are sold this way too.

Why it matters to you. "Ultra-pasteurised" is not the same as "sterilised": according to Tetra Pak, UHT milk loses much less lysine and thiamine than milk sterilised in its package. Store unopened packaging in a cool place, and opened packaging in the refrigerator.