🌡️ Sterilization

Milk sterilisation is prolonged heating above 100 °C, usually in a sealed bottle or can in an autoclave, at 110–125 °C for 3 to 40 minutes. It makes milk “commercially sterile”: without a refrigerator it can keep for months. The price is darkening, a “boiled” taste and larger vitamin losses than with ultra-pasteurisation (135–150 °C for a few seconds).

What commercial sterility means. Tetra Pak and Codex Alimentarius explain: the product contains no microorganisms able to multiply at room temperature, and absolute sterility cannot be achieved. Codex checks this by holding samples for 7 days at 55 °C or 15 days at 30 °C. EU law defines only pasteurisation (72 °C for 15 s or 63 °C for 30 min) and ultra-pasteurisation (not below 135 °C); there is no separate definition of in-container sterilisation.

How it is done. Milk is warmed to about 80 °C, filled into hot bottles, sealed and heated with steam in an autoclave. In continuous lines a hydrostatic tower gives a cycle of about an hour, of which 3–30 minutes at 115–125 °C. Cans of unsweetened condensed milk are sterilised at 110–120 °C for 15–20 minutes; this method appeared in the early 1880s.

Heat has a price. Lysine losses are 6–10 % versus 0.4–0.8 % with ultra-pasteurisation, thiamine losses 20–50 % versus less than 3 %. Tetra Pak sums up that in quality ultra-pasteurised and pasteurised milk are close, while in-container sterilised milk falls behind in nutritional value.

Sterilisation or pasteurisation? Searches usually pit them against each other. Pasteurisation is 72 °C for 15 seconds or 63 °C for 30 minutes, and the milk still needs a fridge. In-container sterilisation heats a sealed pack at 110–125 °C for 3–40 minutes, so unopened milk keeps without refrigeration. Ultra-pasteurisation, 135–150 °C for a few seconds, is a third route to the same commercial sterility; EU law defines only pasteurisation and ultra-pasteurisation, not in-container sterilisation.

On a pack look for “sterilised” (in-container) or “ultra-pasteurised”, “UHT” or “long-life”; the two are not the same process. Either one sits on an ordinary ambient shelf in supermarkets and corner shops, not in the fridge, while pasteurised milk is in the chilled cabinet. Unopened, it needs no refrigerator; once opened, follow the storage advice printed on the label, and check the date.

Why it matters to you. On a package, tell “sterilised” and “ultra-pasteurised” apart: unopened it needs no refrigerator, and after opening the rules from the label apply.