# How is UHT milk heated to a high temperature for only 1–2 seconds?

I've read that UHT milk is pasteurized by heating the milk to a high temperature (e.g. 135 degrees Celsius / 275 °F) for an extremely short period — around 1–2 seconds.

But this glosses over the question of how quickly the milk cools down — surely the milk doesn't cool down instantly, so it must spend a bit of time at 134, then 133 (271 °F) and so on until it gets down to the temperature of the fridge.

So: how quickly is the milk cooled down, and how do they do it?

• 135°C implies it's under pressure (at least 46psi) if prevented from boiling. Dec 14, 2012 at 22:01
• And then, when the pressure is released, some water will vaporize almost instantly. This will create a powerful cooling effect. It is essentially how refrigerators or air conditioners work, with water itself as the refrigerant. The product description I found (in the answer below) implies the use of this effect, with vacuum to magnify its efficacy. Dec 14, 2012 at 22:31
• @J.A.I.L. Water normally evaporates at sea level pressure at 100 C. The only way to heat a water-based liquid above 100 C is increase the pressure and thus the boiling point. When the pressure is removed, if the temperature is above the now-current boiling point, it will evaporate quite quickly until equilibrium is regained at the now-current boiling point. Since this phase transformation requires energy, it lowers the temperature of the remaining liquid. This is how refrigerators work, except they use a low-pressure low boiling point liquid, instead of water. Dec 15, 2012 at 18:43
• @SAJ14SAJ: An explosion is just a faster route to equilibrium!
– jscs
Dec 15, 2012 at 21:01
• @JoshCaswell Sure, ja, you betcha. But the clean up is much rougher. Dec 15, 2012 at 21:09