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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?

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135°C implies it's under pressure (at least 46psi) if prevented from boiling. –  zanlok Dec 14 '12 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. –  SAJ14SAJ Dec 14 '12 at 22:31
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@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. –  SAJ14SAJ Dec 15 '12 at 18:43
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@SAJ14SAJ: An explosion is just a faster route to equilibrium! –  Josh Caswell Dec 15 '12 at 21:01
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@JoshCaswell Sure, ja, you betcha. But the clean up is much rougher. –  SAJ14SAJ Dec 15 '12 at 21:09
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1 Answer

According to this product page from Machine Point:

MachinePoint Food Technologies manufactures injection direct heating systems where high pressure steam is injected into pre-heated liquid by a steam injector leading to a rapid rise in temperature between 80 and 145 ºC for 0,5 seconds. Following the product is flash-cooled in a vacuum to remove water equivalent to amount of condensed steam used, until we reach the 80ºC. Then temperature keep been reduced by a heat exchanger.

Physics is fun.

I suspect the various methods are proprietary, depending on what is being pasteurized, but for economy of scale, they are all going to involve a continuous process similar to the one described, where the product enters a heating phase, then very quickly moves into a cooling phase in the machine.

In these continuous types of processes, of course the product will move continuously through all temperatures, but it will do so very quickly.

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