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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?

J.A.I.L.
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jim
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    135°C implies it's under pressure (at least [46psi](http://www.engineeringtoolbox.com/boiling-point-water-d_926.html)) 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
  • @SAJ14SAJ why does it vaporize after the pressure is released. I thought [changing pressure would just change the milk's temperature](http://en.wikipedia.org/wiki/Gay-Lussac%27s_law#Pressure-temperature_law), but not it's [state](http://en.wikipedia.org/wiki/State_of_matter). – J.A.I.L. Dec 15 '12 at 18:23
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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
  • @J.A.I.L. The law you cited is for a single phase, gas. The state transformation from liquid to gas will first absorb energy, lowering the temperature. This is how pressure cookers work. As more water evaporates, the pressure increases, and so does the boiling point, until equilibrium is reached (for example, if the heat is turned off, or there is a pressure valve), or a catastrophe (exploding vessel) occurs. – SAJ14SAJ Dec 15 '12 at 18:49
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    @SAJ14SAJ: An explosion is just a faster route to equilibrium! – jscs 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
  • @SAJ14SAJ I had always assumed the law was for fluids. It's good to notice, 25 years after I learnt it, that it's for gasses. Thank you. – J.A.I.L. Dec 15 '12 at 23:46

1 Answers1

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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.

SAJ14SAJ
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