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

milkuht

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?

Best 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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