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When steaming milk (for a latte or other coffee drink), it sometimes produces a loud, high-pitched "screaming" sound. I think you can usually prevent it by keeping the tip of the steaming wand on the surface of the milk longer before putting it in deeper, but that's just a guess more than anything I've actually been taught. I've always assumed this noise means the milk is scalding, but I'm really sure.

What causes the screaming noise, and what is the best way to avoid it?

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3 Answers 3

up vote 8 down vote accepted

The screaming noise occurs when your steaming wand can't draw in enough air. The worst that will happen is your milk will taste a little sour. Just pull the wand out of the milk for a second then put it back in.

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Technically, the noise is very likely tiny steam bubbles that collapse with great force as the steam condenses, thereby shrinking to 1/1600 of its volume. This is called cavitation and the forces appearing at the microscopic scale are enormous and present a significant challenge in a number of engineering tasks. For more details, see the Wikipedia article.

Now, with regard to the latte, I'm just a latte consumer, and don't really know anything about how to make it, but the above might still be helpful in connection with what others have written. I would expect cavitation to occur every time, and in my experience, there's always some kind of noise associated with the steaming. However, that high-pitch screaming might mean there's more cavitation, for instance, because the steam is hotter, the milk is colder, or there is more steam etc. Each of which may very well affect the result. Since milk is quite a complicated emulsion, I'm certain with those tiny implosions, there's a lot going on at the microscopic level.

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'Screaming' or 'Hissing' is a common occurrence when making a latte - it is an indicator of milk expansion. It can scald your milk, so it's best to use the scream as an audible warning to adjust your steaming process. Once you hear the scream, start rotating your milk jug.

Here are some general tips on steaming milk from my website http://www.perfectlatte.com/

Steam wand should be approx. 2cm in from jug and no more than 1cm beneath the surface of the jug. Turn the steam wand on and adjust the jug with you hand until you hear the milk “hissing” or in other words expanding. When the milk begins to expand, it must be going in a counter clockwise motion around the jug allowing small bubbles to form. The bubbles should not be big. Once the milk has been expanded enough (remember 1cm foam) and has reached a temperature of 80ºF, bring the steam wand into the centre of the jug and continue the spinning motion until the milk reaches 150-160ºF.

I hope that was helpful for you. Remember, rotation should help you avoid scalded or burnt latte milk. Just don't let the scream go unnoticed.

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Why counter-clockwise? –  jontyc Feb 14 '13 at 4:48

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