On my Gaggia machine I have an issue where after I am done stretching the milk the milk level continues to very slowly raise. The tip is definitely placed down far enough into the pitcher that it is done stretching and the tell tale sound is absent as well. My guess is that the steam is still wet even though I purge it before steaming and it is slowly introducing water into the milk. This is frustrating because even if I leave plenty of room in the small pitcher after stretching eventually it keeps rising to the top and I get anywhere from a small amount of spillage to a large amount. Also, if it is water my milk is becoming slightly diluted.

Is this normal for espresso machines or is something happening like water in the steam?

Thanks
Kurt

Weigh it before and after to see how much water has been added to the milk.

Most of the Gaggias have 'froth aid' wands. A plastic sleeve over a steel wand. That sleeve has a small channel in it which allows air to be drawn into the wand and injected into the milk so you do not need to surf the tip. If you have one of those, even after you plunge the tip deep in the milk, it will continue to inject air and stretch the milk. The fix is to cut down the wand to remove the air channel or remove the plastic extension and just use the inner tip of the wand, or replace the wand if one is available.

Steam is water vapor. It is water that is over the boiling point at the given pressure. When you release it into cold milk, it quickly condenses and mixes with the water.

All steam is wet... Some 'wetter' than others. I understand this to relate to how hot the steam is: hotter ought to seem more dry, as the steam is provding more heat and force with less water than steam at a lower temperature. Anyway, the answer I think you're looking for is that yes, you will experience some small expansion even after stretching, because steam condenses into the milk during the whole process, stretching or not.

laservet wrote:Weigh it before and after to see how much water has been added to the milk.

Excellent suggestion! I've noticed the same phenomenon, thought I'd give it a try. About 12g (12ml) extra water was introduced by my Spaziale S1 double boiler. The S1 is a very powerful steamer, but the steam is on the wet side. Still, this amounts to only 10% of 4oz milk by weight, even less by volume after stretching, and doesn't fully account for the increased volume. I'm thinking white matter...*

* Microfoamed dark matter. Is this is the key to the missing mass problem? Only time will tell.

The milk-steam-air mixture continues to be heated after the initial "stretching" phase - wouldn't this heating account for some additional expansion? For me the milk rises about another 10-15% in volume after the stretching phase.

Hans, did you read my explanation? Steam is water vapor. It cools on contact with milk. It turns into liquid water, and it is in solution with the milk you started out with. Steaming, at all phases of the steaming process, introduces water to whatever you are steaming (unless you're steaming something whose temperature is greater than the boiling point of water where you are). You will naturally observe this increase in volume.

If you are realistically concerned about diluting your milk, I would suggest a more powerful steamer (you can turn up your pressurestat if it's adjustable), or using a heavier milk. But hey, a little dilution is healthy, right? if you started out with a 3.56% solution of milkfat, now you probably have 3.25%. That's going to help you shed those unwanted pounds...

Thanks Nicholas. What you say makes sense. But doesn't the overall continuing heating of the steam/milk mixture account for some expansion? Also air is being added in the frothing process - so would that change in volume as the mixture heats up?

Of course you're right about the frothing--introducing air during the frothing process will dramatically increase the volume of the milk. However, I don't think that heating the milk is going to significantly expand it. My main point was that the increase of volume during steaming (not the stretching/air introduction) is almost entirely due to the introduction of water from the condensed steam.

If you want to test the hypothesis, simply try heating the milk to the same temperature without steaming it: e.g., in the microwave or on the stove. Be sure to cover it in either case. You check the volume before and after heating...