If I had to guess, I would say it has something to do the beans. I've never had big bubbles form, even after 5 minutes, using CC Toscano or Rustico. Even only 2 days after roast. And I've definitely foamed milk poorly before.

My other guess would be that it also has a little to do with the steaming. For me, when I see soft edges (like one picture above) it means that I didn't mix/roll the milk well enough. Rolling is a good term because vertical rotation is more important than a pure horizontal swirling.

Maybe... when the foam isn't completely and evenly incorporated it lifts the crema instead of mixing slightly with it. This would then make the brown portions less stable, especially in "dissipating prone" blends? Milk foam seems to be more stable than crema and I would imagine milk foam would mix more easily with crema that liquid milk.

This would explain why pre-mixing foam with the crema might help, and also why even good looking microfoam with no large bubbles could sometimes be a problem.

Of course, I'm really new to all of this so my semi-convincing explanation might be totally off.

I found the problem and it's actually quite silly as it wasn't even supposed to be this way in the first place. We were using UHT milk as we were out of fresh milk and UHT is much more common here so we just bought that instead. It's back to fresh now and no problems whatsoever.

Hi,

I've had this 'problem' with both UHT and fresh, so I don't think it's that. Perhaps it has something to do with what's in the milk ?

Regards,
dsc.

You may be right, as I did change brands in the process.