Tomato soup ... with scum?

[pastameshugana]

So I (semi) regularly make a roasted tomato soup for the family. It's easy, freezes well, and everyone loves it. Tomatoes are just coming into season here (South Africa), so I got a couple kg's worth and made a big batch a couple of days ago.

 

It's very simple: I chop and roast tomatoes, carrots, onion, garlic, red & yellow bell pepper, and a couple of small chillies (birdseye this time) under the broiler, toss with salt & pepper and a little olive oil, then into a pot to simmer, then blend with an immersion blender. At the very end I add cultured buttermilk before we eat it.

 

This time, for the first time ever, while simmering (after blending) I got a persistent layer of scum on the top that had to be scraped off. Similar to when you're making a stock and you have to skim it. Because the soup itself is so thick, it was quite hard to get the gunk off the top without losing a lot of the soup itself.

 

The only difference this time: Normally, when I chop the tomatoes, I dump the insides (seeds and goo) into the trash, but this time I thought I would try to get some of that 'tomato water' so I put them into a fine sieve over a bowl while I did the rest of the prep. I ended up with about two cups of very delicious tasting juice that I added to the mixture.

 

Could that tomato water/juice be the culprit? Or is there something else I'm missing?

 

Thanks in advance!