Soup science help needed!
One of my favorite summer dishes is tomato soup. I make a version, based on a recipe by Barbara Kafka, that is simply a lump of butter, some sliced onion, and a lot of chopped tomatoes, cooked and run through a food mill. Add some basil chiffonade, salt and a splash of cream. Delicious! Also tasty without the cream, but you know how it is...
Last year, when very fresh and tasty whole milk became available at my farmers' market, I tried using milk instead of cream. It tasted good, but the milk really didn't disperse into/emulsify the soup. It seemed to stay in little little droplets, suspended in the soup. Not attractive.
I was making my first batch of this summer yesterday, and, since I always have cream on hand now that I am constantly making ice cream, I decided to add some cream. Same problem! The cream was not ultra-pasteurized and was fresh and not past its sell-by date. When I first started making this soup, the cream blended in beautifully, making a uniformly dark pink pot of soup. Now I can't seem to get it right. Could it be the acidity of the tomatoes? The heat of the soup? Can anyone advise? Many thanks in advance!
The only thing I can think of is that the little "droplets" are actually coagulated milk protein (casein, I believe) and that the coagulation occurs due to excess acidity from the tomatoes. I believe that too much heat can cause the same coagulation. As an experiment, you could try taking a sample of your tomato mixture, prior to adding the milk/cream, and raising the pH (i.e., lowering the acidity) with some baking soda. If the "droplet" phenomenon doesn't happen I think that would be definitive. It's likely that the soup won't taste very good with very low acidity, so it may take a bit of experimentation to strike the right balance with a production batch. If lowering the acidity doesn't change things then I'm stumped.
Good question LindaMc. I have added milk or cream to even COLD tomato soup and it has curdled (or glumped up).
FlyFish is likely correct. Last year was my first crop of tomatoes at my "new" home. The soup was MUCH more acidic than soups made from tomatoes grown at my previous home--just miles away!
Baking soda did reduce the acidity, but be VERY careful, and add only a tiny bit at a time, or you will lose the tomato "essence".
I don't know if this will work, but it will be easy to try. Maybe "temper" the cream with a tiny bit of tomato pureee, whisk, add a little more, whisk, etc. SImilar to thickening a hot sauce with an egg yolk.