Why would yellow plums turn super-tart when baked?

[Live_to_cook]

This morning I bought a load of little yellow plums at the farmer's market. Marveled at how sweet-tangy they were. Cut up a couple cups of them to top a puff pastry fruit tart. Mixed the fruit with 1/8c sugar. Loverly.

After the tart was baked, however, it was almost too TART to eat. Even with another sprinkling of sugar and brushed-on peach jam glaze.

What gives? Was it these particular plums? Would it be any stone fruit? (I've never made a tart like this before, only seen it done.) Shame to go through all that work and end up with something I had to brace myself to eat.

[momoreg]

Did they taste tart when they were raw? I've never heard of htem turning tart in the oven.

[Live_to_cook]

That's the weird thing: Raw they tasted sweet, though there was definitely an acid tang on the backside. My girls ate them and asked for more.

After they were baked into the tart, though, the girls would have spit it out, for sourness.

Live and learn, I guess.

[momoreg]

That is bizarre. I'm curious to see if anyone else here has experienced this. Do you know what type of plum it was?

[Live_to_cook]

Mystery solved. According to the garam masala thread, garam turns bitter when cooked. There's my mistake.

[momoreg]

[Live_to_cook]

Sorry, momoreg, I forgot to mention that I added a couple pinches of garam masala to the plums along with the sugar.

Doh.