It's the internal form of blossom end rot. There are some images of internal BER at 1:10 of this video.

Agree. While external BER is far more common the causes when it is inside the fruit are the same.

Dave

Thanks! That's very interesting. I've never heard of internal BER. I didn't see it on the ends of any of my tomatoes this year, so it's surprising to me that that's what this is. Thanks for the YouTube. That was a helpful explanation.

This paste tomato gave me so many tomatoes compared to other kinds I've tried, but now I wonder if Tiren is prone to this. I have been just cutting around it so we're using them just fine.

I find that dry BER doesn't affect taste much, so it's not a big deal unless you need aesthetic fresh presentation. I just cut it away. The BER that turns wet and mushy (internal BER can do that, too) usually makes the entire fruit taste funky in a bad way, though. Bleah.