I too am curious about what you saw.
It's just hard to imagine how, if it was not a cloud of vapor, a real liquid stream would be visible that far away -- and moreover, how a plane could be putting out so much liquid to make it visible that far away? If only we could have a picture or a picture similar to what you were seeing? One stream, you say?
Is it possible that a reflection or refraction of the sun at a certain angle into a vapor trail made it look like liquid?
I've watched thousands of flights on approach and haven't seen this before, which is why I immediately turned to FT. It's certainly possible that it had something to do with the angle of the sun, but the planes are close enough to be obvious it wasn't coming from the product of combustion (not the engines), but the tail. I have, and will continue scouring Image search for something similar. Have also emailed a relative who is retired 757/67 pilot.