what makes red tomato sauce turn orange in colour? I have noticed that tomato sauce (having started with deep red tomatoes) can turn orange.

The colour change isn't from mixing in an ingredient of different colour such as cream. It seems to occur in these observed circumstances:


Blended using a blender or an immersion blender (boat motor).  The colour change is noticeable quickly (30 seconds or so)
Cooked tomato sauce for a long time (hours)


The sauce stays nice and red if taken off the heat quickly and not blended.

Some of the ideas are:


Oxidization: the immersion blender doesn't incorporate much air so this is unlikely.
pH change: this usually requires adding of other items such as vinegar.
Breakdown of seeds or something else in the tomatoes releasing something that causes the change in colour.  Although I think I've seen seedless tomato sauce also go orange.


This question also seems to have been asked on the serious eats forum without a substantive answer.
Update:

It's been a few years but there now is a definite answer to this question.  

@PegDat is correct.  The Tomatoes Oxidize when they are exposed to air and turn orange.  

This was proved using an experiment with vacuum blender  blending tomatoes with air and with most of the air pumped out.  

When you reduce the air in the blender chamber, the tomatoes remain bright red.