Yellow underarms help!

[birdbrain21]

I have quite a few white tops that are really old and the underarms have turned yellow. I thought this was just because they are old and have been worn too many times, so I treated myself to a new one a few weeks ago. I've only worn it a few times and I washed it on a 40 degree white wash after every couple of wears but the underarms look like they're turning yellow already.
How can I stop this happening? And even better how can I get the yellow stains out of the tops I already have? I tried soaking in a bleach solution and that didn't help...

I've changed deodrant brands a few times but that didn't make any difference either.

Thanks

Bumping as I come on to ask this, dh bought brand new cream tshirt, one wear on holiday and neck and armpits are yellow so I linked it to sweat not deodorant but I can't get marks out.

I think that spraying the underarms of tops with white vinegar before washing should help to remove deodorant + niff.

White vinegar will sort it out.

Anther thread said Sanex deodorant prevents this. Drying in the sunshine also helps in my experience.

Thanks will try some vinegar. How can I stop it going yellow in the first place?