I had a similar problem with a nylon t-shirt.  I used a spray bottle of something called Power House Fabric Refresher.  It works great and doesn't harm the fabric.

I'd try the simpleist remedy first - I'd just hang the pack out in the sun and air for a couple of weeks, at least

the sun should either break down the smell, or at least accelerate the cause, I'd think - nothing to lose

don't know from experience but I wonder if baking soda, hydrogen peroxide, and a small amount of dish soap would do.   same formula we've used on the dog when she gets skunked.  my brother had an old beat up tarp with that vomit smell. i soaked it in a bleach solution which did not help. didn't think to try the skunk solution. threw the old tarp out

Try some Hydrogen Peroxide, it takes the smell out of just about anything.  It took fish oil out of the carpet, it should work.

Check out the Mcnett(dot com) website!  They have good instructions (on how to deal with the smell) there. The vomit/cheesy smell is the PU coating breaking down.

I believe when we smell that sour "vomit" smell what we are picking up is acidic in some form or another. Why not try several boxes of baking soda and water, seal it in a plastic bag and set it in the sun. Shake whenever you remember to. Leave it for a week.
I don't remember if anyone asked but you have checked to see it isn't actual remains of someones urp, Right?
Best Wishes, Rob

I've always wondered what nylon vomit smells like...

BARF bag, BARF pack what did expect for free, smells like a great deal
and I don't think I would try the piss test neither!
Happy camping

//Windsailor

Firechief,

Try a product calle Mirazyme by McNett - as someone else mentioned, it's enzymatic - uses microbes and enzymes.  I've used it successfully on foul smelling tents, sweat stained packs and dry bags - even worked well on my "trash kit" dry bag that gets all the garbage from the trip - keeps the mess and smell from everything else.  It' says it safe for almost anything and it beats peeing on stuff...except snow.  It runs about $4-5 for a 2 oz. concentrate bottle.  If that don't get it out, maybe you should re-gift it!

portage dog

Urine decomposes to ammonia when exposed to air - I'd guess that ammonia is the odor-removing compound in your technique.

But urine decomposition usually produces a number of other by-products, some of which have their own unpleasant odor. You may have noticed this (if not, your wife probably has).