^ Flu got it right, it's the chlorophyll that is causing that smell, which is due to the water content in your plant.
From memory (don't quote me on this), you are suppose to lose about 80% in weight from water when you dry the plant. My numbers may be off but the point is there is A LOT of water you need to get rid of.
If you don't have the correct amount of water (under 20% weight left I would guess?), and you start burping, what you are doing is preserving that "grassy" smell that is naturally in the plant. If you continue to burp with all that extra water content, mold will eventually form completely ruining your bud.
It sounds like your bud is salvageable at this point, so do what you can to finish drying it out and then start burping again once you know it is dry. Next time you do this you should have a better idea of how dry it needs to be and when you start burping, you will know you did it right when your first burp makes you go "omg fuck yeah".
I'd imagine the drying racks you are referring to would help with the uniformity issue I was talking about or at least make it easier/quicker than using a paper bag, but if you a paper bag works just as well as long as you are using the same technique that we were discussing.
EDIT:
In case I didn't make it clear...YES you should have let them stay in the paper bag for longer.