A question came up in another thread concerning the merits of allowing game meat to develop a "crust" (a relatively dry surface) while hanging in the field. I thought a separate thread on that issue would be a good idea. Here's my take.
Meat goes bad one of two ways; from the inside out (bone sour) or from the outside in (dirt, surface bacterial contamination, fly eggs, etc). The primary purposes of a game bag are, as I see it, two-fold.
1. Keep the meat clean. A good game bag should protect your meat from dirt, keep flies out, etc. This is why I don't favor a cheesecloth-type bag that allows fly eggs to penetrate the material when stretched.
2. Keep the meat dry. Meat-spoiling bacteria require a mostly moist environment in which to multiply. Therefore, a bag that facilitates drying of the surface of your game meat is preferred over one that does not. So what kind of bag facilitates drying? That's where the controversy with synthetic bags lies, for the most part. In my opinion (and I have used synthetic and cotton bags in the field, even on the same meat pole), a game bag must breathe well. This allows moisture molecules to be carried away from the surface of the meat by air movement (wind, breezes, etc). It has been my experience that the interstices between the fibers on the synthetic bags are simply too small to allow moisture to freely pass. Rather, I have experienced the opposite; in warm weather a humid environment forms inside the bags, which actually speeds up bacterial growth, rather than inhibiting it. The first generation of synthetic bags were much worse than the new ones you see these days, but even the second generation material lacks the airflow I'd like to see. Hopefully the manufacturer is seeking out a material that allows better airflow, which would eliminate this problem. This would make the bags more useful in a variety of field situations. Second, the bag should be made of an absorbent material. Absorbent bags (cotton) will attract moisture as it evaporates off the meat, facilitating the formation of a dry crust on the meat. Synthetic bags do not absorb moisture, period.
HUMID / RAINY WEATHER
It should be mentioned that if conditions are very moist (rain, high humidity, etc), you are going to have a battle on your hands trying to get your meat to form a dry surface no matter what bags you use. This is when it is especially important to use bags that absorb moisture, rather than those that don't.
REMOVING THE BAGS
There are times when the best thing is to simply remove the bag while the meat is hanging. If flies are a problem, build a smudge fire to keep them off, and keep a watchful eye on the meat to ensure no eggs are laid on it. One of the best ways to ensure this is to remove the bags at dusk, if it is cool enough that flies are not flying. Then bag the meat first thing in the morning. This may allow your meat to form a dry surface.
USES FOR SYNTHETIC BAGS
So is there a use for synthetic bags? I think there is. Cotton bags take up a lot of space, and are much heavier than synthetics. So I carry the synthetic bags in my pack as I am hunting / guiding, so I am always prepared to bag meat immediately at the kill site. When I get the meat to camp, I change out to cotton bags so the meat will form a dry surface.
I have encountered a few folks that will defend synthetic bags in any and all circumstances, even saying that the formation of a crust is not important. I think this is bad advice and may reflect limited field experience. Yes, there are times when the synthetics are all you need. This is particularly true if the meat is going to a processor immediately or within a day or two. But it does not hold on longer expeditions where the meat may hang several days in the field until a pilot can pick it up and deliver it for processing. Cotton bags have served this purpose very well for many years. So while synthetic bags may have a place in certain circumstances, they are not the universal end-all, be-all, do-all that some claim. This is not about personalities or opinions so much as keeping meat clean, cool and dry.