Larry.the directions I got with the citric acid said 3 oz per 25 lbs of meat.I have used it making slim jims and it gives just the right level of tang.Be careful and dont use more than that or it will make the meat mushy.I tried fermento and was not happy with it at all.I dont know where you get yours but I found it at Allied Kenco Supply for a good price.I dont think it does anything to preserve the meat,its more of a taste thing I believe.Im getting ready to make slim jims next weekend,Ill let you know how it goes and post some pics.
I use 1oz encapsulated citric acid to 10lbs meat. Add after grinding, while mixing. Capsules are designed to melt at 135 degrees so you need to hold the internal temp at 135 for a couple of hours for the acid to do its job, thats pretty easy to do while your smoking. I guess you can try alittle more if you think you need more tang. This amount seems to be enough for my taste.
Get mine at PS Seasoning in Iron Ridge WI