I'm by no means an expert Pops is referring to, but here is how I see it.
With dry curing we trying to preserve meat by removing water from meat thus denying harmful bacteria the essential ingredient to grow in. But the drying has to be done in environment with relatively high humidity, which may seem counterproductive, but it serves the purpose of preventing the surface to dry out and trap the water in the middle part. Water has nowhere to go and meat may get spoiled.
I think that 30% loss in weight is a guide only and means that enough water was lost to consider the product to be dry enough. Given proper temps and RH was maintained, the cross section should by consistently dry i.e the water from inside migrated to the outside and was removed.
It may work in most cases but sometimes it is just necessary to go by feeling the firmness or cutting out the sample a have good look and sniff. If if doesn't look or taste dry enough, give it extra couple of weeks of hang time and don't worry about the weight.
The 30% weight loss rule may not work consistently because it depends how much water is actually in the product before drying. If for example a salami receipe call to add bactoferm disolved in water it sure would have more water than a lean muscle cut. So if lets say the muscle has 30% of water in weigth when it drops weight by 30%, that means it is completely dry. If salami had 40% of water in total weight, by dropping to 30% weight the 10% of water is still there.
[/end of rant]
Edited by Laszlo - 4/17/12 at 4:48pm