From http://www.extension.umn.edu/distribution/nutrition/DJ5968.html : ------------------ "It has been shown that during the aging process certain changes take place in portions of the structure of collagen and muscle fibers. Currently, it is thought that enzymatic-caused changes in the structure of muscle fibers are largely responsible for the increase in tenderness. It is known that tenderness decreases immediately after slaughter while rigor mortis takes place (taking 6 to 12 hours to complete); then tenderness increases gradually. Tenderness continues to increase up to 11 days, after which there is no increase in tenderness..." "Some research has demonstrated that as fresh meat ages, the activity of the various enzymes decreases and protective action against oxidation declines, thus increasing susceptibility to oxidation. This suggests that oxidation of fresh raw meat becomes increasingly important the more meat is aged. During the aging process, one can also expect a loss of weight of the product. Because the lean (exclusive of trimmable fat and bone) is approximately 70 percent water, it's easy to see why there is a weight loss. The weight loss is caused by dehydration of the lean and fat. The weight loss occasionally occurs at tremendous proportions depending on relative humidity, amount of air flow and temperature of the aging cooler. During chilling of the hot carcass immediately after slaughter, the carcass will lose 2 to 3 percent of its weight because of moisture loss. Aging the carcass beyond this time will result in additional tissue shrinkage of 1 to 1.5 percent for each seven days. Carcasses with a thin external fat cover will lose more moisture than carcasses with a heavy fat cover. One study observed an 18 percent trim and shrink loss from loins aged 14 days in a 36 degrees F cooler."