That question has been nagging me in recent weeks after some of the sandwich meats I purchased seemed more watery than usual. I've known for years that many meats in the store are water-injected, but I wondered if the percentages had increased recently. After a search today, I found an excellet article at the USDA Food Safety and Inspection Service. Some salient points:
Naturally Occurring Moisture Content of Meat and Poultry
The muscle is approximately 75% water (although different cuts may have more or less water) and 20% protein... The percentage of naturally occurring water in meat varies with the type of muscle, the kind of meat, the season of the year, and the pH of the meat...
Water Content of Meat and Poultry
The amount of naturally occurring water, or moisture, present in meat and poultry may surprise consumers (see chart [at the link]). An eye of round roast is 73% water before cooking. The same roast after roasting contains 65% water...
Since water is a component of protein (but not fat), a leaner cut will contain slightly more water on a per weight basis.
Leaner Beef Contains More Water
Hotline callers sometimes comment that today's beef contains more water and also doesn't taste the same as in the past. One reason for this is that today's animals are bred to be leaner. Meat from these animals is naturally leaner and contains more water. The fat in meat contributes to flavor, so a leaner cut will taste different than a fattier cut. Some of these leaner cuts are enhanced with a flavor solution.
Enhanced Meat and Poultry Products
Enhanced or value-added meat and poultry products are raw products that contain flavor solutions added through marinating, needle injecting, soaking, etc... Typically, this information will be on the principal display panel or the information panel.
Freezing Meat and Poultry
The water that is outside the cell wall freezes first. As it does, it leeches water from inside the cell walls. When it thaws, the original balance does not return to normal. The thawed product will have lost some of its natural springiness...
The faster meat and poultry freezes, the smaller the ice crystals will be. Smaller ice crystals will do less damage. Products that are flash-frozen by the manufacturer will have superior quality to fresh products frozen by the consumer.
Moisture Loss from Meat and Poultry
Beef is often ground while partially frozen. Because ice crystals are in the frozen beef, there may appear to be more liquid in it...
At the grocery store, the products are displayed in refrigerator cases as cold as 26 °F. At this temperature, the cells of the product will "loosen up" somewhat and some of the moisture will melt and gradually seep out. The production of this visible meat or poultry juice is known in the industry as weep or purge... In a home refrigerator set at 40 °F or below, even more liquid will seep out of the product. The longer a product sits in the refrigerator, the more liquid will be released from the muscle cells.
Packaging of Meat and Poultry
While the package sits in the refrigerator case, the vacuum is still in effect, extracting the juices out of the meat. Because these packages are airtight and leak proof, the juices accumulate in the package. In contrast, the plastic wrapped packaging, typically used by most supermarkets, allows a certain amount of evaporation...
Retained Water in Raw Poultry Products
Poultry is not injected with water, but some water is absorbed during cooling... During this water chilling process, turkeys and chickens will absorb some of the water, and this amount must be prominently declared on the label. It is not unusual for poultry to declare 8 to 12% retained water on the label.
Cooking Meat and Poultry
In general, the higher the cooking temperature, the more moisture will be lost in cooking. It is not unusual for a beef roast to lose 1/3 of its original size and weight when cooked at a high temperature or cooked too long.
The fact sheet is dated 2007, but presumably the same basic principles still apply.