Reading the Label
When you pick up a can of tuna, there are three to five terms* that may appear on the label that let you know what you are buying. The label will tell you the:

(1) Physical composition of what is in the can
(2) Color designation of the meat
(3) Media in which the tuna is packed
(4) Seasonings or flavorings that are added to the fish.
(5) Other Ingredients

Examples:
Chunk Light Tuna in Water
Solid White Albacore Tuna
Grated Dark Tuna in Oil

Physical Composition
(1) Solid Pack. Sometimes you will just see the word solid. These fish are filleted (or quartered) into sections and the fillets are placed into the can without any free fragments of tuna. There may be small pieces added to fill a container, but this must be less than 18% of the can's contents and must be pieces broken from the tuna loins.
(2) Chunk. A mixture of varying size pieces of tuna. A test is performed in which the tuna is spread out and pressed through a 1/2 inch mesh screen. After the test, there must be 50% or more of the tuna remaining that did not go through the screen in order to qualify as chunk tuna. The fish will retain its structure, so you will still be able to tell that your can contains pieces of meat.
(3) Flake. The same test is performed as with chunk, but if more than 50% of the fish goes through the mesh screen, the tuna is flake. You will still be able to tell that it is pieces of meat.
(4) Grated. The fish is made into small particles, all of which will go through a 1/2 inch mesh screen. You will be able to distinguish between the particles. The fish will not be paste-like.

Color Designation
(1) White - Albacore is the only fish that can be sold as 'White' tuna.
(2) Light - The color of the fish cannot exceed a Munsell value of 5.3
(3) Dark - The color of the fish is more than a Munsell value of 5.3
(4) Blended - At least 20% is light or white, and the remander is dark.

Packing Media
(1) Oil - Any type of vegetable oil but not olive oil.
(2) Olive Oil (might say 'Tonno' on the can)
(3) Water

Added Seasonings or Flavorings
A variety of flavorings and seasonings can be added including:
(1) Spices, spice extracts, or spice oils.
(2) Garlic
(3) Lemon flavoring
(4) Vegetable Oil (up to 5% of the contents of the can may be vegetable oil, and the tuna will still qualify as water packed)
(5) Hydrolized Protein
(6) Monosodium Glutamate (MSG)
(7) Salt
(8) Vegetable broth which must have at least two of the following vegetables: beans, carrots, garlic, parsley, potatoes, red bell peppers, tomatoes, cabbage, celery, onions, peas, green bell peppers, or spinach.
(9) If the tuna is smoked, it will say smoked on the can.

If the tuna is seasoned with anything that is not on the list, it will say 'Seasoned with _________' and the seasoning item will fill in the blank.

Other Ingredients
If the can has sodium acid pyrophosphate, the label will say 'with added pyrophosphate' or 'pyrophosphate added.'

Can Contents - How much tuna is in there?
The new standard size can of tuna is five ounces. This is a reduction from the 'normal' size of six ounces, which was the standard for several years. Prior to the six ounce standard, tuna came in seven ounce cans.

The FDA has the following guidelines for the amount of tuna left in the six ounce can after you squeeze out the liquid:

Solid - 4.47 ounces
Chunks - 3.92 ounces
Flakes - 3.92 ounces
Grated - 3.96 ounces

It is important to note that soy and vegetable broth make the tuna behave like a sponge. The meat will soak up to as much as 20% additional water weight, and the smaller the chunks or flakes in the can, the more water weight the tuna will soak up. As a result, if you have tuna with soy or vegetable broth, the meat weight may be inflated up to 20% with water.

*Occasionally the label will tell you the species of fish that is in the can, but this is not typical. For instance, the most popular canned tuna is 'chunk light.' This can be a mixture of skipjack, yellowfin or bigeye. Any tuna that is called 'white' can only be albacore. Since albacore is the premium eating canned fish, it is usually listed by name on the label.