A friend sent me this article/paper on what is really in cat food and what the different ingredients are there for... it answers a lot of questions that are asked on here so I thought I would share.
Its a pdf and about 20 pages
Cat Food Uncovered
If you don't want to download the whole thing - these are the basic bits:
Meat or Meat-Based Ingredients
Animal by-products (US)
AAFCO define these as parts not used for human consumption e.g. kidney, lung and tripe. By-products are secondary or incidental products of the meat industry e.g. feathers, hair. Poultry by-products contains head, feet, underdeveloped eggs, intestines, feathers and blood. Fish by-products are fish process residues and can contain heads, tails, intestines and blood. Meat by-products can include hair, hooves, viscera and also the blood soaked sawdust.
Animal by-products (UK)
Unprocessed fresh or frozen slaughterhouse material. Processed material including blood meal, meat meal, meat and bone meal, greaves (the dry remnants left over after fat rendering)
Animal by-product meal (US)
Made by rendering those animal tissues which do not fall into the US categories listed here.
Animal digest (US)
Powder or liquid (soup, slurry) made from dndecomposed animal tissue, broken down using chemical or enzymatic
hydrolysis. The type of meat used is specified e.g. chicken, turkey, beef. Digests are ingredients not soluble in their natural state, but made soluble (hence useful as ingredients) with the use of heat, moisture and or
chemicals/enzymes e.g. "Poultry Digest" may be processed chicken feet.
Fish meal (UK)
Dried processed whole fish and fish offal (e.g. cod heads).
Highly Pigmented Slurry (UK)
Mechanically Recovered Meat (UK) pulp. Contains varying amounts of bone. This slurry is reformatted into chunks and may be texturised.
The ground or pulverised composite of animal feed-grade
ingredients e.g. poultry by-product meal consists of the ground, rendered, clean parts of the carcass of slaughtered poultry, such as necks, feet, undeveloped eggs, and intestines, exclusive of feathers, except in such
amounts as might occur unavoidably in good processing practices. Meal contains nothing humans would term meat.
The clean flesh of slaughtered cattle, pigs, sheep or goats. Does include muscle meat, tongue, some organs, fat and skin of the animal. AAFCO define "meat" as the "clean flesh of slaughtered mammals as is limited to the striate muscle with or without the accompanying and overlying fat and the portions of the skin, sinew, nerve and blood vessels, which normally accompany the flesh."
The flesh, including fat, skin, rind, gristle and sinew in amounts naturally associated with the flesh used, of any animal or bird normally used for human consumption. Does include diaphragm, head meat (muscle meat and associated fatty tissue only), heart, kidney, liver, pancreas, tail meat, thymus and tongue. May (depending on intended use of product) include brains, feet, large and small intestines, lungs, oesophagus, rectum, spinal cord, spleen, stomach, testicles, udder. (Meat Products and Spreadable Fish Products Regulations 1984)
Meat by-products (US)
The clean parts of slaughtered animals, excluding meat as defined above in "Meat (US)". Does include lungs, spleen, kidneys, brain, liver, blood, bone, fatty tissues, stomach and intestines. Does not include hooves, teeth, horns or hair.
Meat by-products (UK)
Offal e.g. liver, kidney, tripe, melts, lights. Also blood, bone, heads, feet, whole rabbit/chicken carcasses, other carcasses from which flesh has already been stripped for human consumption. Includes poultry by-products. (Waltham Book of Dog and Cat Nutrition)
Meat Derivatives (UK)
Rendered carcass material (I could not find a precise description)
Meat meal (US)
Rendered meal (dry) made from animal tissues. Does not include blood, hair, hoof, horn, skin, manure, stomach or intestinal contents, except for those small amounts unavoidably included during processing (contaminants). AAFCO define "meat meal" as "the rendered product from mammal tissues, exclusive of any blood, hair, horn, hide trimmings, manure, stomach and rumen contents."
Meat and bone meal (US)
Rendered meal (dry) from meat and bone. Does not include blood, hair, hooves, horn, skin, manure, stomach and intestinal contents, except for very small amounts that may be unavoidably included during processing.
Mechanically Recovered Meat [MRM] (UK)
Meat (UK) obtained by mechanically stripping flesh from bones. MRM includes meat recovered using combinations of grinding, steam and high pressure. Contains bone
marrow, cartilage and ground up bone.
Beet pulp (US)
Dried residue of sugar beets from the sugar production industry.
Brewer’s rice (US)
Small pieces of rice kernels sifted out of the larger kernels of milled rice.
Cereal by-products (UK)
By-products of the cereal industry. Includes wheat, barley, oats, rice, rye, maize (sweetcorn), some sorghums. Sago and tapioca are considered as cereals although they are processed cassava root. (Waltham Book of Dog and Cat Nutrition)
Cereal food fines (US)
By-products of breakfast cereal production; particles of food.
Cornmeal, Corn chop, Ground corn (US)
Meal made from the entire maize (sweetcorn) kernel.Must contain no more than 4% foreign material.
Corn gluten meal (US)
Residue from the manufacture of maize (sweetcorn) syrup or starch. Gluten is a sticky substance which gives wheat starch or maize starch its tough elastic quality. It is used to bind or hold together other ingredients.
Dried kelp (US)
Dried seaweed. The percentage of salt and minimum percentages of potassium and iodine must be stated on the label.
Dried whey (US)
The thin part of separated milk, dried (powdered). Must not be less than 11% protein nor less than 61 percent lactose.
Mill run (US)
See vegetable by-products
Textured Vegetable Protein [TVP] (UK)
Made from de-fatted soya bean meal.
Vegetable by-products (US)
The residue left after the primary food product has been extracted during milling e.g. "Corn Mill Run" is a pulverised blend of maize husk and corn-cobs left over after the sweetcorn kernels have been removed.
Keeps dry ingredients (flour, salt) free-flowing, prevents clumping
Prevents growth of bacteria
Preservative, prevents spoiling
Turns murky brown slurry/meal a more appetising colour!
Binds water and fat together so they do not separate
Turns slurry into a meaty or jelly texture
Modifies flavour, may do this by chemical effect on brain itself!
Adds flavour to unappetising slurry/meal
Flour treating agents:
Help at various stages in recipe
Turns slurry into a meaty or jelly texture
Makes the food taste moist.
Raising agent used in flour
Sweetener which adds no nutritional value.
Oxidizing and reducingagents:
Used in rendering, processing and cooking.
pH control agents:
Control pH of food or modify pH of urine.
A texturizer - for chewiness and some reformatted meat products
Used only to aid or ease the manufacturing process
Dissolve flavouring or colouring so they are evenly distributed
Stop food separating, curdling or breaking down; thickens food
Surface active agents:
Surface finishing agents:
Modifies texture of food e.g. by affecting proteins