Dry Food
The vast majority of dry food is made with a machine called an extruder. First, materials are blended in accordance with a recipe created with the help of computer programs that provide the nutrient content of each proposed ingredient. For instance, corn gluten meal has more protein than wheat flour. Because the extruder needs a consistent amount of starch and low moisture to work properly, dry ingredients—such as rendered meat-and-bone-meal, poultry by-product meal, grains, and flours—predominate.
The dough is fed into the screws of an extruder. It is subjected to steam and high pressure as it is pushed through dies that determine the shape of the final product, much like the nozzles used in cake decorating. As the hot, pressurized dough exits the extruder, it is cut by a set of rapidly whirling knives into tiny pieces. As the dough reaches normal air pressure, it expands or “puffs” into its final shape. The food is allowed to dry, and then is usually sprayed with fat, digests, or other compounds to make it more palatable. When it is cooled, it can be bagged.
Although the cooking process kills bacteria in the ingredients, the final product can pick up more bacteria during the subsequent drying, coating, and packaging process. Some experts warn that getting dry food wet can allow the bacteria on the surface to multiply and make pets sick. Do not mix dry food with water, milk, canned food, or other liquids.
A few dog foods are baked at high temperatures (over 500°F) rather than extruded. This produces a sheet of dense, crunchy material that is then broken into irregular chunks, much like crumbling crackers into soup. It is relatively palatable without the sprayed-on fats and other enhancers needed on extruded dry food.
Semi-moist foods and many pet treats are also made with an extruder. To be appealing to consumers and to keep their texture, they contain many additives, colorings, and preservatives; they are not a good choice for a pet’s primary diet.
Bottomline: low quality wet food is better than the highest quality dry food, for your cat's overall long-term health.
Wet Food
Wet or canned food begins with ground ingredients mixed with additives. If chunks are required, a special extruder forms them. Then the mixture is cooked and canned. The sealed cans are then put into containers resembling pressure cookers and commercial sterilization takes place. Some manufacturers cook the food right in the can.
Wet foods are quite different in content from dry or semi-moist foods. While many canned foods contain by-products of various sorts, they are “fresh” and not rendered or processed (although they are often frozen for transport and storage). Wet foods usually contain much more protein, and it’s often a little higher quality, than dry foods. They also have more moisture, which is better for cats. They are packaged in cans or pouches.
Freeze-Dried Food
Freeze-drying is a technique that preserves raw ingredients [such as fresh meat] to deliver a finished pet food product which is safe, shelf stable and lightweight. Freeze-dried cat & dog food is technically raw and full of nutrients that would normally be lost during the Dry food preparation process but are retained during freeze-drying.
Most Freeze-dried food goes through HPP (high pressure processing) to naturally destroy harmful bacteria while assuring the food’s nutritional value is not affected. Dogs and cats, especially those with compromised immune systems, can greatly benefit from a raw diet that doesn’t carry the risk of bacterial contamination.
We at the Gargoyle Catterie swear by Freeze-dried, we mostly use a combination of Stella & Chewy, Vital Essentials, and Purpose. Since it tends to be on the spendier side of things & we have multiple cat foster households, we do a mixture of Wet food with Freeze-Dried mixed in.
Comparing Food Types
Because of the variation in water content, it is impossible to directly compare labels from different kinds of food without a mathematical conversion to “dry matter basis.” The numbers can be very deceiving. For instance, a canned food containing 10% protein actually has much more protein than a dry food with 30% protein.
To put the foods on a level playing field, first calculate the dry matter content by subtracting the moisture content given on the label from 100%. Then divide the ingredient by the dry matter content. For example, a typical bag of dry cat food contains 30% protein on the label, but 32% on a dry-matter basis (30% divided by its dry matter content, 100-6% moisture = 94%). A can of cat food might contain 12% protein on the label, but almost 43% on a dry-matter basis (12% divided by its dry matter content, 100-72% moisture = 28%). Dry food typically contains less than 10% water, while canned food contains 78% or more water.
In summary: 90% of your cat's diet should be canned paté.
Links:
The Downside of Free-Feeding Your Cat | Hill's Pet
Why Not Free-feed Your Cat | NutriSource
Cat Feeding Myths | Dr. Jean Hofve's website
10 reasons Why Dry Food is bad for your pet | Little Big Cat
How to Select Catfood | Gargoyle Catterie The Perils of Free Feeding | Gargoyle Cat
General Feline Nutrition info | the Gargoyle Catterie