Cooking pig food is something I do. But why, you ask? Why not just pour the pellets into the feed hopper as is? First of all, I don't use commercial pig pellets in this farm, nor much in the way of commercial feed. Thus I need to take a few steps in preparing the pig food.
Some foods I don't cook. Foods that the pigs like "as is" include many fresh fruits, old bread, milk that is beginning to turn bad, stale cereal, leftovers from my own meals, stale beer and other drinks. And haycubes, which I soak in water but don't cook. (I don't buy these, but am given a bag or two occasionally because they got touched with mold and were no longer safe to feed to horses.) But other things I do indeed cook for a variety of reasons.
#1- cooked food is more digestible. It's a known fact in the hog industry that heat treated feed is somewhat more digestible, resulting in faster and greater weight gain. Some cattle feedlots also steam treat their grains to improve and shorten finishing time for their cattle. Thus cooking pig feed = better weight gain.
#2- cooked food is often more palatable. There are foods that my pigs turn down when raw, but readily eat when cooked. I find the exact same situation with myself. While I don't like raw onions, I adore cooked ones. So cooking means that the pigs waste less. In fact, there isn't much they won't eat once it's cooked.
#3- cooking kills bacteria and fungi growing on the food. While food grown on my own farm appears to be "clean" from the various food poisoning forms of E. coli (etc), outside foods might be contaminated. Thus cooking off-farm food sources will help prevent introducing dangerous organisms into my own farm livestock. It's not a 100% guaranteed protection, but it helps (sort of like washing your hands to prevent bringing diseases and parasite eggs to your dinner plate).
#4- cooking prevents bringing in diseases and parasites that could be transmitted to humans, which could sometimes be found in off-farm waste foods. I often collect waste foods. It comes from local restaurants, stores, and friends. I don't know how that food was previously handled. Dropped on the floor? Contaminated by commercial meat liquids? Coughed or spit in? Food half eaten by unknown persons, that is, scrapings off restaurant plates? Were those people sick? I can't naively assume that everyone is disease and parasite free, thus I cook slop to prevent passing these problems onto my pigs. So anything that has the potential to carry a problem gets cooked.
On this farm, I use a wood fired rocket stove to cook livestock food. If I only have a little stuff to cook, I'll use a large soup pot. But for larger volumes I use an old jumbo pot big enough to fit a large Thanksgiving Day turkey. In fact, that's exactly what I'm using -- discarded turkey fryer pots. I've acquired several over the years. They're perfect for the task.
Most foods heading to become Mom's Famous Slop & Glop get ground up before cooking. It makes cooking time shorter, plus the pigs clean it up better (that is, less waste). Ground up food makes mixing the slop with the haycubes easier. When I first started out making slop for a few chickens, then later, one pig, I used a blender. I'd cook the foods first then blenderize it. But that became time and labor consuming as the number of chickens (and pigs) increased. So I switched to using a garbage disposal which I set up in my outdoor processing area. The disposal discharges out to a strainer set atop a five gallon bucket. Crude but it does the job. I keep thinking I should make an easier system to use, but it's not high on my priority list.