Dehydrated dog foods are probably not even considered by most dog owners, which I think will mainly be because they are not highly promoted as a healthy quality alternative dog food.
In essence the only true comparison to this form of dog food option is the Dr Ian Billinghurt's style of BARF (meat plus some fruit and vegetables) but because BARF comes frozen it is far bulkier.
Sure you have to add warm water and wait on average 20 minutes but those minutes can also be taken to re-hydrate say a couple of days food, providing you have refrigeration storage available, for the unused amount.
In dehydrated dog foods I am including the two main methods of dehydration which are the usual air drying method plus also the "freeze dried" method.
Our dogs might not care that much or be that critical but it is worthy of consideration. You could try your dog with both versions and let your dog be the judge!
How it differs.
Dehydration of food works by removing only the water content from the food. All other nutrients etc. remain and are unaffected by the process, in fact the products are still considered as "raw". Exceptions are meat and eggs which are dehydrated at higher temperatures and more commonly considered as being the "cooked" version of dehydration.
Freezing, baking and high pressure processing affects food enzymes whereas in dehydrated food they are just suspended, until re-hydration with water happens.
It's Not New!
Drying of food (dehydration) for preservation has been around for a very long time, probably since the beginning of our world as we know it.
I've seen a couple of mentions that scientists in Africa have documented the live birth of fish from eggs that have been dehydrated by the sun in times of drought. Life cycle and hatching of the egg rekindles again when rain falls, the rivers start to run, lakes reform and fill again. Nature re-hydrates the "suspended" eggs which allow the life cycle to continue as if nothing had happened!
If it's good for Mother Nature then it's also okay by me!
Buyer Beware - as usual!
One thing to point out is any comparison being made of dehydrated dog foods needs to take into account the amount of water suggested by the manufacturer to re-hydrate the food, and then how much dog food results thereafter.
Extra water will make extra food but decreases the calorie content which means your dog will need to eat more to get the same food energy benefit, as it would from another product using less water.
An important point about changing your dog’s food.
Diet changes must be done gradually to avoid discomfort to your dog and changing to dehydrated dog foods is no different.
When you change either the quality, or style, of your dog’s food you need to do so over a number of days to allow the dog’s system to adapt.
Your dogs' large intestine bacteria becomes accumstomed to a certain level of PH within the intestine and any abrupt diet change will trigger a reaction from that bacteria to the intestinal lining to release water, and bring the PH back into the previous balance.
This results in your dog having stomach ache plus diarrhea, or extremely loose stools, and a reluctance to eat.
I would suggest the following 7 day example as a minimum changeover plan, because if you can do it over a longer period it is even better.
DAY (1) Add 10% of new food with 90% old style / quality. DAY (2) Put 15% new with 85% old. On DAY(3) it is 25% new and 75% old then on DAY(4) new becomes 40% and old is 60%. Follow this on DAY(5) with 60% new and 40% old and DAY(6) will become 80% new with 20% old and on DAY(7) the switch is complete with 100% new food.