As any-one who's tried to cook them will know, kidneys smell pretty awful. In deference to the preferences of my fiance, I've tried to reduce the urine smell by soaking them (as lots of recipes recommend) and then discarding the liquid before cooking. This works pretty effectively to remove the smell, but I wonder, am I also discarding most of the nutrients (which is, tbh, the only reason to cook the kidneys in the first place?
I've not seen any studies on this, unsurprisingly, but wonder if any-one has any insight?
I would be surprised if there was significant loss of nutritional value in a soak in water.
The fat soluble vitamins are very unlikely to leach out as they are hydrophobic so vitamin A, D, E, and K are safe.
Most of the metals are going to be bound up in either enzymes or metallothionein, a sulfur rich protein that chelates metals produced in large amounts in the kidney and liver. As far as I understand it, they often use some type of chelating agent like EDTA to remove the metals from proteins as they don't tend to come out easily. This suggests that little of the metals will be lost in the process of a water soak.
The water soluble vitamins do obviously pose a potential risk for loss. The vitamins that are water soluble will most likely have some polar groups like aldehydes or hydroxyls that cause it to like water in the first place. Proteins tend to have a significant amount of these polar groups as well, which leads to hydrogen bonding keeping them closely attached. In order for them to leach out into the water, those bonds would have to be hydrolyzed, which isn't necessarily impossible and may be favorable entropically. However, reactions tend to be significantly slower at lower temperatures, and I would imagine you aren't using hot water to soak the kidneys. The time spent soaking also makes a big difference as this is a diffusion process. If you don't leave them in hot water for a long time I suspect you'll get the vast majority.
Even if you are boiling them, the cells that contain the vitamins would need to be broken, then the vitamins would have to diffuse out of the food into the water. Most likely any considerable loss would be on the outside layers. You could always cook it sous vide and drink the broth, then no loss.
You're not likely removing nutrients. Fat-soluble vitamins are going to associated with the fat, which won't be dissolving in your soaking liquid. Water-soluble vitamins/minerals like to stick to proteins, so again, you're not likely to pull appreciable amounts of nutrients away from the meat.
Getting rid of ammonia, urea, other nitrogenous (and stinky) compounds via a soak sounds like a win to me. In the end, if it means you're more likely to eat the food, they'll be getting more nutrients in!
I don't think the uric acid in them can be considered a "nutrient" in the good sense. Hypothetically, if the solution you soaked them in was isotonic to the cells of the kidney, and yet still broke down the uric acid, you'd probably have most (if not literally all) of anything you'd considered a nutrient still there.
I don't bother soaking kidneys, however I like to eat small kidneys (rabbit, etc). The flavor at that size is "good" to my senses. A quick pan-fry, a dash of salt and thyme - BOOM - delicious kidneys.
I read (somewhere) a long time ago, that it was "overcooking" the kidneys which caused the urine smell/taste. A quick pan fry as described by greymouser might be the solution.
Funny coincidence, because I actually buy a few kilos of (pig) kidneys weekly and feed them raw to my cats. They are 1,50 euro/kg where I live, from freshly killed pigs.
Nutrition: Kidneys are a good source of the B vitamins, which are water soluble. I doubt that soaking them in water significantly reduces their vitamin content, however. The USDA database indicates that kidneys cooked by simmering in water are somewhat less rich in B vitamins than raw kidneys, but even the simmered kidneys are B powerhouses.
Raw, 100 grams http://nutritiondata.self.com/facts/beef-products/3466/2
Simmered, 100 grams http://nutritiondata.self.com/facts/beef-products/3467/2
Smell: If soaking doesn't kill the kidney scent for you, try cleaning and scoring the kidneys in a criss-cross fashion and then tossing them with 1 tsp of soda bicarbonate. Let stand for 20 minutes, rinse well, then toss the kidneys with 2 tsp of vinegar and 1 tsp of salt. Let stand in a colendar, draining, for 30 minutes. Blot dry before cooking. I learned this technique from Ken Hom's "Chinese Cookery," and it's the only technique I've used since.
Real "destruction" of nutrients really only occurs at high-temperature or long exposure to heat methods. Even then, the compounds often eliminated are easily replaced by other sources. Make it taste good, then supplement with alternative sources before freaking out.
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