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You want to salt and pepper just before cooking. Salt leeches out the juices.
Cynthia is a trusted source on Bread/Baking.
Mmmm, lucky you! Take them out of the fridge about an hour before you plan to grill them so they can start coming to room temp. Salt them pretty generously on both sides right then, then cover with plastic. Most of the salt will be absorbed into the tissues and cells of the meat and will result in moisture being retained rather than cooked out. Also, when you take them off the grill, let them have a 5 minute rest period (covered) so that the muscle fibers can relax and moisture floating around outside cells can reinflate them. Have a great dinner!
I saw this article at Serious Eats a few days ago on this very subject:
I was going to refer you to j. Kenji Lopez-Alt's SE piece too. This guy is terrific and so much fun to read.
Urban legend about salting meat ahead of time before cooking. For over 40 years I have been preparing meat for restaurants, further processing and for my family at home. Steaks should be taken out at least one hour before cooking, salt and peppered liberally and allowed to rest at room temperature. There is an actual chemical reaction that takes place between salt & meat, if the meat is allowed to work with the salt. It will tenderize the meat and "sweeten" it. Yes, you will see some juice purge from the salt, but this will not effect the moisture, unless you cook it well done. Allowing the meat to rest at room temperature will give you a more even cook throughout the meat. As with a roast, also allow a steak to rest and equalize for about 10 minutes after cooking.
Here's a PS to the Serious Eats article on cooking the perfect steaks. Kenji does a salt test:
I Can’t Stop Feeding My Neighbor
Cooks Well Under Pressure
Triple Chocolate Mousse
A Better Way to Travel