Put all ingredients except the ice in a large stockpot. Bring to a boil on high heat. Once boil is reached, lower heat, simmer for 5 minutes. Pour into a large container and add ice until you have 3 gallons of liquid brine. Once cool, submerge pork rack completely in brine. Leave in brine for 48 hours. 

Remove pork loin from brine, rinse and dry completely. With a sharp knife, make a shallow incisions in a crosshatch pattern on the fat-covered side of the roast. Season the roast liberally with kosher salt and pepper. Preheat oven to 225°F. In a medium prep bowl, whisk together the sorghum and whole-grain mustard. Place the pork loin in a large roasting pan and into the oven. With a digital-read thermometer check the thickest part of the pork loin every hour until it has reached an internal temperature of 125°. Remove the roast and increase the oven temperature to 450–500°, depending on how high your oven goes. Once the oven has preheated, place roast back in the oven for 15–20 minutes, until the outside is a deep golden brown. 

Lower oven temperature to 325°. Pull the roast out and brush with sorghum-mustard glaze. Place roast back in the oven for 5 minutes to bake on the glaze. Pull roast out when internal temperature reads 135°. Brush the roast with more glaze and allow to rest for 20 minutes. Slice and serve.