And rub. Thanks in advance :)
A pork shoulder that large can take a while to roast. Here is a recipe for an 8-10lb shoulder that starts at a very high temperature of 500 F, and then is lowered to 300 F, http://www.ibreatheimhungry.com/2012/08/easy-roasted-pork-shoulder-3.html. This method would help cut some of your cooking time and also give the shoulder a nice crust. Is this a boneless or bone-in pork shoulder? A bone-in shoulder will take longer to cook.
A bone in
Also It is boneless
While it may sound like you'll be cooking the meat for a long period of time, the fat in the shoulder will keep it nice and moist. You're in for a delicious meal, especially with that cajun rub!
It is boneless
225 = 1hr per lbs or more. Your looking for an internal temp of 190. At that temp and time, you'll get the best results, trust me.
I have to politely disagree with Dave on this one. In the old days the FDA wanted you to hammer the snot out of a pork roast. You don't need to do that. An internal temp of 140F is just about fine---and of course it needs to rest because it's going to get a heat boost as all those sizzling juices spread back out again. Use a good instant read thermometer.
Dave, I am with you, that's a recipe for heaven. You can definitely go longer than one hour per pound, the longer the merrier.
Pierino, I do not think that higher internal temperature of slow roasts has to do with FDA. 140F is perfect for pork tenderloin, but I believe that high internal temperature of slow roasts has to do with collagen turning into gelatin with prolonged cooking at around 160F and above.
Yes, this seems to be the most thoughtful answer.
Queensashy is dead on. I love my pork chops to come off right at 140, but a shoulder is a different concept. Remember, it was considered the worst piece of meat on the pig because it was so fatty, and was given to the slaves during those days. But, being smarter than their owners, they learned that slow roasting the shoulder until all the fat was melted created delicious pulled pork. Hence the birth of BBQ in the south (Carolinas). If its bone in, it's not done until the bone pulls away from the meat. For boneless, it's right around 190.
If you are cooking it with the skin on which is the best part, it should cook on med heat until skin becomes crispy then turn down and cook on low heat for about 3 hours.
If you stop cooking a pork shoulder at 140°, it will be like trying to eat leather. The reason for the 190° (some barbecue chefs even recommend 200-205) is that you want to break down the collagen in the meat so it isn't so tough, which requires higher temperatures than just what is required to make the meat safe. Pork chops are great at 135, but a pork shoulder at that temperature will be inedible