Milk has fats in it as well as the liquid, so you may experience a drier muffin (and thus also a bit less flavorful), but liquid is liquid. If you have a milk allergy, go ahead. Otherwise, the tested recipe is likely going to be best.

What kind of pineapple juice? From a can of pineapple tidbits? Heavy syrup, or actual juice?
No matter the juice is sweeter and more acedic than milk and so is the pineapple more acedic and wetter than raisins. Can you use dried pineapple instead of raisins? If youa re using the juice you may also need to adjust the ratio of BP to BS, and maybe add more flour to fix the excess moisture issue
Frankly I would do one of the other....either use the pineapple in the muffins or use the juice and raisins...
And I really don't like the flavor of rum extract....I would use real rum or vanilla extract.

-Fruit juice will generally work as a milk substitute in something like muffins since there is not a lot of it.

-Fruit juice will also add a new flavor, so choose something that will compliment the ingredients list (white grape juice is a neutral, orange juice can get bitter so I'll use apricot juice instead of orange - since you are using pineapple, I'd use pineapple juice). I often substitute almond milk or coconut milk for cow's milk (whatever I happen to have in the refrigerator).

-There is more than enough fat (butter and eggs) that you wouldn't need to adjust it if using fruit juice.

-You don't say whether you are using fresh, canned, or dried pineapple. Those choices may make a difference, albeit a small one.

-Things you usually need to be concerned with is the proportion needed to maintain between ingredients that are tenderizers (fat and sugar) and those that are structure builders (flour and eggs). Milk can be a tenderizer, but not in this recipe.

-I usually tend to change no more than one (possibly 2) things in a recipe; and I keep track of those alterations on a standardized sheet I improvised just for this purpose. I've used this method for decades when I alter recipes so I can track each iteration of the recipe. I include a copy of the original recipe, things I've changed (and why), a critique (I use food judging standards I use when judging foods at fairs), and things I might need to change the next time (and why).

-You can usually change one dried fruit for another without any other alterations. I usually use currants instead of raisins (and can usually use less currants than I would raisins), and I like to use apricots snipped into 1/4" bits using my OXO kitchen scissors.

-Grainlady

Sweetened or unsweetened, it will make the result sweeter, so reducing the sugar by a bit is good idea, if you're not into super sweet foods. I do it all the time, since I like to add more fruit than called for in many recipes.

For example, the other day I made a banana bread recipe in muffin cups & added more than a cup of diced strawberries - came out great too - very moist & tender. I also used olive oil instead of butter/shortening & plain yoghurt instead of sour milk.

I usually just pencil in any 'adjustments' to a recipe on the original - that way, I can erase & replace if I change it again in the future.

On further consideration, I think more of the problem might be subbing pineapple (tidbits, not dried) for the raisins. And I am using real rum, I have some of that to use up too. I think I'm either going to have to use up the pineapple or the juice. Problem is, I wanted to use up both, because they go together . . .

Use the pineapple juice and a little vinegar for extra acidity, in salad dressing. Mix with dry mustard and flour and cook for a sweet hot mustard dipping sauce, marinate pork chops, baste a ham with it.

What if you used the pineapple juice, and subbed unsweetened coconut for the raisins? Or, make pancakes instead of muffins and use the pineapple as a topping?

Sally

Here's how it went down. I took inspiration from Lizbeth's pineapple coleslaw post and used the pineapple chunks in coleslaw I made for my dad. He loved it. I found a box of raisins I also had to use up so I marinated the raisins in pineapple juice and rum overnight. I made the muffins this morning. I strained out the marinating juice and topped it off with half and half to get 1/3 cup. I followed the recipe more or less as-is other than that, except I didn't top them with maple syrup, I didn't want them to get that sticky. Instead, I added 1 TBLSP maple syrup to the batter. I topped them with sugar in the raw instead, which I often do for muffins. Here's a photo. I'm starting a "Muffin Monday" tradition. I don't have to be at work first thing on Monday's and making them on Mondays will give me enough to last the week with my morning or afternoon coffee. I also share them with my landlady, she does a lot of nice things for me, and she said no need to repay her other than keep her supplied with muffins!

These rum raisin muffins were just OK. If I do it again I will skip the pecans, they're expensive and didn't add that much to the flavor. I guess maybe toasting them might make a difference but I don't much care for nuts in muffins anyway. They are OK just out of the oven but don't keep all that well. I'd double the rum (I used 1 TBLSP) and use 1.5 cups raisins, and use brown sugar for the sweetener.

L, they look pretty darned good to me, and I like "Muffin Monday". I'd do that, but then I'd have to eat all those muffins and soon I'd have (even more of) a "muffin top", LOL.

Annie

I did that years ago....made oat bran muffins,wtih egg white applesauce nuts and some sort of dried fruit....forget what all.

I wonder how using coconut milk would have worked.

They DO look "pretty darned good" ;-)

We have muffins every Saturday (Sunday - loaf quick breads; Monday - pancakes; Tues. and Thur. - peanut butter toast; Wed. and Friday - whatever is left in the freezer - oat farls, cooked oatmeal scones, French toast, waffles, coffeecake; all gluten-free). I keep muffins in the freezer wrapped in foil pop-up sheets (purchased at Sam's Club). I bake them during the day, as needed, and reheat for serving.

We have a warming center (burner) on our electric stove. We place foil-wrapped baked goods on this burner and cover it with a glass bowl, and in 10-12 minutes you have warm goodies.

-Grainlady