I've never had trouble making Tuna and Pineapple casserole before coming to Turkey. After making the white sauce and adding the cheese, tuna and fresh pineapple the whole mixture turns sour. No amount of adjusting with salt or sugar can save it. This has happened at least 3 times now. Could the cheese be making it sour or some other chemical reaction. I used to use canned pineapple in Australia but fresh is more readily available here. Could that cause the sourness because there is less sugar?
One cup of fresh pineapple chunks has 82 calories, 21.7 grams of carbohydrates, 2.3 grams of fiber and 16.3 grams of sugar. One cup of drained, canned pineapple chunks has 109 calories, 28.2 grams of carbs, 2.4 grams of fiber and 25.8 grams of sugar. The calories, carbs and sugar in canned pineapple are higher than in fresh pineapple because the canned fruit is typically packed in fruit juice.
I think that it is probably the pineapple affecting it. If you can buy pineapple juice, adding some of that may help the problem.
Canned pineapple is often HEATED (mildly) when canned. Heating the pineapple prevents the souring. This idea is used in some other dishes as well.