There are many reasons why you might want to substitute ingredients in a recipe, here are the one's I find to be more common, but by-no-means all of them:
1. Cannot find the specific ingredient
2. Do not have the ingredient in your pantry (and do not want to go to the store)
3. Want to include a healthier ingredient
4. Do not like a specific ingredient
5. Allergic or food-sensitive to a specific ingredient
No matter what your reasoning is, let's figure out how to substitute and not drastically alter a recipe or the flavor. For baking you can easily substitute ingredients like butter and oil and you do not need to change measurements. If you have a cup of butter or oil, substitute 1 cup of unsweetened applesauce or cooked pumpkin (not pumpkin pie mix). I've used applesauce in place of butter and oil in many baking recipes, and just recently tried substituting pumpkin for butter in a whoopee pie recipe. The pumpkin was a great substitute, especially since I was making the whoopee pie's for Halloween. Applesauce and pumpkin make cookies, muffins, breads and the like moist and are great choices for vegans. I haven't come up with a great alternative to eggs, if you are looking for a truly vegan substitution.
|Creamed Pumpkin and sugar|
|Whoopee pie with pumpkin substituted for butter|
Now I wouldn't recommend substituting applesauce or pumpkin if you are making any pie crusts or pastry dough. For those I still use butter and I might try coconut oil, but that will be for another day.
For cooking, I substitute or exclude ingredients depending on the what the recipe is. For instance, I recently tried Giada's Orecchiette with Roasted Fennel recipe, but did not have any sun-dried tomatoes. I could have substituted purely sun-dried tomatoes that were not in oil, however I did not have those either. So to make sure I still had a tomato flavor in the dish, I simply added about a cup or so (to taste) of marinara sauce. The result was fantastic and I think I might always make this recipe with the substitution. In other cases you can substitute different cuts of meats or even types of meat. I often substitute chicken breast for recipes that call for chicken thighs or drumsticks. Other times, when I cannot find an ingredient, for instance harissa paste, I found a recipe to make my own and used that as a substitute. Sometimes if can be a simple substitution baby bella mushrooms for button mushrooms or vegetable stock for chicken stock.
Substituting ingredients should not be difficult and remember it is possible to substitute, just make sure you like what you are replacing. You can find some of my "experiments" with substituting Hemp Seed Here and Here.