How does one alter a recipe that calls for one fruit with another fruit of differing water content?

There's no trick, there's math. 100g of banana is about 75g of water (1g of water is 1ml, so easy to measure), 12g of sugar, and 13g of fiber and other stuff. A pumpkin is about 92g of water and 3 grams of sugar, leaving 5g of other stuff.

My banana bread recipe calls for 2 medium bananas, that's about 250g of banana. That's 188g of water and about 30g sugar. If you add 250g of pumpkin instead that's 230g of water and about 8g of sugar, so pumpkin would add 42g of water (42ml, a bit less than 1/4 cup), and take 22g of sugar. That's enough of a difference to really throw your recipe out, so you'd want to remove 42ml of water, milk or other liquid and add 22g of sugar (4g of sugar is about 1 tsp, so 22g is about 5.5 tsp, or just under 2 tbsp).

Baking is part science, part alchemy, and part luck. There are so many variable you can't control when you bake that you really need to use your senses and instruments to determine when something is done.

If you are substituting in a recipe, you definitely want to account for the water difference, so this is an astute question.

If you can estimate how much additional water is in the substitution (or how much less), then you can adjust for that with other ingredients. Sometimes you can add in juice or water or milk, but keep in mind that it's not only the water you're changing when you alter the recipe. You are also altering the sugar content which can affect a bake, and adjusting the amount of milk in a recipe to account for water difference in fruit is also altering fat and protein.

Many recipes are fairly robust, and you are fine making these changes and may just need to adjust the cook time to make sure the product is not over- or under-baked.

I'd suggest thinking through the alterations you want to make and do your best to make appropriate adjustments and then see how it comes out. Is it baked well? How is the crumb? Did it not rise as much as normal or too much? Is it too sweet? You can then make other adjustments the next time you bake it to account for these side effects.

What's great about baking is that it's like working in a lab and experimenting, and in the end you get to eat pumpkin bread. And even if it's not perfect, it's probably still pretty good. And you have an excuse to make it again.

I'm sure that there area a zillion recipes for both breads. I found a recipe for two 9x5 loafs.

• Pumpkin bread using One 15-ounce can pure pumpkin puree.
• Banana bread using eight very ripe bananas (unpeeled, about 32 ounces?).

Most of the ingredients are similar, but the seasoning varies. I assume that the desire with the pumpkin bread recipe is to have the pumpkin bread taste something reminiscent to pumpkin pie.

+--------------------------------------------+-------------+-----------+
|                                            | Pumpkin     + Banana    +
+--------------------------------------------+-------------+-----------+
| cups all-purpose flour                     |    3 1/2    |    4      |
+--------------------------------------------+-------------+-----------+
| cups sugar                                 |     3       |    2      |
+--------------------------------------------+-------------+-----------+
| cup vegetable oil                          |     1       |   1/2     |
+--------------------------------------------+-------------+-----------+
| large eggs, lightly beaten                 |     4       |    4      |
+--------------------------------------------+-------------+-----------+
| cup water                                  |    2/3      |           |
+--------------------------------------------+-------------+-----------+
| teaspoons baking soda                      |     2       |    2      |
+--------------------------------------------+-------------+-----------+
| teaspoon baking powder                     |     1       |    2      |
+============================================+=============+===========+
   SPICES / SEASONINGS                             
+============================================+=============+===========+
| teaspoons fine salt                        |     2       |    2      |
+--------------------------------------------+-------------+-----------+
| teaspoon ground nutmeg                     |     1       |           |
+--------------------------------------------+-------------+-----------+
|  teaspoon ground allspice                  |     1       |           |
+--------------------------------------------+-------------+-----------+
| teaspoon ground cinnamon                   |     1       |    2      |
+--------------------------------------------+-------------+-----------+
| teaspoon ground cloves                     |    1/2      |           |
+--------------------------------------------+-------------+-----------+

So the pumpkin bread uses about half the fruit, and actually adds water.

Cooks Illustrated solved this problem in their pumpkin cheesecake recipe by removing the water from the pumpkin puree. They simply spread the puree on a towel or paper towel.

another method for completeness that also uses math:

try to match the target water percentage

subtract water: weigh out pumpkin weight of (banana recipe grams of banana) * (.90/.72) and dehydrate to reach that weight

likewise for fruit with less water if the substituting dried figs at about 24% water, weigh out figs = (banana recipe grams of banana) * (.24/.72) water = (banana recipe grams of banana) - (figs weight) mash that weight of figs into that amount of water. with figs and water, finely chop the figs, add water, and leave covered in the refrigerator overnight for the figs to absorb the water, mash until it's a consistent mush, ends may not rehydrate, so discard any remaining hard bits

you're right on the other parameters

1. between different fruits, there may be difference in acidity, as in pH not sourness, that will affect how much baking powder is needed
2. sugar content can be adjusted similarly to the weight method above etc.

here's the big one, though: flavor. pumpkin, like apple, is relatively a much weaker flavor than banana. to get the same amount of flavor, you'll want to add more pumpkin that's more dehydrated, but that will definitely affect the way you calculate the amount of water in your final recipe that will take some data gathering:

• how much weight in water is lost during baking a normal loaf of banana?
• how much more reluctant to giving water up is pumpkin than banana? (affects cooking time and temp, so cook to minimum internal temp, but at a lower cooking temp so the crust is still edible)
• etc.

matching the flavor concentration of banana bread may not be achievable without seriously negatively affecting the texture of the loaf, so that's when you start to consider other ways to add more pumpkin flavor, like pumpkin flavor extract. but again, this sometimes leads to qualities that "just don't seem right" after you've smeared butter on a still-warm slice from the loaf.