adding peanut butter to a recipe

Question

Ok, so I saw another question answered here that said:

Natural Peanut butter is about 50% fat, butter is about 80% fat. If you want to maintain the original amount of fat in the recipe, you would decrease the butter by 60% of the amount of peanut butter you used.

I however have a recipe for oatmeal cookies that calls for 3/4 C of butter and I'd like to use a full cup of peanut butter, How do I adapt this correctly? I'm really bad at math and don't want to mess it up, haha.

Also if I'm adding that much peanut butter do I need to adjust any of the other ingredients like the flour or oats quantity?

Thanks in advance for your answers!

Answer 1

The answer to this is that you use a conversion equation. If you did even the most basic chemistry in school, this is one you hopefully learned and is one of the most useful equations you can use in a lab or when cooking - very useful for dilutions and working out final concentrations in solutions.

V1 * C1 = V2 * C2

Where V = volumes and C = concentrations, 1 and 2 refer to before and after. Some people use i (initial) and f (final) or something along those lines instead of 1 and 2

It works on the fact that if you take an amount from one side of the equation and put it into the other side, you have the same amount. Again if you did any chemistry, you might remember the equation N = V * C, where N = number of what you want - but notice the similarity to the first equation and that if you take both sides you have N = N.

So, onto your calculation:

Butter fat concentration (C1) = 80%, volume (V1) = 0.75 (3/4) cups. Fat concentration in PB (C2) = 50%:

80% * 0.75 cups = 50% * V2 cups

60 = 50 * V2 - divide both sides by 50 and cancel appropriately to get V2 alone.

V2 = 60 / 50

V2 = 1.2 cups of peanut butter.

Therefore, assuming that your PB is 50% fat, you would need to use 1.2 (1 and 1/5th) cups of PB to equal butter in total.

However, as you only wanted to use 1 cup (in 2017, so I guess you made the cookies by now...) you have 0.2 cups worth of 50% fat left over to compensate for. So... you use the equation again to work it out. This time V1 = 0.2, C1 = 50% and C2 = 80%

0.2 * 50 = 80 * V2

10 = 80 * V2

V2 = 0.125 (1/12th) cups of butter.

As there are 16 tablespoons in a cup, 1/12th of a cup = 16 * 1/12 = 1.3 tablespoons of additional butter needed.

Answer 2

That's a loaded question. Your recipe calls for 3/4 cup butter, which is 80% fat, so 3/5 or (6/10) of a cup total fat. A cup of regular peanut butter is 70% fat so is 7/10 of a cup of fat. So since your recipe is calling for 6/10 cup of total fat, you would completely omit the butter and cut that cup of peanut butter down by a couple of tablespoons (an exact measurement gets wonky). However, since butter is 15% water and regular peanut butter is only 1% water and has salt and sugar. If the recipe calls for unsalted butter and adds salt separately you would cut the salt by about 1/4 tsp, you might also want to adjust the sugar down slightly. And you would want to add an additional approximately 1.5 tbls. water since you are omitting the water in the butter.