In what ratio would you combine skim milk and higher-fat dairy to reach an equivalent of e.g. whole milk? I ran out of whole the other day making bread, tried to add a splash of half-and-half to 2%, but it didn't come out that well - but it must be possible in the right ratio, yes?
If you want a fat fraction of f, starting from cream with a fat fraction c and milk with a fat fraction of m, then the fraction of cream to use is (f-m)/(c-m).
All you have to do is multiply that by the total volume to get how much cream to use, and then fill in the rest of the total with milk.
For example, if you want to approximate 1 cup of 3.25% whole milk using 36% heavy cream and 0% skim milk, you need (3.25-0)/(36-0) * 1 cup = 0.09 cups of cream, or 1.5 tablespoons. So take 1.5 tablespoons of cream, and add skim milk to reach 1 cup.
Or if you want two cups of 10.5% half and half using 33% whipping cream and 2% milk, you need (10.5-2)/(33-2) * 2 cup = 0.55 cups, or about 1/2 cup + 1 tablespoon of cream. So take 1/2 cup plus 1 tablespoon of cream, and add 2% milk to reach 2 cups.
Unfortunately it's hard to provide a really useful table of examples, because fat content of dairy products varies around from country to country, and even within countries. Probably the most useful single thing to know is that you can use Google, e.g. search for (10.5-2)/(33-2)*2 cups in tablespoons to do the calculation from the previous paragraph.
For example, in the US, I believe:
- whole milk - usually 3.25%
- half-and-half - usually 10.5% or maybe 12% (can be up to 18%)
- light cream - usually 18% (can be up to 30%)
- whipping cream - usually 33% (can be 30-36%)
- heavy whipping cream - usually 36% (can be higher)
Resources for finding fat contents if not given:
Derivation is pretty simple. If x is the fraction of cream, then the resulting fat fraction is:
f = x * c + (1-x) * m.
This is simply a weighted average of the component fat ratios: x is the fraction of cream, so 1-x is the fraction of milk. (If you really wanted to convince yourself of this, you could write out the amount of milk in each component and the total, and the volumes of everything, and divide to get this.)
Solving for x yields the formula given at the beginning of the answer.
Credit to Karl S on Chowhound:
Add the following to 1 cup of skim milk to approximate 1 cup of:
- 1.5t heavy cream= 1% milk
- 1T heavy cream= 2% milk
- 2T heavy cream= whole milk
- 5T 1t heavy cream= half-&-half
- 9T heavy cream= light cream
- 1T light cream= 1% milk
- 1T 2t light cream= 2% milk
- 3T light cream= whole milk
- 5 oz light cream= half-&-half
- 2T half & half= 1% milk
- 3T half & half= 2% milk
- 4T half & half= whole milk
This chart is unfortunately vague - is that "1 cup skim plus X makes >1 cup Y, take 1 cup of result" or "X, add skim to total 1 cup"? I'm not sure how to check, but an earlier comment on the page that shows math seems to indicate the former.
vA is volume of A
fA is fraction of milkfat in A
vB is volume of B
fB is fraction of milkfat in B
fD is the desired fraction
fD must be between fA and fB
fD = (fA*vA + fB*vB) / (vA + vB) fD*vA + fD*vB = fA*vA + fB*vB vA*(fD - fA) = vB*(fB - fD) vA/vB = (fB - fD) / (fD - fA)
vA/vB is the ratio
So to make .5 from .25 and .75 you should mix 1:1