Cream based substitute for milk
I typically have heavy cream on hand but not milk so I'm frequently in need of a method to substitute the milk in recipes with cream. It seems to me that adding some amount of water and sugar to the cream would get it closer to milk in composition. I realize that the exact method and appropriateness of the substitution depends on the specific recipe but am looking for general substitution guidelines.
I've used evap milk + water for pancakes. Works great!
Yes, you can often do this substitution and I've done it successfully many times when I've run out of milk unexpectedly, but you should know that the substitution is never going to be perfect.
On a purely mathematical basis, let's say your heavy cream is 36% M.F. You need partially-skimmed 2% milk for your recipe. If you go purely by weight (which is almost 1:1 for volume when talking about mostly water), then 250 mL of 2% milk will contain about 5 g of milk fat. To get that from 36% cream, you only need about 14 mL, or approximately 3 teaspoons of cream. (Note - don't do this, read on!)
Of course, diluting cream at a ratio of 15:1 is ridiculous and you'll end up with something that's just really watery, and that's because milk is more than just water and fat. I only provide the above calculation for illustrative purposes, to show why there's no hard rule or even rule of thumb for this particular substitution.
With normal cream, most people go with half water, half cream, or if they're trying to approximate skim milk then maybe 1 part cream to 2 parts water. If you have double cream, I would probably use 1 part cream to 3 or 4 parts water; that will give you a reasonable approximation of the consistency of whole or skim milk without making it too watery.
Please keep in mind that different recipes/preparations use milk for different reasons, so this isn't going to work everywhere. I certainly wouldn't do it in baking, but for many stovetop recipes, the main purpose of the milk is just moisture, so you're fine to substitute cream + water and it doesn't really matter if you're "exact" - which you can't be anyway.
I suppose you could keep non-fat milk powder on hand and use that to get even closer, based on @aaronut's formula for finding the right fat percentage.
Although the question didn't specifically ask for alternatives, now that we're on the subject, the best thing you can keep around if you're prone to running out of milk (or don't use it very often) is evaporated milk. It keeps forever and unlike cream, the main difference between evaporated milk and ordinary milk actually *is* the water content.
I can barely call the flavor to mind, but doesn't evaporated milk taste quite a bit different than fresh milk when reconstituted (as does milk power, but I was hoping that mixed with cream it might be better).
@Michael: Absolutely - just about anything that's been canned will taste different from fresh! Still, I'd consider evaporated milk + water to be closer to milk than cream + water. The cream version might taste *better* but it won't have the same properties as an ingredient.
Thanks for the useful follow-up discussion. I've tried powdered milk in the past but thought the flavor it added was disgusting in situations where the milk flavor is in the forefront so I've been fearful of alternatives like evaporated milk. Guess I should by a few cans and give it a try.