What's a Cup?
A recurring question, posed for many reasons, is, "What's a cup?"
Some people in other countries want to know what a cup is so they can use recipes on American web sites and in American cookbooks and magazines. One English baker asked plaintively in a newsgroup, "What's a sodding cup?" But even Americans who live with cups have some uncertainty about the matter.
A quick and easy answer is that a cup is about 238 milliliters. If you weigh a cup of water, it should weigh about 238 grams. Other liquids may weigh more, or less, depending on their specific gravities. A liquid containing lots of alcohol will weigh less, while honey or molasses will weigh more. However, any cup of these liquids will weigh as much as any other cup of the same liquid.
The big issue is that granulated solids, unlike liquids, are compressible. Flour isn't a so much a solid as a granulated solid or a powder. And powders can be compressed or fluffed up. Some people fill their cups by scooping the flour out of its sack, which will tend to compress the flour. Other people sift the flour, spoon it into the cup, and then scrape the excess off the top. This latter method the one endorsed by most cookbooks and flour companies. However, many bakers feel this is more trouble than it is worth.
The practical differences between these approaches were clearly shown in a test in one of the Usenet baking oriented newsgroups. A large number of people weighed a number of cups of flour. Cups varied from less than 100 grams to more than 200 grams. Worse, scoopers had as much as a 25% cup to cup variation. All of this makes it very hard to share recipes in cups and have any confidence that the reader will have a bread remotely similar to the one I made.
A recent test I ran showed that the sifted cups were very close to one another, ranging from 110 to 130 grams, with most quite close to 120 grams, which is what the flour companies insist a cup of flour should weigh.
Emily Buehler, in her wonderful book, "Bread Science," points out that weighing flour is a better predictor of how many molecules of flour you have put in the mixing bowl than measuring by cups.
So, if you have to use cups, which we discourage, we suggest sifting, spooning and scraping. Our recipes are being adjusted for the 120 gram cup of flour.
But, please don't misunderstand, you can make good bread measuring flour with cups. You can even make good bread without measuring at all - if you're good enough. At the end of the mix, all the professional bakers I know measure by weight, mix the dough, and then feel the dough to make sure it feels right and taste it to make sure it tastes right. When you know what the dough should feel and taste like, measuring just becomes a way to get to that point more quickly. For this web site, measurement is a tool that helps us feel confident that you will make a dough similar to what we intended you make.
If you use cups, we suggest the following strategy. If the recipe you are using calls for a range of flour, such as 4 to 6 cups, use the smaller amount to start with. If the recipe tells you to use a certain number of cups, start with about 1/2 to 3/4 the amount called for. Mix the dough. Add more flour until the dough is too stiff to mix by hand. Then turn the dough out onto a lightly floured surface. Knead, or develop, the dough pretending that you are Ebeneezer Scrooge and that flour is the most expensive ingredient in the world. Dough would rather be a bit too wet than a bit too dry. If you use more, or less, cups of flour than the recipe calls for, that's OK, as long as the dough - and bread - come out the way you want them to.
What about whole wheat flour? To our surprise, a cup of whole wheat flour weighed the same as a cup of white flour.
Finally, what about sourdough starters or other starters? To no small extent this depends on the ratio between water and flour. With the starters we make, we find that a cup of starter weighs about 260 grams.