I posted this on the bread newsgroup, alt.bred.recipes, and thought it might be helpful here, too.
When I’ve converted recipes from English to metric or vice-versa, I’ve always been aware of the two methods of doing it.
1. Exactly. This results in some strange amounts, like 563 grams or 14.45 ounces, but if you can hack the units, it will work as well as the original.
2. Proportional. This means that you round the flour or water up to the nearest whole unit, like 500 grams of 32 ounces, and scale all the others accordingly.
You can test these methods by looking at a book like “Cook’s Guide to Bread,” by Ingram and Shapter. They post all recipes in English, metric and volume. The amounts are not exact conversions, but the amounts are all scaled to work no matter which units you use. You do have to be careful with a recipe, since working partly with metric and partly with English can give you some errors if you aren’t careful.
I don’t think that a few grams here and there will be make-or-break in most cases.
When I work with salt and yeast, I keep a few numbers in mind:
A tablespoon of salt is 22 grams and also 0.80 ounce.
A packet of yeast is 1/4 ounce and also 7 grams and also 2 1/4 teaspoons.
Good starting point for salt is 2% of the total weight of flour.
Good starting point for yeast is 1% of the total flour.
Both by weight.
There has to be something distinctive in the recipe for me to get too far away from these numbers.
I don’t think the difference between dry and damp kitchen affects the salt too much, so I don’t worry about it. I’m sure a p-chemist could make a convincing case for allowing for it, but I simply can’t be bothered.
As for the cup problem, that’s a real poser. The “traditional” methods of filling a cup range all over the place, from scoop and dump to fluff and fill. I use weight. When I have to convert a cup recipe to weight, I do the following.
1. Consider what I am trying to bake. Is it a ciabatta or a bagel? Is it a dry bread or a wet bread? Is it a lean bread or an enriched bread?
2. Does the recipe rely on “sprinkles of flour” to achieve the final dough? This is backward of what one should do. The final adjustment, as figured out long ago, should be done with the liquid, not the flour. If one uses extra flour, there is a risk that it won’t get properly hydrated going into fermentation. The other consideration is that the proportions of the ingredients are worked out with a specific amount of flour. Changing the amount of flour by adding more can imbalance the recipe. Adjusting with water is a bit of a mess, but it’s worth it.
3. What does the recipe say about final dough condition? Most of them say something like “smooth and elastic,” which is misleading most of the time. Don’t forget that flour and water make paste, so all dough will be sticky, some just more so than others. My hunch is that all those recipes that call for “smooth and elastic” are really saying “Have a coating of un-hydrated flour on the surface to mask the stickiness.”
Once I have these in hand, I have a pretty good idea of the final hydration I should have. I then figure out how much liquid there is in the final recipe, making allowances for eggs, butter, oil, rum, whatever. From there, it’s easy to figure out how much flour I need. For future reference, I divide the amount of flour by the number of cups in the original recipe to find out what the recipe writer was thinking a cup of flour weighs. It’s interesting to work with a book and find that the writer is using different equivalents in different recipes. And that most of the time, when a writer has a table in the back of the book that gives equivalents, the numbers are not congruent with what he uses in the book.
I then make the dough. I keep track of any extra flour I add and modify the recipe to take this into the recipe at the start.
That’s my method. It works for me and prevents me from having a lot of problems. I still get blindsided sometimes, especially when there is an error in the recipe, but I no longer make dumb mistakes like I did when I started out.