Cookbooks often caution about the need to adjust the amount of flour
in a bread recipe depending on the humidity. A bit more flour is
needed when the humidity is high and a bit less is appropriate in the
winter when the humidity is low. I've never seen, however, any hard
numbers quantifying the size of this adjustment. I recently used my
bread recipe analysis spreadsheet to help answer this question.
According to the California Wheat Commission,
<http://www.californiawheat.org>, the moisture content of wheat flour
is usually within the range of 12-15%. (For what it's worth, the
data in the USDA National Nutrient Database,
<http://www.nal.usda.gov/fnic/foodcomp/search>, are within this range
for wheat flour but the water content of oat, rye and triticale
flours are closer to 10%.)
Using my spreadsheet, I calculated the true hydration of my standard
weekly bread recipe, using 1209g of 'average' flour with 13.5% water
and 87.5% dry solids, as about 90% (vs. a hydration of 65% using the
traditional formula). I then created an equivalent recipe using
'wet' flour that had 15% water and 85% dry solids. With the second
recipe, I needed to use a total of 1265g of 'wet' flour to get a
dough with the same true hydration of 90%.
In other words, my analysis said that, in theory, you need to add or
subtract up to 4.5% of the flour specified in a recipe to compensate
for the humidity. (For a pound of flour, this corresponds to about
3/4 oz; for a cup of flour, it's about 2 teaspoons.)
Since it was a very humid day, I decided to test my analysis. I made
a test batch of bread by simplistically weighing the flour, but using
3.5% more flour than specified in the recipe. As hoped and expected,
I found that the resulting loaf was just like those I bake under more
normal conditions.
Dan