The flour you use today may have either more or less moisture than was in the supply you used yesterday, last week, or last year. Flour tends to lose moisture the longer it is stored. Also, moisture content usually varies with the brand of flour and the season, and a flour will pick up moisture if you knead it on a humid day.
For these reasons, you must sometimes make adjustments in the amount of flour (or liquid) you use in order to keep the dough or batter's flour-liquid ratio in proper balance. If your flour has more than an average amount of moisture, use more flour. If it is on the dry side, use less.
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