Baking Soda's the Culprit

• The most common reason muffins are bitter has to do with the leavener. Baking soda, which is highly alkaline, needs an acidic ingredient, such as fruit puree, brown sugar or buttermilk, to counteract its bitterness. Without an acidic ingredient, it won't dissolve completely and you may notice a bitter, soapy flavor. This can also happen if the baking soda doesn't get blended enough with other ingredients or if you use too much. To prevent this problem, add an acidic ingredient with baking soda and use the leavener specified in the recipe. Baking powder is used more frequently in muffins than baking soda and is a little less temperamental. Use only the amount of leavener specified in the recipe -- typically about 1/4 teaspoon of baking soda or 1 teaspoon of baking powder per cup of flour. Stir the leavener into the flour before you add the wet ingredients.

The Fruit Factor

• Other ingredients, such as fruit or fruit purees, may also cause bitterness. For example, when making banana muffins, you want bananas that are very ripe and almost black. If the bananas are beginning to rot, though, they may add a bitter or slightly yeasty flavor to the muffins. Always use fresh or frozen fruit that is of the best quality. Discard any fruit that has signs of mold. Use bananas that are soft and brown, but not oozing liquid. If you smell a bitter or alcohol-like odor, toss them out.

Oils and Nuts

• Oils and nuts can go rancid, potentially causing bitterness in muffins. To prevent this problem, store oils in a cool, dark pantry or the refrigerator, and replace them at least once a year. Delicate oils, such as hazelnut or grapeseed, stay fresh for only about three months once they're opened. Smell your oil before you use it. A bitter, unpleasant odor indicates the oil has gone bad and should be thrown out. Store nuts in the refrigerator for up to one year, or the freezer for up to two years. At room temperature, they become rancid in as little as four months. Taste nuts before you add them to muffins, and discard them if they taste bitter.

In the Oven

• Overcooking muffins can cause bitterness, although this won't be the only problem. If the muffins cooked too long, they may be burnt, dry or coarsely textured. When cooking muffins, preheat the oven to 375 or 400 degrees Fahrenheit. Bake the muffins on the center rack with the timer set for five minutes before you think the muffins will be done. Pop a toothpick in the center of the muffins when you think they're done. It should come out with just a few moist crumbs. Reduce the heat by 25 degrees if you're using glass or dark metal bakeware, or if you have a convection oven.