There are a few possible reasons why your whole wheat brown bread might taste bitter when heated up in an oven:
Overheating: Whole wheat bread contains natural oils, which can become rancid when exposed to high temperatures for too long. If the bread is heated at too high a temperature or for an extended period, these oils can break down and lead to a bitter taste.
Stale bread: If the bread was not stored properly or is past its prime, it may have become stale. Heating up stale bread can intensify its off-flavors and result in a bitter taste.
Burnt crust: If the crust of the bread gets too dark or burns during the heating process, it can contribute to a bitter taste.
Ingredient interactions: Some ingredients in whole wheat bread, such as certain bran particles or whole wheat flour components, may interact with heat to produce a bitter taste.
Reactions during baking: During the initial baking of the bread, certain chemical reactions may occur that produce bitter compounds. These compounds can be concentrated when the bread is reheated.
To improve the taste of your whole wheat bread when reheating it in the oven, consider the following tips:
Use lower temperatures: Try heating the bread at a lower temperature for a shorter time to avoid overheating and prevent the natural oils from turning bitter.
Wrap in foil or damp cloth: To prevent the crust from burning or getting too hard, you can wrap the bread in foil or place a damp cloth over it while reheating.
Store properly: Ensure that your bread is stored in an airtight container or a resealable bag to maintain freshness and prevent staleness.
Freshen up with steam: If you're just looking to freshen up the bread without toasting it, you can sprinkle some water on the crust before heating it in the oven. The steam will help soften the crust without causing it to become bitter.
Remember that the quality of the bread you start with also plays a crucial role in the final taste after reheating. If the bread is already past its prime or has been stored improperly, the reheating process may not be able to fully restore its original flavor and texture.