Oregano's flavor, including its potential bitterness, is primarily a result of its natural biochemistry. The plant produces a complex array of volatile organic compounds, primarily phenols like carvacrol and thymol, which are responsible for its characteristic pungent, earthy, and slightly bitter taste. This bitterness is an evolutionary defense mechanism. These potent compounds deter herbivores and insects from feeding on the leaves and protect the plant from fungal and bacterial infections. Therefore, a certain level of bitterness is inherent to oregano; it is a sign of a healthy, chemically-active plant protecting itself.
The growing conditions of the oregano plant dramatically influence the concentration of these flavor compounds. Stress factors, such as intense sunlight, high heat, and limited water, signal to the plant that it needs to bolster its defenses. In response, it increases production of phenolic compounds. Consequently, oregano grown in a hot, sunny, and somewhat dry Mediterranean climate will have a much stronger, more pungent, and potentially more bitter flavor profile compared to plants grown in cooler, shadier, or over-watered conditions. The plant is essentially concentrating its essential oils for survival, which directly intensifies its taste.
The plant's life cycle is crucial for flavor. The concentration of essential oils peaks just before the plant flowers. This is the point at which the plant is putting maximum energy into its reproductive strategy, and the leaves are at their most potent. If harvested too late, after the flowers have bloomed, the plant's energy shifts away from leaf production and maintenance. The leaves can become tougher, less aromatic, and the balance of compounds can shift, sometimes leading to a harsher, more unpleasant bitterness. For the best flavor, leaves should be harvested in the morning after the dew has dried but before the afternoon sun is at its peak, and ideally before the flower buds have fully opened.
How the plant material is treated after harvest is equally important. The method of drying oregano significantly impacts its final flavor. High heat, such as from an oven or dehydrator on too high a setting, can cook the leaves rather than dry them. This heat can break down the delicate volatile oils, destroying the bright, aromatic notes and leaving behind a dull, hay-like, or intensified bitterness. The plant's compounds become degraded and oxidized. Furthermore, improper storage where the dried leaves are exposed to light, heat, or air will cause these same essential oils to evaporate or spoil, further diminishing flavor and potentially increasing a stale, bitter taste.
From the plant's perspective, its chemical structure is altered when introduced to heat during cooking. The essential oils that create oregano's flavor are volatile. When subjected to prolonged cooking, these complex compounds break down. The more delicate, aromatic top notes evaporate first, leaving the heavier, more robust, and bitter compounds behind. This is why adding oregano at the beginning of a long-simmering sauce will often result in a bitter flavor, whereas adding it in the last few minutes of cooking preserves its brighter, more complex character, allowing the plant's full intended flavor profile to shine.
