Not preheating an oven can have several consequences related to cooking performance and the final outcome of your dish, but it does not produce harmful chemicals like an improperly ventilated microwave.
Uneven Cooking: Preheating an oven ensures that the entire cooking chamber reaches the desired temperature before you place the food inside. Without preheating, the heat distribution may be uneven, leading to parts of the dish being undercooked while others are overcooked.
Extended Cooking Time: If you skip preheating, you'll likely need to extend the cooking time to compensate for the initial lower temperature. This can result in the food taking longer to cook than intended, potentially affecting its texture and taste.
Altered Baking Results: When baking, especially with delicate recipes like cakes, cookies, and pastries, preheating is crucial. It helps to create the right conditions for proper rising, browning, and texture development. Skipping preheating might cause baked goods to spread too much or not rise correctly.
Inconsistent Roasting: When roasting meats or vegetables, preheating the oven ensures that the outer surfaces of the food sear and brown properly. Without preheating, the food may not develop the desired crispy exterior.
Delayed Cooking Time: Preheating an oven takes some time, typically a few minutes, but it's an essential step for many recipes. By skipping preheating, you're adding time to the overall cooking process.
Regarding harmful chemicals, unlike a microwave, an oven does not produce or emit microwaves during the cooking process. Therefore, there are no concerns about harmful chemicals being released from not preheating an oven.
However, it's important to note that using an oven that is dirty or hasn't been cleaned in a long time might result in smoke or unpleasant odors due to food debris or spills burning off at high temperatures. This can be easily resolved by cleaning your oven regularly and maintaining good ventilation in your kitchen.
In summary, not preheating an oven can affect cooking performance and the quality of your dish, but it does not produce harmful chemicals like an improperly ventilated microwave. To ensure the best cooking results, it's generally recommended to follow the recipe instructions, including preheating the oven when specified.