If recipe says 400 degrees for 45 min and you want to cook at 300 how long must I it?

You can't simply adjust the cooking time proportionally when changing the oven temperature. Here's why and how to figure out a good estimate:

Why it's not a simple conversion:

* Heat penetration: Lower temperatures take longer for heat to reach the center of the food.

* Chemical reactions: Cooking involves chemical reactions, and different temperatures affect these reactions in different ways.

How to estimate a new time:

1. Consider the food: The type of food (meat, bread, etc.) and its thickness will influence how much the time needs to change.

2. Start with a rough estimate: A good rule of thumb is to double the cooking time when reducing the temperature by 100 degrees. So, in your case, you might start with a rough estimate of 90 minutes (45 x 2).

3. Monitor closely: It's essential to keep a close eye on your food. Use a thermometer to check for doneness and adjust the time as needed.

Example:

Let's say you're baking a cake that normally cooks at 400 degrees for 45 minutes. You'd likely need to bake it at 300 degrees for at least 90 minutes, but you'd want to start checking for doneness around the 75-minute mark.

Important: Always check the recipe for any specific guidance on temperature adjustments. Some recipes might offer instructions for different oven temperatures.