It's best to wash chillies *before* freezing them. This removes dirt and debris, ensuring they are clean and ready to use after thawing.
Washing chillies before freezing is the recommended practice for several reasons. First, it removes any dirt, pesticides, or other contaminants that may be present on the surface of the chillies. Freezing these contaminants along with the chillies could potentially affect the flavor and quality of your dishes later. Second, washing before freezing allows the chillies to dry completely before being frozen. This reduces the likelihood of ice crystals forming on the chillies, which can make them mushy when thawed. To wash chillies, simply rinse them under cool, running water, gently scrubbing away any visible dirt. Pat them dry thoroughly with a clean towel or allow them to air dry completely before placing them in freezer bags or containers.
To prevent freezer burn and maintain the quality of your frozen chillies, consider flash-freezing them individually on a baking sheet before transferring them to a freezer bag. This prevents them from sticking together and makes it easier to use only the amount you need.