I am puzzled by baking and roasting recipes that advise setting the temperature 20C lower if you are using a fan-forced oven. But not all recipes do recommend this and I wonder if it is still current advice? If the recipe doesn’t stipulate what sort of oven setting you are using, should you still reduce the time when using the fan-forced option?
Almost all new ovens these days have a fan-forced option. This can be a bonus to the home cook. Your electricity bills should be lower due to a lowered cooking temperature, but you are using a fan to move heat around so it doesn’t come completely free.
The way a fan-forced oven works is just as it sounds — there is a movement of hot air forced around the oven by the fan, and as it passes over food it releases its energy into the food. If the fan isn't on then the hot air circulation moves much more slowly and less heat is able to be passed to the food as quickly. So the more movement of air that happens in the oven "box'', the more heat is passed into the food more quickly, and therefore a lower temperature is required, or a shorter cooking time.
Interestingly, an Aga (Swedish designed in 1922) cooks food in a totally different way. It relies on radiant heat and I think it is still the only oven that uses this principle. The “box’’ of the Aga oven cavity is dense with heat which is transferred on to any surface it comes into contact with — which in an oven is the roasting dish of chicken, a cake or a tray of duck-fat potatoes. My friends are a mixture of love Agas or loathe them. I’m quite a fan, but doubt I’ll buy one while living in a city, rather than a country home. I once cooked on weekends for a family who owned a Wiltshire village (really, complete with the house called Fosbury Manor).
In the kitchen there was both an Aga and a typical electric oven and I used both for different things at different times of the year. In cooler months the Aga was brilliant when used to bake quinces, roast legs of lamb and braise venison stews over long periods of time, often overnight, at lower temperatures. The results were spectacular. The food came out succulent and moist and the kitchen was always warm and cosy. Come summer however, the kitchen was too warm for my liking, pots of water wouldn’t boil quick enough and because the food I was cooking was simpler, fewer stews and long roasts, I tended to use the regular oven more. Mind you, the Aga makes a mean pavlova and great cakes all seasons of the year.
Lowering the temperature in a fan-forced oven, compared with a non-fanned one, therefore makes sense as the heat is distributed much more evenly and efficiently. This is especially true when it comes to an oven crammed full with many trays — whether it be three shelves of biscuits; a roast bird plus veges and a cake; or a shelf of biscuits, one of a roasting dish of jerusalem artichokes and one of apple chutney.
With various shelves in use and the need to make sure everything cooks evenly, you really do need either a fan to blow the heat around, or a solid fuel beast like the Aga which will pump out heat at a constant rate without any drop in temperature.
So, to answer your question, even if the recipes don't say to drop the temperature, you can do so quite confidently.
The only recipe I’d tell you not to would be something like a souffle where you need to make sure it gets to a high core temperature which prevents it collapsing. So long as there is not an overly generous amount of sugar (which burns at high temperature) then I’d suggest you stick to the temperature the recipe tells you and keep an eye on it.
Food will colour more quickly, so just use common sense if you need to take it out early. Otherwise, cook a little less and turn the oven off to save fuel.
In our Ask Peter series, executive chef Peter Gordon answers your curly culinary questions. If you're stumped over something food-related, send your question to firstname.lastname@example.org and keep checking in for answers. You can read more on Peter on his website, have a read of his Ask Peter articles or check out his recipes on our site.