Fan oven temperature conversion and biology/chemistry


Many recipes call for a lower temperature in a fan oven (usually by 20°C), which makes sense as these ovens transfer heat more effectively. However there would appear to be exceptions:

  • When the conventional oven temperature is only just high enough for browning – the Maillard reaction needs ⪆ 140°C while caramelisation of most pure sugars needs ⪆ 160°C. A recipe reducing from 160°C to 140°C would therefore seem to result in much less browning when equally cooked through. But keeping the non-fan temperature would result in overcooking.

  • Sterilising jam jars in the oven (common/official advice for hot-packing jams, chutneys etc.) relies on the surface getting hot enough to kill anything that could be living on them.

I'm sure there are others.

How can we identify and deal with such cases? Is there a good rule of thumb for when to lower the temperature and when not to?

Best Answer

I think that the feature that you would use to identify the ones that might require the same or higher temperature in fan assisted ovens is removal of water.

In the case of a fan oven, the water is more efficiently removed from the food surface by elimination/reduction of the boundary layer through circulation of air in the enclosed space. The boundary layer is a diffusion rate-limiting layer of still air that limits diffusion away from any surface (e.g. Washed clothes dry faster on a windy day than on a still one when hung outside). In conventional ovens, the boundary layer is larger as the air is largely still and the environment is isothermal.

In most recipes the cooking involves removal of water from the items being cooked, however the Maillard reaction and sterilization of (dry) glassware both rely on heat alone.