I do a lot of frying and roasting and I have recently heard that some oils, such as extra virgin oil, should not be used for such purposes. If I am roasting potatoes, which oil is best? For Christmas I use goose fat as suggested by Nigella, but I would like to use an oil for more everyday cooking. Thanks for you help!
Generally olive oil, especially extra virgin olive oil, is not recommended for frying at high temperatures as it has a low smoke point. It also has a strong flavour, which can affect the flavour of the cooked food, and is also fairly expensive when compared with other oils.
The smoke point of an oil is the temperature at which it starts to emit smoke. It is important as beyond the smoke point the components of the oil degenerate and the flavour can become bitter. The smoke point for extra virgin olive oil is 190c (375F) and for regular olive oil is around 216c (420F). For groundnut (peanut) and sunflower oils the smoke point is around 230c (450F) and for expeller-pressed rapeseed/vegetable (canola) oil is is around 240c (470F).So if you are frying items at 180c or above, or roasting potatoes at 200-250c then olive oil is not the best choice, though if you are cooking chopped onions in a frying pan as the base for a dish then regular olive oil would be OK as the temperature is unlikely to get over 200c.
For roast potatoes goose fat is wonderful, or you could use beef dripping which also imparts flavour. The milder groundnut/sunflower/vegetable oils tend to be the best option if you want to use a more everyday type of oil and you can always add a little extra flavour by adding a couple of tablespoons of olive oil to the milder oil if you like - the olive oil won't burn as readily if it is mixed with oil that has a higher smoke point.