Were the potatoes in this image baked or roasted?
We ‘bake’ a cake, but ‘roast’ a chicken. I wonder why that is, given that both are put in an oven at around the same temperature. And what do those bake/roast settings on the oven knob mean anyway?
Not much really. Strictly speaking the oven doesn’t roast – the chicken is being baked just like the cake.
Roasting refers to cooking by radiant heat, that is, direct heat (infra-red radiation) emitted from something very hot. It is one of the oldest forms of cooking (think meat on sticks around campfires). If a chicken is put on a rotisserie at some distance from hot coals, then it is being roasted. Oven grills, where food is placed near a very hot element (typically heated to ~1,000C) positioned above or below the food, is also a form of roasting.
In an oven most heating is by convection. That is, the oven elements heat the air that then flows around the food and heats it. The oven walls, although heated by the elements and radiating, are rarely at temperatures much higher than ~250C, and radiant cooking is minimal.
Cooking this way is time-consuming because air is a very poor heat conductor. You can safely put your hand in an oven heated to 100C for some time, but would be instantly and severely burned if you did that with 100C (boiling) water, although the temperature is the same. Curiously, oil is not as good a heat conductor as water either, it just seems that way because it can be heated to temperatures much higher than 100C.
The process of cooking in an oven can best be described as drying-out.
Baking is controlled dehydration. Which is why ovens were invented in the first place. Their purpose was to dry out mud-bricks. Then they started to be used for dehydrating food to preserve it. Only later were they used to dry out larger food items so as to heat the interior and crisp the exterior. This probably started with bread.
When baking a roast, conduction carries heat towards the interior of the meat, while near the surface water is turned to steam, dehydrating the surface and allowing temperature to rise above the boiling point of water (100C) so that browning reactions can take place (at >130C).
The aim is not complete dehydration of course, and judgement as to when to stop this process is needed for success. Although the oven might be set to 180C (to ensure surface browning), the desired interior temperature is not likely to be more than 70C (a well-done temperature). This is a huge surface-to-core differential, and oddly it is usually judged not by measuring temperature, but by estimating time. A probe thermometer takes away this guesswork.
This temperature differential is also why meat should be rested, to allow temperature to equilibrate somewhat. It also means that core temperature will rise during resting, adding further complexity to guessing oven doneness. Again, this is because air is a terrible conductor of heat – it’s easier for heat to redistribute within the food (from the hot outer layers to the cooler inner core) than it is for heat to escape from the food.
The dehydration process can be taken further with cakes and breads, depending on the degree of moistness required at the centre of the food.
So ‘baking’ cake, bread and meat are all part of the same spectrum, and follow the same physical principles.
And those knobs? The roast setting usually just switches on another element in an attempt to increase radiant heat. In some ovens, the upper and lower elements are cycled in opposition to endeavour to reduce convection heating. Which makes fan-forced roasting a contradiction. Either way, ovens don’t roast.