Vol. XX · Vegetables, Fruit and Legumes Plate 5 of 12roasting vegetables, roast potatoes, crowding, spacing, browning, caramelised vegetables, sheet pan, oven temperature, cut size, oil, 烤蔬菜HT

Roasting Vegetables.

Two trays of the same potatoes in the same oven: one golden and crisp, one pale and soft. The only difference is that one was crowded.

A vegetable is mostly water (XX.1). In a hot oven its surface cannot brown until that water has gone, because a wet surface stays near the boiling point (VIII.2). Crowded on a tray, pieces touch and trap their steam: the air around them is humid, the water on each surface has nowhere to go, and the tray holds a puddle. They steam soft before they dry, and they brown late, if at all. Spread out in one layer with gaps, on a preheated tray, with a thin coat of oil, the surfaces dry quickly and then brown and caramelise (VIII.1, VII.8).

Size sets the balance between inside and outside (II.2): small pieces brown before they are tender only if the oven is very hot; large chunks are tender inside before the outside is coloured unless the oven is hot enough. Most vegetables roast well at 200–230 °C. Sugar-rich roots (carrots, parsnips, sweet potatoes, onions) brown fast; starchy potatoes benefit from parboiling and roughing up the surface, which makes a starchy paste that crisps. The workshop roasts potato chunks.

Below: the browning and softening of potato chunks of the size you choose in a 220 °C oven, spread in one layer or crowded (thermal bench: crowding modelled as more water on the surface and slower heat transfer), against minutes.

What you will learn
Workshop
Thermal bench: potato chunks as spheres of the size chosen, from 20 °C in a 220 °C oven; spread: the usual surface water and heat transfer; crowded: five times the surface water and half the heat transfer (a teaching stand-in for trapped steam and shading). Browning index as in Vol. VIII; softness from the course vegetable kinetics at the centre.

One layer, hot tray, dry surfaces

In this model 30 mm chunks spread on a tray dry at the surface after about 3 minutes and are half-browned by about 21 minutes; crowded, they stay wet for about 12 minutes and take about 33 minutes to brown as much, by which time they are soft through and losing more water. Real ovens and trays vary, but the order is always the same: dry first, then brown.

Method for roast vegetables: heat the oven to 220 °C with the tray in it; cut pieces evenly; dry them; toss with oil and salt; spread in one layer with gaps, using two trays if needed; roast, turning once when the underside is brown. For crisp roast potatoes: parboil chunks in salted water 8–10 minutes until the edges are soft, drain and steam-dry, shake in the pan to rough the surfaces, then roast in hot fat at 220 °C for 40–50 minutes, turning once or twice. A pinch of baking soda in the parboiling water (XX.2) softens the surface further and makes it crisper.

At the stove

Roast the same cut vegetables on two trays: one crowded, one spread with gaps. Pull both at 30 minutes and compare colour and texture.

Sheet-pan dinners — protein and vegetables roasted together on one tray — work only when the pieces are cut so they finish at the same time and are spread far enough apart to brown; the tray is a scheduling problem as much as a recipe (XXIII.5).

Terms

Crowding
Pieces packed so closely that their steam cannot escape (XV.3).
Parboiling
Partly boiling before roasting or frying.
Caramelisation
Browning of sugars on their own (VII.8).
Preheated tray
A tray heated in the oven so pieces start sizzling.
Vegetables brown only after their surfaces dry. Spread them in one layer on a hot tray, cut evenly, oiled and dry; crowded they steam first and brown late.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.