Vol. IX · Fats and Oils Plate 6 of 10deep-frying, chips, fries, crust, steam, oil uptake, bubbles, tempura, 炸, double fryingHT

How Deep-Frying Works.

The bubbles round a chip in hot oil are steam, not boiling oil. While they stream out, oil cannot get in.

Lower a potato stick into oil at 175 °C and the oil round it erupts. Those bubbles are the potato's own water, boiling off at the surface. While steam streams out it keeps the surface near 100 °C and keeps oil out; the outer layer dries, its starch gelatinises and then crisps into a crust (VII.1), and once it is dry it rises above 100 °C and browns (VIII.1). Inside, the food is cooked by its own steam and never passes 100 °C: a good chip is crisp outside and fluffy inside because the two regions are cooked by different media.

The bubbling slows as the crust thickens and less water reaches the surface; then the food is nearly done. Oil uptake is mostly a matter of what happens next. As the food leaves the pot and cools, the steam in its pores condenses, the pressure falls, and the oil clinging to the surface is drawn into the crust. Drained well and shaken while hot, fried food carries much less oil. The workshop fries a potato stick.

Below: the surface and centre temperatures of a potato stick in hot oil, a cross-section with its crust at the end, and the water it loses and the oil it takes up (thermal bench; oil uptake is a teaching ratio).

What you will learn
Workshop
A slab of potato of the thickness you choose, from 20 °C into oil (thermal bench; course heat-transfer coefficient falling as the surface dries). Crust is the depth of the layer that dried above 100 °C. Oil taken up is a teaching ratio of half a gram per gram of water lost, about four-fifths of it after removal from the oil.

Steam out, oil in later

A 10 mm stick in oil at 175 °C has its centre cooked through in about 2.5 minutes, yet the centre never passes 100 °C; after five minutes it has a crust about 1.9 mm deep and has lost 15 % of its weight as water, ending with about 8 % oil in this model. At 150 °C the crust is only 1.3 mm in the same time; at 190 °C, 2.6 mm and browner. That is the logic of double frying: a first, cooler fry cooks the inside through without browning, and a short, hot second fry, often after a rest, builds the crust.

Keep oil uptake low by frying hot enough that steam keeps streaming (too cool and the food sits soaking), frying in batches small enough that the oil stays hot (IX.7), lifting food out when the bubbling slows, and draining it at once on a rack or paper, shaking off the surface oil while it is hot. Tempura batter, made with cold water and barely mixed, forms a thin, lacy crust because little gluten forms (VI.10) and its water flashes off fast.

At the stove

Fry a few potato sticks and listen: a loud sizzle at first, fading as the crust forms. Lift one out every minute and cut it; the moment the sizzle has softened is close to the moment the inside is done.

Chinese cooks distinguish 炸 (deep-frying), 油泡 (a quick plunge in hot oil to set the surface) and 过油 (passing food through warm oil before stir-frying, XVIII.6), choosing the oil temperature for each by eye.

Terms

Deep-frying
Cooking food submerged in hot oil.
Crust
The dried, crisped and browned outer layer.
Oil uptake
The oil a fried food absorbs, mostly as it cools after frying.
Double frying
Frying first cooler to cook through, then hotter to crisp.
In frying, steam leaves and the surface dries into a crust while the inside stays below 100 °C; oil enters mostly as it cools.

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.