Vol. XVI · Moist Heat Plate 2 of 11blanching, shocking, ice bath, green vegetables, chlorophyll, olive, enzymes, crisp-tender, broccoli, 焯水, carryoverMT

Blanching and Shocking.

Broccoli blanched for three minutes and left on the board goes on cooking: soft and olive ten minutes later. Plunged into iced water, it stays bright and crisp.

Blanching (焯水) is a short boil — a minute to a few — with three uses. It sets the colour: heat drives air out of the tissue, so green vegetables turn suddenly brighter; it kills enzymes that would otherwise brown and toughen vegetables in the freezer (XIII.3, VIII.6); and it softens them partway, to crisp-tender, so they can be finished quickly in a stir-fry or dressed in a salad. Lots of water at a full boil matters, so the vegetables do not cool it much (IX.7 in water).

The catch is carryover (II.9). Hot vegetables keep cooking as they sit: their cell walls go on softening, and the green chlorophyll goes on losing its magnesium to acids in the tissue, turning to olive pheophytin (XX.3). Shocking — plunging them into iced water straight from the pot — stops both within a minute or two. The workshop blanches broccoli and compares shocking with leaving it on the board.

Below: softening and olive colour (course reaction rates, averaged through the piece) for broccoli stems blanched for the time you choose and then either shocked in iced water or left in room air for ten minutes (thermal bench).

What you will learn
Workshop
Thermal bench: broccoli stems as cylinders of 10 mm, from 20 °C, in boiling water for the time chosen, then 3 minutes in iced water or 10 minutes in room air. Softening (pectin breakdown) and chlorophyll loss are course first-order reactions tracked through the piece; enzyme inactivation near complete in all cases.

Into the boil, into the ice

Blanched for three minutes and shocked, broccoli stems are about 37 % softened in this model and 20 % turned olive; left on the board for ten minutes, they reach 72 % and 49 % — the same pot, twice the cooking. That is why restaurant kitchens keep an ice bath beside the blanching pot and why vegetables for a buffet or a salad are shocked even if they will be served warm later.

Method: bring a large pot of well-salted water to a rolling boil; add the vegetables in a quantity the water can absorb without dropping far below the boil; cook until bright and just tender (taste one); lift them into iced water until cold; drain and dry well. For freezing, blanch by the times tested for each vegetable (they are set to kill enzymes) and freeze spread on a tray (XIII.3). Leave the lid off: acids that escape with the steam would otherwise condense back into the water and speed the olive colour (XX.3).

At the stove

Blanch two handfuls of green beans together for three minutes. Shock one handful in iced water and leave the other in a bowl. After ten minutes compare their colour and bite.

In Chinese cooking 焯水 is used as much for meat as for vegetables: a brief boil of pork ribs or beef draws out blood and scum (血沫) so a later braise or soup stays clear.

Terms

Blanching
A short boil to set colour, kill enzymes and partly soften.
Shocking
Plunging hot food into iced water to stop cooking.
Crisp-tender
Cooked through but still with bite.
焯水
Chinese blanching, of vegetables or meat.
Blanch in plenty of boiling water to set colour, kill enzymes and soften; shock in iced water at once, or the vegetables keep cooking.

Sources

  1. Chlorophyll to pheophytin in heated green vegetables: first order, activation energy about 50–60 kJ/mol, much faster in acid (S. J. Schwartz and T. V. Lorenzo, Crit. Rev. Food Sci. Nutr. 29, 1990). Teaching values within these ranges.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  3. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.