Blanching: Keeping Vegetables Green

Why the green disappears

The green in vegetables is chlorophyll, and chlorophyll is vulnerable to heat and acid. Heating releases organic acids from the cells; those acids strip the magnesium out of the chlorophyll, and the colour shifts to olive and then yellow.

Knowing that, the technique writes itself: cook briefly, dilute the acid, and give the acid a way out.

Change the pigment and acid changes sides

This section is about chlorophyll, and other pigments react to acid the other way round. Betanin, the red in beetroot, breaks down to brown when heated at neutral pH but holds its colour in the presence of acid. So Borscht adds vinegar or lemon while the beets cook, and Šaltibarščiai — where the colour bleeding into white dairy is the dish — rests entirely on it.

So acid is not the enemy of colour; it depends which pigment. Green vegetables need the acid let out; red beetroot needs it put in.

Use plenty of water

At least five times the volume of the vegetables. Two reasons:

1. The water temperature drops less when the vegetables go in. In a small pot it takes much longer to return to the boil, and the vegetables cook that much more. 2. The acid coming out of the vegetables is diluted.

Leave the lid off

With the lid on, volatile organic acids condense and fall back into the water. Lid off and they escape. This rule applies specifically to green vegetables.

Bean sprouts, though, want the lid on

Bean sprouts appear in the times table below, and they sit on the other side of this rule. They have no chlorophyll and so no colour to protect; what has to leave is the raw green smell that comes off asparagine, an amino acid they carry. Lift the lid while that is volatilising and the steam cools and drops it straight back in — which is what Kongnamul-guk means when it says opening the lid mid-cook makes them smell raw.

So the lid depends on what you are trying to get rid of. Off to let organic acids go; on to hold the raw note in while it cooks off.

And the sprout window is narrow. The raw smell is mostly gone in the first minute, and past three minutes the snap is gone too — so a dish where the snap is the point, like Kongnamul Gukbap, stops at sixty to ninety seconds.

Salt it properly

Roughly 10g per litre. Salt in blanching water seasons, but it also narrows the osmotic gradient, so less of the vegetable's own substance leaches into the water. Skipping it is the most common reason Sigeumchi Namul or Kongnamul Muchim tastes flat.

What the ice bath is for

Drained vegetables keep cooking in their own residual heat. An ice bath stops that instantly. Arresting overcooking is the main purpose — preserving colour is a side effect.

• Squeeze the water out thoroughly. For a dressed side dish especially, leftover water makes the seasoning slide off and taste dilute.
• Do not leave them in longer than about 30 seconds, or flavour leaches into the water.

The thirty seconds follows from the purpose

That cap is not a property of cold water. It follows from the job named in the sentence above it: if all the bath has to do is stop residual cooking, you need the surface cold and nothing else, and every second after that is leaching you did not want. Ask something else of the same bowl and the number moves — in both directions.

• Where the cold is there to set something, the clock belongs to the core. A surface at 0°C over a warm middle has arrested the cooking and set nothing. Yam wun sen calls the iced water "not optional either" and gives the reason — "the setting is part of the dish, so faster is better" — and its step runs until completely cold. What is setting is the amylose in mung bean starch recrystallising as it cools, which cooking noodles explains. Hainanese chicken rice holds a whole poached bird in it for five minutes, because the skin's bounce is gelatin from its collagen firming, and gelatin firms at a temperature rather than at a moment (serving temperature). So arresting tells you the earliest you may lift something out, and setting tells you the latest. They are not the same instruction.
• And where the leaching is the point, the bullet warning about it reads backwards. Kamaboko stirs chopped white fish into iced water for thirty seconds, tips it into a sieve, changes the water and repeats three times — "the cloudier the water, the better it is going", because what leaves is the fat, the blood and the water-soluble proteins while what stays is the muscle-fibre protein. The same page says flatly that the number of rinses is the springiness. Cold is what keeps that selective: the muscle protein the paste is built from comes out only to salt, and the paste must stay under 10°C throughout. Rasgulla gets both jobs from one bowl — rinsing the lemon out of the curd, and stopping the curdling dead.

So before setting a timer on a bowl of iced water, settle what the water is there to do. The thirty seconds above is the vegetable case, and it is the case where time in the water is nothing but cost.

Times by vegetable

Do not blanch different vegetables together; they do not cook at the same rate.

• Spinach, chives, perilla leaf: 20–30 seconds. Out the moment the colour deepens.
• Bean sprouts: 3–4 minutes — long enough to lose their raw edge.
• Broccoli, asparagus: 2–3 minutes.
• Carrot, potato: 5 minutes or more, or use another method entirely.
• Courgette: 1 minute. Longer and it goes soft.

When the rules invert wholesale

Everything above assumes a mild vegetable with no bitterness, cooked briefly to keep its colour. Change the material and four of those rules reverse at once.

Greek Horta boils wild bitter greens for 15 to 20 minutes — where spinach is 20 to 30 seconds. It is then not squeezed: hot leaves take up olive oil, while cooled and wrung-out ones only carry it on the surface. The cooking water is not discarded either; it is kept and drunk as zoumí. And the colour is deliberately olive-drab. Horta that arrives bright green is undercooked.

So the rules above are not a law about vegetables; they are the rules for that objective. Where bitterness has to leave, where oil has to get inside, and where the cooking water is a broth, they go the other way one by one. Sigeumchi Namul must be wrung dry (its sesame oil is a small amount and must not sit on the surface) and horta must not be — the same principle from both ends.

Blanching things that are not vegetables

Blanching is written up as a vegetable technique, but the reason for the brief boil splits several ways.

• To take something out. Blanquette de veau brings the veal up from cold water, skims the grey scum and throws that water away. The opposite of a vegetable whose water becomes a broth: here discarding it is the point, because the next liquid becomes the sauce and cannot be cloudy.
• To set a surface so it holds on. Polbo á feira dips the octopus in and out of boiling water three times — asustar. The surface sets first, and the skin stays attached to the flesh.
• To get water out. Dambou boils moringa leaves, cools them and wrings them dry, because wet leaves would put the steamed grain back together into a mass. Wrung for exactly the opposite reason to horta's.
• To make something come apart. Iraqi Dolma blanches a whole onion, not to cook it but to loosen the bond between one layer and the next so the leaves peel off singly. Forced apart raw they tear; boiled properly they go soft and are useless for wrapping. Dipping a tomato to slip its skin is the same idea.
• To make a surface. Sunday Roast parboils its potatoes for 8-10 minutes "until the outsides are just starting to break up", drains them dry, puts the lid on and shakes the pan hard to rough the surfaces up — "that rough exterior is where the crust comes from; smooth potatoes never get there." It is the asustar bullet above run backwards on the same variable: one sets the surface so it holds together, the other breaks it down so there is more of it.

Taking fat off the outside is a different operation

"To take something out", above, is written for something that has to come out of the interior. The veal comes up from cold water so that what is inside has time to coagulate and rise to where it can be skimmed; the cold start and the slow climb are what make that possible.

Where what you are taking out is fat sitting on the outside, neither is needed, and the dishes that do it finish in seconds. Eomuk bokkeum blanches shop fish cake "briefly — or pour hot water over it in a sieve" to take off the surface oil, and Budae jjigae gives the ham and the sausages the same brief dip before they go into the pot. Boiling water straight over the top, no cold start, because nothing has to travel.

And the reason to do it at the sieve is that this fat never presents itself for skimming. Stocks and broths has you taking the cap off in the first ten to fifteen minutes, and braising notes that the next day's set fat lifts straight off — both of which assume the fat has risen to the surface of a liquid you can reach. The fish cake's film never enters a liquid at all: it stands between the sauce and the cake, and that page's line for it is that "a sauce cannot cling to fat it cannot displace." The stew's would go into a broth people are eating straight out of the bubbling pot, which is the wrong moment to be holding a spoon over it.

When to keep the water

• Discard it: for vegetables high in oxalic acid, like spinach, or anything with a raw bitterness.
• Keep it: bean sprout water is a broth as it stands. The water from blanching vegetables for Japchae can go into a soup. Horta goes further still: the cooking water is served alongside and drunk.

When not to blanch at all

Blanching does not suit every vegetable. Watery, mild ones — aubergine, mushrooms, courgette — only take on water and lose flavour. Roast or sear those instead; see Browning and the Maillard reaction.

Aubergine most of all. Yuxiang Eggplant salts it in brine for five minutes, pats it dry and fries it plain — its starch goes into the sauce at the end, never onto the aubergine; Caponata cooks each vegetable separately to drive the water off. Both move away from water entirely, because an aubergine is a sponge: put it in water and it comes out holding that water.