Blanching is a short boil followed immediately by an ice bath. The boil switches off the enzymes that would otherwise keep working inside the vegetable, dulling its colour and softening its texture over days or months. The ice bath stops the cooking at the exact moment you chose, rather than five minutes later when the residual heat has finished the job for you.
That second half is the one people skip, and it’s the half that decides whether your green beans stay bright or go the colour of old cabbage.
What the boil is actually doing
Vegetables are still metabolically busy after they’re picked. Enzymes — the main culprits are peroxidase and lipoxygenase — carry on breaking things down: chlorophyll degrades, cell walls slacken, flavours turn grassy or hay-like. Freezing slows this to a crawl but doesn’t stop it, which is why unblanched frozen broccoli tastes flat and looks olive after two months while blanched broccoli from the same head still looks like broccoli.
A 60 to 180 second boil, depending on the vegetable, denatures those enzymes. It also drives out air trapped between the cells, which is a side effect worth knowing about: that trapped air scatters light and mutes colour, so pushing it out is why blanched green vegetables look almost artificially vivid the second they hit the ice.
Steaming works too and holds onto more water-soluble vitamins, but it takes roughly half again as long and the timing is harder to judge. I use boiling water for everything except spinach, which I do in a colander with a kettle poured over it because a whole pan for spinach feels absurd.
Why the water has to be at a hard rolling boil and taste salty
A gentle simmer is not blanching. It’s poaching, slowly, and by the time the enzymes give up the vegetable is overcooked. You want water moving hard enough that it doesn’t stop moving when the vegetables go in.
Salt it like pasta water: about 1 tablespoon per 2 litres, enough that it tastes clearly seasoned. Two things happen. The vegetable seasons from the inside rather than sitting under salt sprinkled on later, and the salted water pulls slightly less of the vegetable’s own soluble compounds out into the pot. Unsalted blanching water tastes green and vegetal afterwards, which tells you exactly where that flavour went.
Use more water than feels necessary. Four litres for 500g of vegetables is a reasonable floor. The volume is thermal mass — it’s what stops the temperature crashing when cold produce hits it.
Small batches, or you’re just simmering
Drop 800g of cold green beans into 3 litres of boiling water and the water stops boiling for well over a minute. Your two-minute blanch has become a two-minute sit in 85°C water, which softens without setting the colour. Everything comes out limp and faintly khaki.
So: no more than about 200 to 250g of vegetables per 4 litres at a time, and wait for the water to come back to a full boil before you start the timer. On a domestic hob that recovery takes 20 to 40 seconds. Time from the boil, not from when they went in.
It feels slow. Four batches of green beans is maybe eight minutes of hovering. But I’ve done it both ways for the same freezer session and the single-big-batch beans were visibly duller, so I stopped arguing with it.
One pot, one bowl of ice water, and a spider or slotted spoon so you’re lifting vegetables out rather than draining and refilling. Same water does five or six batches happily.
Times by vegetable
Timed from the moment the water returns to a full boil, for vegetables cut to the sizes given.
| Vegetable | Prep | Blanch time |
|---|---|---|
| Spinach | Whole leaves | 30 seconds |
| Peas | Shelled | 60 seconds |
| Asparagus | Trimmed spears | 60-120 sec by thickness |
| Broccoli | Florets, 4cm | 90 seconds |
| Cauliflower | Florets, 4cm | 2 minutes |
| Green beans | Topped, whole | 2-3 minutes |
| Sugar snaps / mangetout | Whole | 60-90 seconds |
| Carrots | Sliced 5mm | 3-4 minutes |
| Kale | Leaves, stemmed | 2 minutes |
| Brussels sprouts | Halved | 3 minutes |
Thin asparagus at pencil width wants 60 seconds. Thumb-thick spears want the full two minutes and I’d peel the bottom third first. That range in one vegetable is why timing charts should be read as a starting point rather than a rule — pull one out at the low end and bite it.
Doneness for blanching is not doneness for eating. You want the vegetable a shade under: bright, hot through, still with resistance. It will finish in the pan, the oven or your mouth later.
The ice bath, and why cold tap water fails
Roughly equal volumes of ice and water, in a bowl big enough that the vegetables are submerged rather than floating on a crust. For 250g batches I use about 1kg of ice to 1 litre of water and top up the ice every couple of rounds.
Cold tap water comes out around 12 to 18°C in most kitchens, warmer in summer. Hot vegetables at 100°C dropped into that will pull it up past 30°C in seconds, and at 30°C nothing stops — the carryover heat keeps cooking the middle for another two or three minutes. That’s the difference between a green bean with snap and one that bends.
Leave them in only as long as it takes to go cold, which is about the same as the blanch time. A 90-second broccoli floret needs about 90 seconds in ice. Longer and you’re leaching flavour into the water for no reason. Fish one out and feel the thickest part — if the centre is still warm, it’s still cooking.
I keep a bag of ice in the freezer specifically for this. Trying to do a big blanching session with three ice cube trays is a mistake I made exactly once.
Drying matters more than people think
Wet vegetables are a problem in two directions.
For freezing, surface water freezes into a shell of ice crystals that puncture cell walls and glue everything into one block. Spread the blanched vegetables on a clean tea towel, pat the tops, then freeze them in a single layer on a tray for an hour before bagging. They stay loose and you can pour out a handful instead of chiselling.
For anything hitting a hot pan, water is the enemy of browning. Blanched broccoli straight from the ice bath into a wok drops the pan temperature and steams — this is the most common reason a home vegetable stir fry comes out wet rather than seared. Dry the florets properly and they’ll take colour in 90 seconds. A salad spinner does this faster than any towel; I run green beans and broccoli through mine and it saves five minutes of patting.
Same goes for a quick veggie stir fry where the carrots and green beans have been blanched ahead. Bone dry, or don’t bother.
When to skip it entirely
Blanching earns its pan in four situations. Freezing anything green — this is not optional if you want it to be worth eating in March. Prepping ahead, where you blanch on Sunday and have vegetables that reheat in a minute all week. Crudités, where you want carrots and beans and asparagus to be tender enough to eat raw-ish but still crisp, and where the colour is half the point. And getting even texture in a mixed stir fry, since carrots and broccoli and mangetout cook at wildly different rates and blanching lets them all go in at once.
It’s a waste of effort for anything you’re going to roast. Roasting is dry heat doing exactly the job you’d be doing twice, and a blanched vegetable going into a hot oven for a roasted vegetable medley just comes out softer with less browning. Same for anything going into a long braise or soup. Same for cucumbers, tomatoes, peppers and lettuce in a greek salad — nothing there benefits.
The one genuine grey area is cabbage for coleslaw. A 30-second blanch and shock makes it slightly more tender and stops it weeping into the dressing quite so fast, and some people swear by it. I don’t do it. Raw cabbage crunch is the point of the dish and I’d rather salt the cabbage, let it sit twenty minutes and squeeze the water out.
If you only ever blanch one thing, make it the bag of green beans going into the freezer in July. That’s the batch where you’ll notice the difference six months later.