Blanching is the technique of plunging ingredients into heavily salted boiling water, then transferring them immediately into ice water to halt cooking — a thermal shock that deactivates enzymes, sets colour, and firms texture in a controlled window of seconds to minutes. For green vegetables, blanching preserves the vivid chlorophyll colour that would otherwise turn olive-drab through prolonged heat exposure, which converts chlorophyll a and b into pheophytin (a magnesium-stripped, grey-green pigment). The salted boiling water — 30–40g salt per litre, roughly the salinity of seawater — raises the boiling point marginally but, more importantly, seasons the vegetable from the inside during its brief submersion. The ice bath must be at 0–2°C (32–36°F), heavily iced, and large enough to absorb the thermal mass of the blanched food without warming above 5°C. This is where the dish lives or dies: an inadequate ice bath merely slows cooking instead of stopping it, and the carryover heat turns your bright green haricots verts into army-surplus drab. Timing by species: haricots verts (Phaseolus vulgaris), 60–90 seconds; English peas (Pisum sativum), 30–45 seconds; broccoli florets (Brassica oleracea var. italica), 90–120 seconds; asparagus spears (Asparagus officinalis), 60–120 seconds depending on thickness; spinach leaves (Spinacia oleracea), 10–15 seconds; tomatoes for peeling, 15–20 seconds until the skin visibly splits. Sensory tests during blanching: the colour intensifies dramatically in the first 20–30 seconds — dull green vegetables turn almost neon as the air between cells is expelled and the chloroplasts are exposed. This visual brightening is your timer’s starting gun. When the colour peaks and just begins to stabilise (no further brightening), transfer to ice water. The texture should be crisp-tender: firm resistance when bitten, followed by a clean snap, with no raw squeak against the teeth. Blanching also serves a preparatory role. Bones for stock are blanched for 2–3 minutes in unsalted water to remove blood, impurities, and scum — producing a cleaner, clearer stock. Almonds, pistachios, and hazelnuts are blanched briefly to loosen their skins. Potatoes are blanched at 75°C (170°F) in a lower-temperature variant to activate the pectin-strengthening enzyme PME (pectin methylesterase) before frying, which firms the exterior and produces a crispier fry.
Quality hierarchy: 1) Water volume — use at least 4 litres of water per 500g of vegetables. The water must return to a full rolling boil within 10–15 seconds of adding the food. If the volume is too small, the temperature drops below 95°C and the blanching time extends, overcooking the exterior before the interior is done. 2) Salt concentration — 30–40g per litre. This is far more salt than most home cooks use and it is not negotiable. The salt seasons the vegetable internally and helps stabilise chlorophyll by maintaining the magnesium ion within the pigment structure. 3) Ice bath preparation — prepare the ice bath before you start boiling. A 50/50 ratio of ice to water, in a vessel large enough to submerge all blanched food. Replenish ice between batches. A warm ice bath is worse than no ice bath because it gives a false sense of arrested cooking while carryover heat continues to degrade colour and texture. 4) Single-layer blanching — blanch in batches small enough that every piece is fully submerged and surrounded by boiling water. Dumping 2kg of beans into a pot at once drops the temperature catastrophically. 5) Draining and drying — after the ice bath, drain thoroughly and spread on clean towels. Excess water dilutes dressings and sauces and causes sogginess if the vegetables are stored.
The professional line-cook blanch: fill a deep pot with 8–10 litres of water, salt it like the sea, bring to a roaring boil. Blanch each vegetable type separately in sequence, starting with the mildest (peas) and ending with the strongest (brassicas), using the same water. Transfer each batch to the same large ice bath. Once all vegetables are blanched and chilled, drain, dry, and store on sheet pans lined with towels in the refrigerator. At service, reheat in a sauté pan with butter, stock, or olive oil for 30–60 seconds. This is how restaurants serve vibrant, perfectly cooked vegetables consistently. For tomato peeling: score a shallow X on the bottom of each tomato, blanch 15–20 seconds until the skin visibly curls at the X, then ice bath. The skin peels off in sheets. For the PME potato technique: hold cut potatoes at 72–75°C (162–167°F) for 20 minutes before frying. The activated PME cross-links pectin in the outer cell walls, creating a rigid exterior that fries to extraordinary crispness.
Not using enough water — the temperature drop is too severe and recovery time too long, resulting in uneven cooking. Not salting aggressively — under-salted blanching water produces bland vegetables that no amount of seasoning after the fact can fix. The salt must penetrate during cooking. Skipping the ice bath or using a bowl of cold tap water — tap water is 15–18°C (59–65°F) in most kitchens, nowhere near cold enough to arrest cooking. Only a proper ice bath at 0–2°C stops enzymatic browning and colour degradation. Blanching too many vegetables at once — the water temperature crashes and never recovers in time. Work in 200–300g batches. Over-blanching — even 15 seconds too long turns crisp-tender green beans into limp, dull ones. Set a timer and respect it. Blanching and then covering with a lid for service — the trapped steam continues cooking. Leave uncovered or lightly tented. Blanching delicate herbs like basil or chervil — these wilt in seconds and lose their essential oils. Herbs go into the ice bath raw, never into boiling water.