Lemon

Choosing

Choose by weight. Between two lemons of the same size, the heavier one has more juice. A thick, knobbly skin means less flesh for the same fruit. If you want the zest, buy unwaxed — or dunk it in hot water for thirty seconds and scrub.

The juice and the zest are different ingredients

• Juice — citric acid. Its axis is acidity; there is little aroma in it.
• Zest — oil glands holding the aroma compounds (limonene). No acidity at all.

So juice alone, when what you wanted was "lemon", tastes sour and smells of nothing. Zest for aroma, juice for acidity. Grate only the coloured layer: the white pith underneath is bitter.

Juice goes in at the end

Heat drives off the aroma and blunts the edge of the acidity, so the default is after the heat is off.

• A few drops at the finish tightening a soup — bouillabaisse, for instance — is that effect.
• Simmered from the start, the acidity survives but the sense of fresh lemon does not.
• On sequence, see balancing flavour.

It curdles dairy

Lemon juice coagulates the casein in milk and cream. Poured into a hot cream sauce it turns grainy.

Avoid it by lowering the temperature and adding the juice gradually while stirring. Exploiting the same reaction deliberately is how ricotta is made — and the reason yogurt takes lemon juice without splitting is that it is already acidic (dairy and heat).

Do not let it meet green vegetables early

Acid turns chlorophyll drab. Lemon juice sitting on blanched broccoli or asparagus takes it to olive. Dress immediately before serving (blanching).

It also stops browning

The browning of apple, avocado and potato is an enzyme reacting with oxygen, and acid inhibits that enzyme. It is half the reason lime juice goes into guacamole.

Lime is not a substitute

They look interchangeable and are not: the aroma compounds differ, and lime carries much more bitterness from the peel. Swap lemon into the lime in ceviche or som tam and the quantity of sourness is right while the character of the dish has changed. A 1:1 swap in a pinch, yes — but not the same dish.

Vinegar moves only the acidity across. Reckon 1 tbsp lemon juice ≈ 2 tsp vinegar, and do not use it where the aroma was the point (vinegar).

Getting the juice out

• Room temperature, rolled firmly first. Straight from the fridge it gives up less.
• Thirty seconds in the microwave gets more out again.
• One lemon ≈ 2-3 tbsp juice and 1 tbsp zest.

Storing

• Whole, a week at room temperature or three to four weeks refrigerated.
• Cut, wrapped tight against the cut face, two or three days refrigerated.
• Freeze the juice. A tablespoon per ice cube slot keeps six months, and it does not taste like the bottled concentrate.
• Zest freezes too. Grated and sealed, the aroma holds well.