Ingredient

Lemon

A lemon is two ingredients wearing one skin. The juice is acid; the zest is aroma. They do completely different jobs, and a recipe that calls for one will not be rescued by the other.

Zest and juice are different ingredients

The juice is mostly water and citric acid with a little sugar. What it contributes is sourness — the same job vinegar does, with a slightly different accent. It brightens, cuts fat, and balances sweetness.

The zest is the coloured outer layer of the peel, and it is full of aromatic oil. It contributes smell rather than taste: almost none of the acidity, all of the perfume. This is why lemon cake made with juice alone tastes sweet and vaguely sour, and lemon cake made with zest tastes of lemon.

The white pith between them is bitter and should be left behind. That is the whole skill of zesting.

Getting the most out of one

• Zest before you juice. A zested lemon is easy to hold; a squeezed one is not.
• Roll it firmly on the worktop under your palm before cutting. It ruptures the juice sacs and noticeably increases the yield.
• Room temperature yields more than fridge cold. Twenty seconds in the microwave gets a cold lemon most of the way there.
• Cut it lengthways rather than across if you are squeezing by hand — it opens more of the segments.
• Rub zest into sugar with your fingers for a cake or a custard. It drives the oil into the sugar and distributes it far better than folding it in later.
• Freeze zest and juice separately in a small tub and an ice tray. Zest freezes perfectly.

When to add it

At the end, off the heat

Where you want the fresh, bright note. Heat destroys the aromatic top of lemon quickly, so a squeeze at the table is a different ingredient from a squeeze twenty minutes earlier.

During cooking

Where you want the acidity to work — tenderising, balancing a braise, setting a poached egg white, stopping fruit browning.

In a dressing

Softer and more perfumed than vinegar at the same acidity. Roughly interchangeable in quantity; not in flavour.

On fish and vegetables

Both classically, and for the same reason: acidity cuts oil and lifts anything that would otherwise taste flat.

With green vegetables

At the very last moment. Acid turns green vegetables olive within minutes.

Buying and keeping

• Heavy for their size means juicy. Light means dry inside.
• Thin, smooth skin usually means more juice; thick, knobbly skin means more pith and less.
• Unwaxed if you are using the zest — otherwise scrub the fruit well in warm water first.
• In the fridge they keep for weeks; in a bowl on the counter, days. Cut halves dry out fast and are best wrapped or frozen.
• Preserved lemons are a different ingredient again — salted and fermented, and it is the rind you use, not the flesh.

Preserving lemons, and using the whole fruit

A bag of lemons going soft is worth more than it looks.

• Preserved lemons. Quartered, packed in salt and their own juice for a month. The peel softens and the flavour turns deep and savoury, and it is the single most transformative thing you can do with a lemon.
• Zest and freeze. Zest every lemon before you juice it and freeze the zest. It is the part that cannot be bought and the part most often wasted.
• Freeze the juice in ice cube trays, which keeps far better than a half lemon in the fridge.
• Lemon cordial, which is made from the peel oil rather than the juice and tastes nothing like squash.
• Lemon curd, which uses juice and zest together and keeps in the fridge.
• Whole-fruit lemon cake, where the fruit is boiled entire and blended, pith and all.

Getting everything out of one

• Zest before you juice. Zesting a squeezed half is difficult and it is the mistake everyone makes once.
• The zest and the juice are different ingredients. The zest holds the aromatic oils and almost no acid; the juice is acid with little aroma. Most dishes want both.
• Avoid the white pith, which is bitter. A fine grater or a zester takes only the coloured layer.
• Roll it firmly on the worktop before juicing, and use one at room temperature. Both release noticeably more juice.
• Add juice at the end. Acid is volatile and prolonged cooking flattens it. A dish that tasted lively in the pan and dull on the plate usually lost its acid to heat.
• Preserve the spares. Zest freezes well; juice freezes in ice-cube trays; whole lemons can be salt-preserved and keep for a year.

Common questions