Pesto turning brown

Why basil browns

Basil leaves contain enzymes that, once the cell is damaged and oxygen reaches the contents, oxidise phenolic compounds into brown pigments. It is the same process that browns apples, avocados and potatoes.

Three things speed it up, and a blender supplies all three at once: it damages a very large number of cells very quickly, it whips air through the mixture, and the friction warms it.

A mortar and pestle crushes rather than slices, incorporates far less air and generates almost no heat, which is most of why pounded pesto stays green longer as well as tasting different.

How to keep it green

In order of how much difference each makes.

• Blanch the basil for five seconds and refresh in iced water. The heat deactivates the browning enzymes. It costs a little of the fresh aroma and it keeps the colour for days — this is what commercial pesto does.
• Cover the surface with oil. Air is what the enzymes need. A film of oil on top of the jar is the simplest effective measure.
• Add lemon juice. Acid slows the enzymes. Not traditional, and it works.
• Use a mortar rather than a blender, or pulse briefly and keep everything cold.
• Chill the bowl, the blade and the leaves before starting.
• Press cling film onto the surface rather than leaving a headspace.
• Freeze it in an ice cube tray, without the cheese, and stir the cheese in when you use it.

Blanching basil properly

Five seconds means five seconds.

1. Bring a pan of water to the boil and set a bowl of iced water beside it.
2. Drop in the picked basil leaves and count to five.
3. Lift them straight into the iced water. They should stay vividly green.
4. Squeeze them dry — properly dry, in a cloth — or the pesto will be watery.
5. Pound or blitz as normal. The colour will hold for several days in the fridge under oil.
6. The same method works for parsley, mint, tarragon and wild garlic.

What brown pesto is and is not

Not spoiled

Browning is a colour change, not spoilage. Brown pesto that smells fresh is fine to eat.

Flavour changes slightly

Oxidation dulls the aroma over days as well as the colour, so it is a quality signal even if not a safety one.

Keeping

How long any home-made preserve keeps is a food-safety question — follow Food Standards Agency guidance.

Commercial pesto

Stays green because the basil is blanched and the jar is sealed without air.

Freezing

The most effective way to keep both colour and flavour. It freezes very well.

Common questions