Should You Blanch Basil For Pesto?

Short answer
Blanching basil retains its vibrant green colour for longer and helps reduce the speed of oxidation, which is responsible for an unappealing grey or brown tinge. However, the process can cause an unwelcome loss in freshness and aroma, meaning we only recommend blanching basil if you plan to keep your pesto for 24 hours or more.

Long answer
The number one rule of pesto is don't cook it. The number two rule of pesto is don't cook it. It's a proudly raw sauce, so why do people insist on blanching basil before incorporating it into pesto?

Some say that blanching herbs removes bitterness, aids emulsification, and retains flavour for longer, but we've never seen those claims proven. The one thing we know for sure is that blanching delicate herbs like basil before using them in pesto helps the sauce stay greener for longer. That avoids the unfortunate situation of your pesto turning brown between the time of making it and the time of serving it.

So how does blanching work? Well, herbs contain polyphenols, a family of micronutrients that occur naturally in plants. These substances react with compounds in the plant cells to produce brown pigments. That happens at an accelerated speed when the plant cells are damaged or broken and then encounter oxygen. Blanching is an effective way to destroy many of the decomposing enzymes that are responsible for this browning.

The case for blanching basil
Busy lives and busy kitchens mean it's not always possible to make your homemade pesto immediately before eating, even though, in an ideal world, you would.

If you need to make pesto in advance of a dinner party or if you're planning to make a big batch to last for several days, you really should consider blanching your basil. Sure, blanching will momentarily start "cooking" the herb, stripping it of some of its freshness and muting its aromatic flavour slightly in the process. However, provided you blanch the leaves for no more than 15 seconds and immediately shock them in an ice bath to stop the cooking process, the decline in freshness will be negligible at best and completely undetectable by most eaters.

Any loss in freshness is a relatively small price to pay for not serving your diners an unappealingly plate of pasta with a brown sauce that makes the whole meal look disappointingly past its best.

Don't tell our Italian friends, but there's a cheat's way to add freshness back at the last minute, even if the herbs have started oxidising. Simply stir a generous squeeze of fresh lemon juice through the sauce, and the added acidity will magically lift the sauce to make it taste noticeably fresher.

The case against blanching basil
First of all, blanching basil (or any green herbs or vegetables) is a bit of a faff. Admittedly, it only takes ten minutes from start to finish, but who realistically has the time?

Pesto is designed to be eaten raw, and nothing can be fresher than stripping leaves from a living basil plant immediately before turning them into pesto. Even if you blanch herbs for a few seconds and transfer them to an ice bath, you will have started the cooking process and must therefore resign yourself to serving a sauce that cannot possibly boast the same freshness as its raw counterpart.

Ultimately, no matter what tips and tricks you employ to keep your pesto fresher for longer, you will always find that your sauce will start reducing in vibrancy and freshness as soon as it is made, and that will happen whether you blanch your herbs or not.

Still want to blanch your basil? Here's how.
Bring a large saucepan of water to a rolling boil, and make an ice bath big enough and cold enough to ensure that it doesn't rise in temperature too much when you plunge the hot basil leaves into it.

Strip your basil leaves from their stems (ideally working with a live plant and only selecting the smallest, sweetest leaves) and add them to the boiling water in batches. Using a slotted spoon, make sure the leaves remain under the water and don't spend their time floating on the surface, which is naturally what they will want to do.

When fifteen seconds are up, scoop out the leaves, suspend them over the saucepan to drain for a second or two, and then plunge them into your ice bath. Again, make sure to keep them under the surface so that they cool in the fastest time possible.

Bring your saucepan back to a rolling boil and repeat the process until all your basil has been blanched. Once all the leaves are cooled to room temperature or below, remove them from the ice bath, gently squeeze with your hands to remove excess water, and pat them dry with some kitchen roll.

Bonus tips and tricks
If you're a culinary geek and really want to extend the vibrant life of your sauce, then you can add a small amount of sodium bisulfite or ascorbic acid to your pesto. This will help preserve its fresh taste and colour for even longer, although we must admit that we're not fans of adding perfectly safe but slightly scary-sounding ingredients to our dinner.

As much as we adore basil, it's incredibly delicate, which makes it a real pain to work with. Sometimes it feels like we only have to look at it, and it will start wilting. It's therefore important that you handle it with kid gloves and always use a razor-sharp knife when cutting it to avoid excess bruising, which can speed up the rate of oxidation.

Whether you are blanching your basil or not, pesto always tastes better when made with a pestle and mortar. Food processors simply slice ingredients thinner and thinner, while the pounding and crushing motion of a pestle and mortar leads to far more flavourful, aromatic oils being released and results in a noticeably tastier result.

Finally, there is one other magic yet ubiquitous ingredient you have in your armoury, and that's oil. Remember that we're blanching basil to reduce the negative impact of oxidation? Well, when you go to store any leftover pesto, make sure that you add a thin layer of oil to the top of it. This will ensure oxygen cannot reach the herbs, and you will dramatically slow down the amount of time it takes for your pesto to turn brown.

Freshly made pesto lasts in the fridge for five days, but by this point, its quality will be noticeably worse, which is why we recommend eating it within 24 hours.