The executive chef of dine by Peter Gordon at SkyCity answers your cuisine questions.
Can you tell me how to keep pesto green? It is always a bright green for cooks making it on TV, and for bought commercial pots which also contain preservatives. But as soon as I pound or blitz mine it oxidises and looks very dark. Thank you.
The difficulty any products made from chopped or pureed green herbs face with extended storage is that they always darken due to oxidation - which basically means their exposed surfaces (from chopping them open) are attacked by oxygen molecules, causing them to break down and discolour. Though an apple may discolour and pesto may darken, it's no different than your car rusting - those naughty oxygen molecules are just waiting to attack whatever they can. To fight this process you need to engage an antioxidant. For fruit and artichokes, brushing the exposed flesh with lemon juice does the trick, and for your car you apply paint or chrome. Unfortunately neither are suitable for pesto, green herbs or salad. The minute you add lemon juice to pesto it begins to darken as the acidity also destroys the dark green for some molecular reason.
However, what you can do to extend your green-ness is to add ground Vitamin C tablets. I've read it works and it certainly makes sense. Vitamin C, ascorbic acid, is an antioxidant and should do the trick. If you're able to source baobab powder in NZ, it should also help as it's packed full of antioxidants.
However, the fact your pesto seems to start out dark means that perhaps other things are at play. One of the reasons pesto can be dark right from when you make it is that the nuts you've added have been toasted too much initially, or you've added too many of them - obviously the darker the nuts, the darker the pesto. Compare a walnut pesto with a pinenut one and you'll know what I mean. Pinenuts, lightly toasted golden, will barely darken your pesto but add a lovely nutty richness. The more toasted they are the more flavour they'll give, but there comes a tipping point where the pesto just begins to taste of nuts. Another trick is to blanch parsley leaves in boiling water for a few seconds to which you've added some baking soda (which preserves the green-ness). Drain them and plunge into iced water for a few minutes then drain again and squeeze out as much water as you can and add this to your pesto - even just a quarter a cup of leaves will boost the colour.
But lastly I have to say that in reality, a pesto should be eaten within a day or two of making it anyway to retain the freshness and delicacy of it. A new pesto will have a much fresher flavour than one that's been sitting around for a while. The surface layer of a jar of pesto will always darken due to oxidation. You can slow this down by drizzling the top with a thin film of sunflower oil which is less acidic than some olive oils. If you're really bothered by the dark green, use a teaspoon to scoop off a thin layer from the surface and just eat the brighter pesto underneath.