In fact, what stops guacamole going brown – oxidising – is the exclusion of, erm, oxygen, i.e. air. Harold McGee explains it all in his customary engaging and enlightening fashion in The Curious Cook. He describes the stone/pit-burying ritual as ‘one of the looniest culinary myths’, and proceeds to debunk it by means of a carefully controlled scientific experiment.
It’s easy to see how the myth came about – ever cut an avocado in half, used only one half, and put the other back in the fridge – with the stone still in place – to be used up later? Annoyingly, when you come to use up the second half, the cut flesh has gone black. But, wait. When you lift out the stone, you find that the avocado flesh in immediate contact with the stone has stayed beautifully green. This is what has led people to believe that the stone contains some magic, non-browning properties and that the simple inclusion of the stone(s) will prevent guacamole losing its lustre.
McGee’s experiment went like this. He mashed some avocado into a bowl, buried the stone in the middle and waited to see what happened. After an hour or two, the surface turned an unappetising brown. However, the area immediately in contact with the stone kept its pretty green colour. Next, he made a fresh batch of crushed avocado but instead of burying the stone in it, he used a smooth, shiny, glass light bulb (there’s a great sketch in his book of this unlikely experiment). Once again, the surface of the purée discoloured, but the avocado directly in touch with the light bulb stayed truly green.
The magic, he concluded, lay not in the stone itself but in the fact that air had been excluded. And his recommendation for keeping guacamole bright green, even after a few hours? By all means go ahead and bury the stone in the middle, he says – just for fun and because your Mexican friends have always told you so. But if you want to seriously prevent discolouration, take a piece of clingfilm and press it closely into the surface of your guacamole to exclude any air.
Try it. It works like a charm every time and keeps the guacamole fresh and green for at least 3 hours. Once you’ve unveiled it, eat it up pronto – not too much of a hardship, given that guacamole is one of the most irresistible foods known to (wo)man.
STOP PRESS: made a batch yesterday and didn’t use it all up so packed the surplus into a small ramekin and covered it tightly with clingfilm. Next day it was still green – hooray!
1 clove garlic, crushed
1/2 teaspoon salt
1 fresh green chile, de-seeded, finely chopped
a bunch of fresh coriander/cilantro, chopped
2 ripe avocados
juice of 1 lime
3-4 cocktail tomatoes (e.g. Gardener’s Delight), roughly chopped
1-2 spring onions, finely chopped
- If you own a molcajete (Mexican pestle and mortar, pictured above) use this to crush together the garlic, salt, chile and coriander to a fragrant green paste; otherwise, chop (or process) these four together finely
- Cut the avocados in half, remove the stones (reserve them, if only for tradition’s sake) and spoon the flesh out of the skins (or peel skins away)
- Mash (or process) the flesh into the garlic/chile/coriander paste – don’t overdo it, guacamole should not be like baby food but have some texture and character
- Stir in the lime juice, chopped tomatoes and spring onions (and bury in the stones, if you insist)
- Put guacamole in a bowl, press a piece of clingfilm firmly against the surface to exclude air
- Refrigerate for up to 3-4 hours, then serve promptly – with tortilla chips, or with grilled fish or poultry