Guacamole is a surefire and healthy party pleaser, at least for those who arrive at the party early. Stragglers know they're late by the muddy brown shade the dip has assumed in the bowl. Obviously, oxygen is the enemy of guac, as it is for sliced potatoes and apples. The question is, can you delay the oxidation process? (Leaving the pit in the dip, an old myth, doesn't help.)
The solution: A two-part strategy involves using acid to delay oxidation, then doling out the dip as needed from an airtight container. The antioxidant property of ascorbic acid, plentiful in lemon or lime juice, is your first line of defense. Toss cubed avocado in citrus juice, about 3 tablespoons per avocado, then drain before mashing, reserving the juice. After you've mashed all your ingredients, add some juice back to taste. Still, your dip will brown eventually if you serve all at once, so serve in small batches, with the rest stored in the fridge like so: Rub a little olive oil onto a sheet of plastic wrap, then press the wrap, oil side down, onto the surface of the dip—the thin film of oil creates an impermeable barrier, with plastic as a reliable backup. And the dip stays green.