Pesto
Pesto (from the Italian pestare, meaning "to pound") is a vibrant herb suspension. While we often use blenders today, the traditional method uses a mortar and pestle to release the essential oils of the basil without the destructive heat of spinning blades.
Unlike an emulsion, pesto is a suspension: solid particles of basil, pine nuts, garlic, and Parmesan held in a matrix of olive oil. The pounding technique tears the plant cells gently, coaxing out their volatile aromatic oils rather than shearing them apart. This is why mortar-made pesto tastes brighter and more complex than the blender version. The layer of oil on top serves a critical function beyond flavor — it seals out oxygen, preventing the chlorophyll in the basil from oxidizing and turning the sauce from emerald green to a dull brown.
History & Origins
The modern 'Pesto alla Genovese' was first documented in 1863, but its ancestors date back to the Roman 'Moretum', a paste of garlic, herbs, and cheese cheese. Genoa, with its maritime trade, added the aromatic basil and expensive pine nuts, turning it into a luxury condiment.
The Science
Pesto is a suspension rather than an emulsion. Bits of solid (herbs, nuts, cheese) are held in liquid (oil). The high fat content of the oil acts as a barrier to air, which is why covering pesto with oil prevents the basil from turning brown (oxidizing).
Technique
The Mortar and Pestle method is superior to a blender. Pounding tears the basil cells gently, releasing aromatic oils without the shearing force and heat of internal blades, which can make the herb taste 'metallic'.
Common Mistakes
Heat is the enemy of pesto. Never cook pesto in a pan. Always toss it with pasta off the heat. Even the friction from a fast blender can 'cook' the basil, turning it from bright green to dull olive.
Chef's Notes
Toast your pine nuts very lightly. You want a nutty depth, but if they get too dark, they will overpower the delicate basil. For an extra vibrant green, some chefs briefly blanch the basil in ice water before pounding.