Pistachios are widely known in grocery stores and recipes as a type of tree nut, often grouped alongside walnuts, pecans, and almonds. This common culinary classification, however, obscures the actual botanical identity of the food we consume. To understand what a pistachio truly is, it is necessary to move past common usage and examine its origins on the Pistacia vera tree.
The Definitive Botanical Answer
The simple, definitive answer is that the edible part of a pistachio is not a nut, but a seed. This seed is contained within a fruit that is botanically classified as a drupe. Drupes are fleshy fruits that develop from the ovary of a single flower and contain a single seed surrounded by a hardened inner layer, often called a stone or pit.
This places the pistachio in the same botanical category as cherries, plums, and olives, which are all considered drupes. Unlike those fruits where the fleshy outer layer is the part we eat, the edible portion of the pistachio drupe is the seed found inside the hard protective shell. Therefore, when you eat a pistachio, you are consuming the seed of a fruit.
Defining Fruits and Nuts
Botanically, a fruit is the mature, ripened ovary of a flowering plant, and it contains the seeds. This definition includes many items commonly considered vegetables, such as tomatoes, cucumbers, and peppers.
A true botanical nut is a specific type of dry fruit that possesses a hard, woody outer shell, known as the pericarp, which does not naturally open to release the seed. Examples of these true nuts include the chestnut, hazelnut, and acorn. The entire structure—the shell and the contents—is the fruit, not just the kernel inside.
The term “nut” as used in the kitchen is a culinary designation for any large, oily, edible kernel found within a hard shell. This category groups together several botanically distinct items, including true nuts, seeds, and the seeds of drupes. Pistachios, like almonds and pecans, are called nuts for practical reasons, despite being the seeds of a drupe.
The Structure of the Pistachio Drupe
The entire pistachio fruit develops from the single ovary of the flower and consists of three distinct layers enclosing the edible part. The outermost layer is a fleshy, reddish-yellow hull, which botanists refer to as the exocarp and mesocarp.
This outer hull is typically removed soon after harvest, revealing the familiar hard, beige shell underneath. This shell is the innermost layer of the fruit wall, known as the endocarp, which becomes tough and woody to protect the seed inside. In fruits like peaches, this endocarp is the hard pit surrounding the kernel.
The kernel itself, the green, elongated portion we consume, is the single seed of the fruit. As the pistachio fruit matures, the shell often splits open along its seam, a process known as dehiscence, which is a desirable trait in commercial cultivation.