Walnuts are rounded, single-seeded stone fruits of the walnut tree commonly used for food after fully ripening between September and November. The removal of the husk after the ripening stage reveals a browning wrinkly walnut shell, which is usually commercially found in two segments, but three or four-segment shells can also form. During the ripening process, the husk will become brittle and the shell hard. The shell encloses the kernel or meat, which is usually made up of two halves separated by a membranous partition.
The seed kernels – commonly available as shelled walnuts – are enclosed in a brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen, thereby preventing rancidity.
Walnuts have a very slight, pleasant odor.
Walnuts are susceptible to absorption of unpleasant and/or pungent odors. Therefore, walnut should be stored and transported in clean air away from foodstuffs with an intense odor.
Walnut trees are late to grow leaves, typically not leafing out until more than halfway through the spring. They emit chemicals into the soil to prevent competing vegetation from growing. Because of this, susceptible plants should not be planted close to them
Walnuts like other foods will react with atmospheric oxygen and become spoiled. Atmospheric oxygen may enter into an addition reaction (an organic reaction where two or more molecules combine to form a larger molecule) with unsaturated fatty acids in an environment of simultaneous light, heat and certain fat companion substances. Traces of heavy metals may also accelerate the spoilage process.
Rancidity caused by oxidative fat cleavage is more prevalent in shelled walnuts, because the shelling process with exposure to atmospheric oxygen or to the steel parts of container walls or the transporting ship involves damaging exposure. Walnuts must be stored covered in the dark to protect walnuts from oxygen and metal parts, resulting in rancidity — an undesirable state when walnuts become brown-colored and develop a rancid odor and taste.
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