Walnut hull is the outer fleshy covering of the walnut fruit. This natural casing, particularly notable on black walnuts, has played a role in diverse applications throughout history. Understanding this component reveals its unique characteristics and the compounds that contribute to its distinctive nature.
Understanding Walnut Hull
Walnut hull is the green, fleshy outer casing that surrounds the hard shell of the walnut nut. While both English and black walnuts possess hulls, those of the black walnut are more commonly utilized due to their larger size and greater potency. The hull is typically green and round, encasing the nut within, and it darkens to black as it matures or is exposed to air. Removing the hull can be challenging.
Distinctive Compounds and Their Actions
Walnut hulls contain several bioactive compounds. Juglone is a prominent natural naphthoquinone found in various parts of the walnut tree, including the green husks, roots, and leaves. This compound originates as hydrojuglone, a precursor that oxidizes into active juglone upon exposure to air. Juglone is known for its allelopathic effects, meaning it can inhibit the growth of certain other plants. Beyond its impact on plant growth, juglone also exhibits antimicrobial and antifungal properties.
Another group of compounds present in walnut hulls are tannins. These naturally occurring polyphenols contribute to the hull’s astringent qualities. Tannins are responsible for the staining effect commonly observed when handling walnut hulls, and they also possess antioxidant properties.
Varied Uses
Walnut hulls have found diverse applications across history and in modern contexts. Historically, their rich, dark pigments made them a popular natural dye for fabrics, hair, and wood, producing shades of brown.
In traditional folk medicine, black walnut hulls were used as an astringent for various skin conditions, including acne, eczema, and warts. They were also employed as an antiparasitic agent to help expel intestinal worms and for general digestive health. Modern applications for walnut hulls include their use in dietary supplements, available in capsule or tincture forms. Industrially, crushed walnut shells are valued as a biodegradable abrasive in cleaning processes such as sandblasting.
Considerations for Safe Use
Despite their various uses, important safety considerations exist when handling and using walnut hulls. Juglone can cause skin irritation, leading to redness, itching, or even blistering. It also causes a persistent dark stain on the skin that can take weeks to fade. Wearing gloves when handling black walnut hulls is advisable to prevent both staining and irritation.
The allelopathic effect of juglone means planting sensitive species, such as tomatoes, apples, and pines, near walnut trees can result in stunted growth or damage. It is recommended to avoid using walnut tree parts, including hulls, as mulch or compost in gardens where sensitive plants are grown, as juglone can persist in the soil. When considering walnut hull supplements, consult a healthcare professional, as these products are not regulated by the U.S. Food and Drug Administration (FDA). Large quantities of tannins can potentially cause digestive upset, interfere with iron absorption, or, in rare cases, lead to kidney or liver damage.