The reddish-brown, papery layer that clings to the peanut kernel, commonly referred to as peanut skin, was long considered a waste product of the peanut industry. Recent research indicates that this outer layer holds a concentration of health-promoting compounds that far surpasses the kernel itself. This shift in perspective redefines the simple peanut as a whole food whose most beneficial components may be the part traditionally discarded.
Identifying the Peanut Skin (The Testa)
The peanut skin is technically known as the testa, a protective seed coat that shields the inner kernel, or cotyledon, from environmental damage. Structurally, the testa is markedly different from the rest of the legume, most notably for its extremely high fiber content. Approximately 70% of the peanut skin’s mass is composed of dietary fiber, with about 95% of that being insoluble fiber. This is a significantly higher proportion than what is found in the fatty, protein-rich kernel it encases.
The Powerhouse of Polyphenols
The primary reason for the peanut skin’s recognized benefit is its dense concentration of polyphenols, which are plant-based compounds known for their biological activity. This layer contains a richer array and quantity of these compounds than the peanut kernel and, in some cases, other widely celebrated plant foods. Among the most notable compounds is resveratrol, a stilbene also found in grapes and red wine, which is present in the skin.
Phenolic acids, such as p-coumaric acid, gallic acid, and caffeic acid, are also prominent components of the testa. The concentration of p-coumaric acid can be enhanced by certain processing methods, such as roasting. The skin also provides a complex profile of flavonoids, including various catechins like epicatechin and epigallocatechin, along with procyanidins.
Antioxidant Effects and Physiological Benefits
The high density of polyphenols translates directly into a powerful antioxidant capacity that addresses oxidative stress within the body. Consuming the whole peanut, skin included, can nearly double the total antioxidant capacity compared to eating the blanched kernel alone. The compounds neutralize free radicals, which are unstable molecules that cause cellular damage over time.
The physiological benefits extend to supporting the body’s cardiovascular and circulatory systems. Antioxidants from the skin help prevent the oxidation of LDL cholesterol, a process that is a precursor to arterial plaque formation. Furthermore, the anti-inflammatory properties of the flavonoids and phenolic acids can modulate inflammatory pathways. Research has indicated that extracts from peanut skin can suppress the expression of inflammatory enzymes and markers in laboratory models.
Specific human trials involving skin-roasted peanuts have shown measurable improvements in brain vascular function and memory in older adults. The bioactive compounds support the vascular system, leading to enhanced cerebral blood flow, which is important for cognitive health. The considerable dietary fiber content also supports gut health by adding bulk and promoting regularity. However, some studies suggest that peanut skin extracts can potentially inhibit the growth of beneficial gut bacteria, such as Lactobacillus, while promoting certain harmful bacteria in laboratory settings, underscoring the need for further research on its prebiotic effects in humans.
Incorporating Peanut Skin into the Diet
The easiest way to consume peanut skin is by simply eating whole, skin-on peanuts, which are often available as “runner” or “virginia” type peanuts. Many producers now incorporate the skin into peanut butter, which is a straightforward way to boost the product’s fiber and antioxidant content. Grinding the skins into a fine powder allows them to be mixed into smoothies, yogurt, or baked goods without significantly altering the texture of the final product.
The processing method affects the compound profile, as roasting is known to increase the total antioxidant capacity. When choosing peanuts, it is advisable to select unsalted, skin-roasted varieties to maximize the health-promoting compounds. A necessary caution is that the skin contains the same allergenic proteins found in the kernel. The dry-roasting process may actually increase the allergenic potential of the peanut proteins through a chemical change known as the Maillard reaction. Individuals with known peanut allergies must maintain strict avoidance of all peanut products, including the skin.