Discarded Peanut Hearts Could Keep Peanut Butter Fresh, Study Finds

Peanut butter is one of the world’s most beloved spreads, but it carries a hidden weakness: its abundance of unsaturated fats makes it remarkably vulnerable to oxidation. Over time, exposure to oxygen triggers a cascade of chemical reactions that produce peroxides, off-flavors, and rancid aromas, while oil gradually separates from the solid matrix and pools at the top of the jar. Food scientists have long battled this deterioration with synthetic antioxidants such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tert-butylhydroquinone (TBHQ), which are undeniably effective at slowing lipid peroxidation. Yet growing consumer unease about potential health risks linked to these synthetic compounds, including concerns about carcinogenicity and endocrine disruption, has pushed the industry to search for natural alternatives that do not compromise flavor, color, or texture.

A new study published in Food Chemistry: X offers a surprisingly elegant solution, and it comes from a part of the peanut that manufacturers routinely throw away. A research team led by Yuan Gao and Mingqing Wang of Shandong Academy of Agricultural Sciences turned its attention to the peanut embryonic axis, sometimes called the peanut heart. This tiny structure, which contains the hypocotyl, radicle, and plumule and represents only about 2.5 percent of the total kernel mass, is often removed during commercial processing because of its bitter taste. While peanut skins and hulls have attracted considerable attention as sources of valuable phenolics and proanthocyanidins, the embryonic axis has remained largely unexplored territory, despite earlier reports that it contains lower levels of the major allergen mRNAs Ara h 1 and Ara h 3 and higher concentrations of minerals such as magnesium, phosphorus, potassium, calcium, iron, and zinc compared with the cotyledon.

To understand what makes the embryonic axis chemically distinctive, the researchers conducted a comprehensive comparison between the axis and the cotyledon, the fleshy tissue that constitutes the bulk of the peanut kernel and the main raw material for peanut butter. Proximate analysis revealed meaningful compositional differences. The cotyledon contained 46.90 percent fat, whereas the embryonic axis held only 39.35 percent, a reduction of more than seven percentage points. The axis also carried more crude fiber, at 8.07 percent versus 6.31 percent in the cotyledon, and more carbohydrate, at 22.7 percent versus 16.4 percent. Protein content, by contrast, showed no significant difference between the two tissues. This profile of lower fat and higher fiber immediately suggested that the axis could serve as a functional ingredient capable of improving both the nutritional profile and the physical behavior of peanut butter.

The most striking findings emerged from untargeted metabolomics performed with ultra-performance liquid chromatography coupled to tandem mass spectrometry. Using a Thermo UHPLC-Q Exactive system operating in both positive and negative electrospray ionization modes, the team identified 419 metabolites that were significantly up-regulated and 201 that were down-regulated in the embryonic axis relative to the cotyledon. Among the plant secondary metabolites driving this separation, terpenoids dominated, accounting for more than half of the differential compounds, with triterpenoids forming the largest subgroup. Of the top 25 metabolites distinguishing the two tissues, 20 were triterpenoids, and all were markedly more abundant in the embryonic axis. These included compounds with well-documented biological activities, such as beta-elemonic acid, momordicinin, and maslinic acid, which have been associated with anti-cancer and anti-inflammatory effects, along with ginsenoside I, ginsenoside Rh7, and several soyasaponins known for antioxidant and antimicrobial properties.

Armed with this chemical portrait, the researchers formulated peanut butter in which roasted, peeled peanut kernels were partially replaced by embryonic axis at substitution levels of 2.5, 5, 7.5, and 10 percent by weight. The control spread was prepared from 100 percent roasted kernels with no added salt, sugar, oil, or stabilizers, ensuring that any observed effects could be attributed to the axis itself. Proximate analysis of the finished products showed that fat content declined progressively from 51.71 percent in the control to 49.23 percent at the 10 percent substitution level, while crude fiber rose significantly from 6.73 percent to 7.43 percent and carbohydrate climbed to 16.0 percent at the highest inclusion. Protein remained essentially unchanged across all formulations, hovering between 23.7 and 23.9 percent. Because lipid oxidation begins with the substrate itself, reducing the pool of oxidizable fat inherently slows the initiation and propagation of peroxidation reactions.

Texture and rheology measurements revealed that the embryonic axis did more than simply dilute the fat. Hardness increased from 0.79 newtons in the control to 0.94 newtons at 10 percent substitution, while adhesiveness rose from 0.96 to 1.51 newton-millimeters, and springiness, gumminess, and chewiness all climbed significantly as well. Rheological testing showed that all samples behaved as pseudoplastic, shear-thinning fluids, with apparent viscosity decreasing as shear rate increased from 0.1 to 100 reciprocal seconds. The flow curves fitted the power-law model with coefficients of determination between 0.989 and 0.996, and the consistency index rose from 37.29 pascal-seconds to the power of n in the control to roughly 50 at the two highest substitution levels. The researchers attribute this firmer, more cohesive structure to two mechanisms: the reduced free oil that would otherwise act as a lubricant, and the long molecular chains of dietary fiber, which intertwine into a three-dimensional network that binds oil and restricts its movement through the matrix.

The antioxidant evidence was equally compelling. Radical scavenging assays showed that spreads containing 7.5 and 10 percent embryonic axis displayed significantly higher activity against both DPPH and ABTS free radicals than the control, and the 10 percent formulation outperformed the 2.5 and 5 percent versions in the DPPH assay. Spearman correlation analysis confirmed that fiber content was strongly and positively associated with radical scavenging, with correlation coefficients of 0.903 for DPPH and 0.888 for ABTS. The enriched triterpenoid fraction likely contributed as well, since compounds such as maslinic acid, ginsenosides, and soyasaponins have demonstrated free radical neutralization and shelf-life extension in other food systems. Although roasting can partially degrade certain heat-labile saponins, the authors note that degradation is compound-specific and unlikely to have eliminated the overall antioxidant capacity of the enriched spreads.

Storage performance under accelerated conditions provided the decisive test. Samples packed in PET jars were held at 37 degrees Celsius for four weeks and monitored for oil separation and peroxide value. At the outset, all axis-fortified spreads showed markedly lower oil separation than the control, and a clear dose-dependent relationship emerged, with higher substitution levels corresponding to less oil loss over time. The researchers suggest that surface-active triterpenoid saponins may lower interfacial tension and form stable viscoelastic films that inhibit the coalescence and migration of oil droplets, while the denser fiber network physically entraps liquid oil. Peroxide values told a similar story: the control started at 5.25 millimoles per kilogram and climbed to 7.78 by week four, whereas the fortified spreads began between 3.28 and 4.77 and reached only 6.7 to 7.0 after the same period, demonstrating that the axis genuinely retarded lipid oxidation rather than merely masking its symptoms.

Crucially, none of these functional benefits came at the cost of appearance or palatability. Colorimetric analysis in the CIELab system showed no significant changes in brightness, redness, or yellowness across formulations, and the overall color difference relative to the control remained low, between 0.92 and 1.54, well within the range considered visually acceptable. A semi-trained sensory panel of twelve regular peanut butter consumers rated all spreads on a nine-point scale covering appearance, flavor, consistency, taste, spreadability, and overall acceptability. The 2.5 percent substitution earned the highest overall score of 8.0, compared with 7.4 for the control, and also received the best flavor rating at 7.8. Scores declined modestly at higher inclusion levels, likely because triterpenoid saponins impart a characteristic bitterness, but even the 10 percent formulation matched the control at 7.4, indicating that the axis never pushed the product beyond consumer tolerance.

The implications extend well beyond a single spread. The study demonstrates that a bitter, discarded fraction of the peanut, available essentially for free as a processing by-product, can simultaneously reduce fat, boost fiber, enhance antioxidant activity, suppress oil separation, and slow rancidity, all while preserving the clean-label, single-ingredient character that modern shoppers increasingly demand. The authors caution that their metabolomics results are semi-quantitative and based on limited biological replicates, so targeted absolute quantification will be needed to confirm the triterpenoid findings, and sensory testing was performed only at the initial time point rather than after storage. Future work may explore synergies between the embryonic axis and other natural antioxidants and extend the approach to other oil-rich food systems. For now, the humble peanut heart, once destined for the waste stream, has earned a place at the center of a smarter, more sustainable peanut butter.

Subject of Research: Use of peanut embryonic axis as a natural functional ingredient to improve the texture and oxidative stability of peanut butter

Article Title: Composition characterization of peanut embryonic axis and its application in peanut butter: Effects on texture and oxidative stability

Article References: Gao, Y., Fu, B., Wu, J., Yu, L., Bi, J., Song, Y., Wang, L., Jiang, C., & Wang, M. (2026). Composition characterization of peanut embryonic axis and its application in peanut butter: Effects on texture and oxidative stability. Food Chemistry: X, 39, Article 104532. https://doi.org/10.1016/j.fochx.2026.104532

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104532

Keywords: peanut butter, peanut embryonic axis, lipid oxidation, triterpenoids, dietary fiber, natural antioxidants, oxidative stability, oil separation, peroxide value, food by-product valorization, rheology, clean-label foods