Green Solvents Turn Discarded Sea Buckthorn Seeds Into Powerful Antioxidant Microcapsules

Every year, the industrial processing of sea buckthorn berries leaves behind mountains of seeds that most factories simply throw away. A new study published in Food Chemistry: X suggests that this overlooked waste stream may be one of the richest untapped sources of health-promoting plant compounds anywhere in the food industry. Researchers led by Yuxuan Sun and Gang Hao of Southwest Minzu University have developed an integrated green manufacturing route that extracts protective polyphenols called proanthocyanidins from discarded sea buckthorn seeds using biodegradable deep eutectic solvents, and then locks those fragile compounds inside composite microcapsules that survive heat, stomach acid and storage. The work addresses two stubborn problems at once: the waste of a nutrient-dense by-product, and the notorious instability of the very molecules that make sea buckthorn so nutritionally valuable.

Sea buckthorn, Hippophae rhamnoides L., is a deciduous shrub native to the Xinjiang region of China and now widely distributed across Asia, Europe and Canada. Its berries are loaded with carbohydrates, polysaccharides, polyphenols, vitamins and amino acids, and have been linked to antioxidant, anticarcinogenic, anti-inflammatory, antimicrobial and hepatoprotective effects. Proanthocyanidins, or PAs, are oligomeric and polymeric flavonoid polyphenols that accumulate in the fruit peel, seeds and stems. They are prized for potent antioxidant activity, cardiovascular protection, anti-obesity potential through pancreatic lipase inhibition, and enhancement of microvascular integrity. Yet most research has focused on PAs from the pulp, while the seed, leaf and peel fractions, which are routinely discarded during processing, have been largely ignored.

Extracting PAs from plant matrices is technically difficult because plant cell walls resist penetration and polyphenols degrade easily. Conventional approaches such as high-pressure extraction, supercritical carbon dioxide and ultrasonic-assisted extraction all suffer from high equipment costs, heavy organic solvent consumption, large energy inputs and secondary pollution risks. The Chinese team turned instead to deep eutectic solvents, or DESs, a class of designer solvents formed when a hydrogen bond donor and a hydrogen bond acceptor combine into a eutectic mixture. Typically built from cheap, biodegradable and environmentally benign components, DESs can dissolve polyphenols through hydrogen-bonding networks that are stronger than the interactions between water and the solutes themselves, loosening the binding forces that hold phenolic compounds inside the plant matrix.

The researchers tested five choline chloride-based solvent systems in which choline chloride served as the hydrogen bond acceptor, paired with five different donors: 1,4-butanediol, citric acid, lactic acid, urea and malonic acid. Water content proved critical, producing a characteristic bell-shaped yield curve. At low water levels the solvent is too viscous to penetrate the seed matrix efficiently; moderate water addition disrupts inter-solvent hydrogen bonds and lowers viscosity, boosting diffusivity. But beyond an optimum, added water competes with the PAs for hydrogen-bonding sites and hydrates the solvent components, weakening the DES-PA interactions that drive extraction. All five solvent systems significantly outperformed 75 percent ethanol, and the champion, choline chloride with 1,4-butanediol at 50 percent water, reached an absorbance of 0.255, a full 201.2 percent higher than the ethanol control.

With the solvent chosen, the team optimized the process using single-factor experiments followed by a three-factor, three-level Box-Behnken response surface design. The solid-to-liquid ratio exerted the strongest influence, followed by temperature and time, and the interaction between solvent ratio and temperature was highly significant because heating thins the solvent and speeds mass transfer only when enough solvent is present, while excessive heat destroys the thermolabile PAs. The model predicted a maximum yield of 48.025 milligrams per gram at a 41:1 milligram-per-milliliter ratio, 61.11 degrees Celsius and 3.29 hours. Validation runs at practical adjusted conditions delivered 47.591 milligrams per gram, a relative error of just 0.90 percent, with a regression fit of R squared 0.9964. Purification on D101 macroporous adsorption resin lifted the extract to 69.17 percent total proanthocyanidins.

Liquid chromatography-tandem mass spectrometry then revealed exactly what the green solvent had pulled from the seeds. Monomeric catechin and epicatechin appeared at mass-to-charge ratios of 291.08 and 292.08, while the dimeric proanthocyanidins B1 through B4 showed characteristic signals at m/z 579.15 to 581.15. Notably, no trimer or tetramer peaks were detected, indicating that sea buckthorn seed PAs consist almost entirely of monomers and dimers. Antioxidant testing across three independent assays showed the extract scavenging DPPH radicals at 4.61, ABTS cation radicals at 5.53 and reducing ferric iron at 6.24 micromoles of Trolox equivalents per milligram, respectable values that trailed only slightly behind pure ascorbic acid and confirmed the extract retained strong reducing and radical-scavenging capacity.

The second half of the study tackled the compounds’ Achilles heel: poor water solubility, instability under heat, pH swings and gastrointestinal conditions, and rapid metabolism in the body. Using ionic gelation, the team encapsulated the purified PAs within a composite wall of sodium alginate and carboxymethyl cellulose, extruding the mixture into a calcium chloride bath to form microcapsules with an encapsulation efficiency of 89.015 percent. Scanning electron microscopy showed intact, roughly elliptical particles without tears or holes, their surface folds a by-product of freeze-drying that increases surface area and may aid retention.

Spectroscopic and thermal analyses confirmed that the capsule walls protect without chemically altering the payload. Fourier transform infrared spectra showed hydrogen-bond interactions between the phenolic hydroxyl groups of the PAs and the polysaccharide wall, but no new peaks and no shift in the aromatic ring vibrations, proving a non-covalent, structure-preserving association. Thermogravimetric analysis was even more dramatic: free PAs lost mass sharply between 100 and 150 degrees Celsius and left only about 8 percent residual char at 600 degrees, whereas loaded microcapsules degraded gradually like the blank wall material, retaining roughly 26 percent residue and shielding the core from direct heat exposure, a critical advantage for foods and pharmaceuticals that undergo thermal processing.

Simulated digestion told a similar story. In gastric fluid at pH 2.0, free PAs flooded out, releasing 29.58 percent within the first 30 minutes and 53.64 percent by 180 minutes, while microencapsulated PAs released only 2.86 percent early on and 21.99 percent at 180 minutes, a delay of more than 31 percentage points. The team attributes this to the pH sensitivity of alginate, whose carboxyl groups protonate in acid, shrinking the matrix and blocking penetration. In neutral intestinal fluid the balance reversed: microcapsules reached a cumulative release of 45.65 percent at 180 minutes, surpassing the free PAs at 39.57 percent and still climbing, exactly the controlled, intestine-targeted release profile desired for maximizing absorption and bioactivity.

The authors are candid about the hurdles separating bench from factory. The choline chloride-1,4-butanediol solvent is relatively viscous, complicating continuous large-scale extraction, and freeze-drying inflates production costs. They propose screening lower-viscosity ternary solvent systems, coupling extraction with ultrasound or microwaves, blending in cheaper agricultural waste-derived polysaccharides such as corn fiber or rice bran cellulose, and ultimately running in vivo trials of bioavailability, tissue distribution and long-term safety. If those steps succeed, the implications extend well beyond sea buckthorn. The integrated solvent-and-encapsulation strategy offers a reusable blueprint for converting fruit-processing residues of every kind into stabilized, deliverable nutraceutical ingredients, turning industrial garbage into functional-food gold with nothing more exotic than biodegradable chemistry.

Subject of Research: Green extraction and microencapsulation of sea buckthorn seed proanthocyanidins using deep eutectic solvents

Article Title: Extraction and purification and microcapsulation of sea buckthorn seed proanthocyanidins using deep eutectic solvent-assisted extraction

Article References: Sun, Y., Sun, D., Ahmad, M., Li, X., Huang, M., Bi, X., Han, L., & Hao, G. (2026). Extraction and purification and microcapsulation of sea buckthorn seed proanthocyanidins using deep eutectic solvent-assisted extraction. Food Chemistry: X, 39, Article 104394. https://doi.org/10.1016/j.fochx.2026.104394

Image Credits: AI Generated

DOI: Not provided

Keywords: sea buckthorn, proanthocyanidins, deep eutectic solvents, microencapsulation, antioxidants, green extraction, sodium alginate, carboxymethyl cellulose, response surface methodology, in vitro digestion, food by-products, functional foods