The research team, led by Bekri Melka Abdo and Sung-Hyen Lee of the Rural Development Administration’s National Institute of Crop and Food Science, set out to test a deceptively simple question: does the liquid used to presoak onion skin matter? Presoaking is usually dismissed as a washing step, a necessary bit of hygiene before the real work of extraction begins. But the authors argue that soaking can hydrate plant tissue, alter cell wall permeability, and change how soluble compounds diffuse out of the matrix. In other words, the soak itself may be a controllable processing variable—one that food manufacturers could tune to steer the chemistry of the final ingredient.

To probe this idea, the team obtained dried inner skins from conventionally grown yellow onions of a single hybrid cultivar lineage, traced through seed import records to a Dutch-bred variety and farmed in the Mungyeong and Yecheon regions of Gyeongsangbuk-do. The skins were soaked at room temperature in one of five solutions: tap water, 5% vinegar, 1% citric acid, a 1:1 mixture of vinegar and citric acid, or 0.3% hydrogen peroxide. Each soak lasted just four, eight, or twelve minutes—a deliberately narrow window chosen to see whether even minimal预处理 could shift the outcome. After rinsing, drying, and grinding, the powders were extracted in two contrasting systems: methanol, a laboratory reference solvent prized for pulling out flavonol aglycones, and hot water at 95°C, a route compatible with food production.

The results, analyzed by two-way ANOVA with Fisher’s least significant difference comparisons, showed a striking pattern: the identity of the soaking solution had a significant main effect on nearly every major parameter, while the duration of soaking—within that tight 4-to-12-minute range—mattered far less. Solution chemistry, not time, was the dominant lever. Citric acid and the vinegar–citric acid mixture produced the highest extraction yields, boosting total solids recovery to roughly 10.5–10.7% in both solvent systems, compared with about 5.8–7% for plain water. But the researchers caution that yield alone is a misleading metric. The acid-soaked samples recovered more non-phenolic material, diluting the phenolic density of the extracts rather than enriching them.

Total phenolic content, measured by the Folin–Ciocalteu assay and expressed as gallic acid equivalents, reached nearly 491 mg/g in water-soaked methanol extracts—the highest of any condition—while dropping to under 400 mg/g in the citric-acid group. The picture flipped, however, when the team turned to hot water. Here, peroxide-soaked skins yielded the strongest total flavonoid content at 918 mg quercetin equivalents per gram, and the highest quercetin glycoside yields in both solvent systems. Targeted UPLC–PDA quantification confirmed that quercetin, the dominant flavonol of onion skin, partitioned overwhelmingly into methanol—accounting for over 61% of the ion signal in the reference extract—while hot-water extracts were richer in protocatechuic acid and other polar constituents. High-resolution UHPLC–Orbitrap mass spectrometry revealed that the two solvents were not simply recovering different amounts of the same chemistry; they were generating compositionally distinct fractions.

Antioxidant performance tracked this compositional divergence. Using the DPPH radical-scavenging assay, the team calculated IC₅₀ values by four-parameter nonlinear regression and found that peroxide-presoaked extracts showed some of the most favorable radical-scavenging potency, alongside vinegar-treated samples. Exploratory contour plots suggested combined process–response patterns linking solution pH, soak duration, and antioxidant strength, though the authors are careful to note that the soaking solutions differed chemically as well as in pH, so the plots describe trends within the tested range rather than a universal pH optimum. The team emphasizes that assays like DPPH and Folin–Ciocalteu measure electron-transfer chemistry in a plate, not physiological efficacy in a body—useful for comparing treatments, but not proof of health benefits.

To add a biological dimension, the researchers screened representative extracts in RAW 264.7 macrophage cells, measuring metabolic activity with an MTS assay and nitric oxide production indirectly through nitrite accumulation via the Griess reaction. The extracts did not markedly reduce metabolic viability at the tested concentrations of 125 to 500 μg/mL, and presoaking-dependent differences in nitrite accumulation were observed within each extract type. Yet the authors are explicit about the limits: the extracts were tested without an inflammatory challenge such as lipopolysaccharide co-treatment, the two extract types were compared at different doses and durations, and nitrite levels cannot be classified as beneficial or harmful without a defined inflammatory model. The macrophage data are presented as hypothesis-generating screening, not evidence of immunological effect.

The practical implications reach into the growing field of food by-product valorization. Onion skin is a concentrated reservoir of quercetin and related flavonoids with well-documented antioxidant relevance, and converting it into a standardized food ingredient aligns with circular-economy and biorefinery principles. This study suggests that process designers should choose soaking solutions according to their target profile: acids to maximize mass yield, neutral water to preserve phenolic density in organic extraction, or carefully controlled mild oxidation to enhance hot-water flavonoid recovery. Because hot water is the route most compatible with industrial food processing, the finding that presoaking can steer hot-water extract composition is particularly relevant to anyone hoping to upcycle onion waste at scale.

The researchers are equally clear about what their work does not establish. The 0.3% hydrogen peroxide condition was an experimental oxidative treatment, not a food-ready process; residual peroxide, oxidation products, removal efficiency, and regulatory compliance would all need validation before any practical use. The study also did not measure structural changes in the skin matrix, so the mechanism behind the peroxide effect—whether enhanced accessibility or chemical transformation—remains unassigned. Translation to industry, the authors write, will require confirmation across harvests and commercial lots, pilot-scale mass and energy balances, stability and sensory testing, food-matrix performance trials, safety assessment, and techno-economic analysis. What the study does deliver is a framework: a demonstration that a step as ordinary as a few minutes of soaking, guided by the right chemistry and matched to the right solvent, can meaningfully reshape the value recovered from one of the world’s most abundant vegetable waste streams.

Subject of Research: How brief presoaking treatments modulate phytochemical recovery, antioxidant capacity, and macrophage responses of onion skin extracts

Article Title: Simple presoaking modulates phytochemical recovery, antioxidant capacity, and macrophage responses of onion skin extracts

Article References: Abdo, B. M., Song, D., Kang, H. J., Im, J. Y., Hwang, I.-G., Choi, A. J., Kwon, S., & Lee, S.-H. (2026). Simple presoaking modulates phytochemical recovery, antioxidant capacity, and macrophage responses of onion skin extracts. Food Chemistry: X, 39, Article 104402. https://doi.org/10.1016/j.fochx.2026.104402

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104402

Keywords: onion skin, phytochemicals, quercetin, antioxidant activity, food waste valorization, presoaking, hot water extraction, flavonoids, RAW 264.7 macrophages, circular economy, Food Chemistry: X, phenolic compounds

The team focused on three widely distributed brown seaweeds: Sargassum polycystum, Turbinaria ornata, and Rosenvingea intricata. These species are abundant along Indian shorelines, yet they remain largely underexploited as raw materials for nutraceuticals, functional foods, and pharmaceuticals. Brown seaweeds in general are known to be reservoirs of valuable molecules, including phenolic compounds, flavonoids, sulfated polysaccharides such as fucoidan, and the carotenoid fucoxanthin, all of which have attracted intense scientific interest for their antioxidant, anti-inflammatory, and potential anticancer properties. The challenge has always been extraction: how to pull these compounds out of a tough, gelatinous algal matrix efficiently, cheaply, and without resorting to toxic chemicals that compromise the safety and sustainability of the final product.

Hot water extraction offers an elegant answer. Water is non-toxic, inexpensive, non-flammable, and universally available, making it arguably the greenest solvent that exists. The technique sits conceptually close to subcritical water extraction, a method in which heated liquid water becomes an increasingly capable solvent as its dielectric constant drops with rising temperature, allowing it to dissolve compounds that would normally require organic solvents. In this study, the researchers compared extraction performance at three temperatures: 28.5 degrees Celsius, representing ambient conditions, 70 degrees Celsius, and 90 degrees Celsius. The differences they observed were striking and statistically significant, underscoring how profoundly temperature shapes both the quantity and the quality of what can be recovered from seaweed biomass.

The yield data tell the first part of the story. At 90 degrees Celsius, extraction yielded 9.18 percent for Sargassum polycystum, 10.14 percent for Turbinaria ornata, and 8.78 percent for Rosenvingea intricata, each figure significantly higher than what was obtained at the lower temperatures. In practical terms, roughly a tenth of the dried seaweed biomass was converted into soluble extract simply by treating it with hot water. For an industry built on margins, that recovery rate matters. It means that a coastal biorefinery could valorize seaweed harvests with minimal solvent costs, minimal hazardous waste streams, and straightforward equipment, since the process requires nothing more technically demanding than controlled heating and filtration.

Yield alone, however, says nothing about biological activity, and this is where the study delivers its most compelling evidence. Across all three species, extracts obtained at 90 degrees Celsius showed significantly higher antioxidant properties than extracts prepared at lower temperatures, as measured by total phenolic content, total flavonoid content, and ferric reducing antioxidant power assays. The researchers attribute this to the thermal disruption of the seaweed matrix at higher temperatures. Heat softens and breaks down cell walls and liberates bound phenolic compounds that remain locked inside the algal tissue under ambient conditions. In effect, boiling water does mechanically and chemically what harsher solvents do, but without the environmental baggage.

The antioxidant story was reinforced by two free radical assays. Extracts from the 90 degree Celsius treatments showed significantly greater scavenging activity against 2,2-diphenyl-1-picrylhydrazyl, the stable free radical commonly known as DPPH that serves as a standard benchmark for antioxidant capacity. They also performed significantly better in the deoxyribose assay, a test that evaluates both site-specific and non-site-specific radical scavenging by measuring the protection of deoxyribose sugar from degradation by hydroxyl radicals. Because hydroxyl radicals are among the most reactive and damaging species generated in living tissues, strong performance in this assay hints at genuine protective potential in biological systems rather than mere laboratory chemistry.

Perhaps the most visually dramatic evidence came from the DNA nicking assay. In this experiment, plasmid DNA is exposed to Fenton’s reagent, a mixture that generates hydroxyl radicals and converts the intact supercoiled DNA into fragmented, nicked forms that migrate differently on a gel. When the seaweed extracts were added, they shielded the DNA from this oxidative damage, preserving its structural integrity. The result demonstrates that the compounds recovered by hot water extraction are not just chemically active in solution but capable of protecting a biologically central molecule, DNA, from radical-induced breakage. That protective effect strengthens the case for these extracts as candidates for dietary supplements, nutraceuticals, and functional food ingredients aimed at reducing oxidative stress.

The implications extend well beyond human nutrition. The authors point to the potential of these seaweed extracts as immune boosters and growth enhancers for animals, fish, and shellfish, a direction that could matter enormously for India’s vast aquaculture sector. Natural antioxidant additives derived from marine algae could reduce reliance on synthetic antioxidants in feed, improve animal health, and add value to seaweed harvests that currently command low prices. Given that global interest in seaweed farming is rising rapidly, both as a food source and as a tool for climate mitigation, methods that convert raw biomass into high-value products are exactly the kind of technology that could accelerate the so-called blue economy.

There are, of course, caveats and next steps. The study reports crude extracts rather than purified single compounds, so further work will be needed to identify precisely which molecules drive the observed antioxidant and DNA-protective effects, and to confirm that the activity survives digestion, storage, and processing at industrial scale. Temperature optimization will also need balancing against the thermal sensitivity of individual compounds, since prolonged high heat can degrade some bioactives even as it releases others. Nevertheless, the central message stands: for three abundant Indian brown seaweeds, ordinary hot water outperformed milder conditions on every measure of yield and antioxidant potency tested, all with a solvent that costs essentially nothing and harms nothing. In a field often enamored with exotic solvents and high-tech equipment, the humble kettle may turn out to be the biorefinery’s most underrated tool.

Subject of Research: Hot water extraction of antioxidant bioactive compounds from Indian brown seaweeds

Article Title: Optimization of Hot Water Extraction as a Green Approach to Valorise Bioactive Compounds from Indian Brown Seaweeds: A Comparative Study of Sargassum polycystum, Turbinaria ornata, and Rosenvingea intricata

Article References: Optimization of Hot Water Extraction as a Green Approach to Valorise Bioactive Compounds from Indian Brown Seaweeds: A Comparative Study of Sargassum polycystum, Turbinaria ornata, and Rosenvingea intricata. (n.d.). https://doi.org/10.1007/s12649-026-03789-4

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03789-4

Keywords: brown seaweeds, hot water extraction, green extraction, antioxidant activity, bioactive compounds, Sargassum polycystum, Turbinaria ornata, Rosenvingea intricata, DNA protection, nutraceuticals, phenolic compounds, waste valorization