Plant Polyphenols and the Effect of Steam Explosion on Their Extraction and Antioxidant Activity
Plant polyphenols are a class of secondary metabolites containing multiple phenolic hydroxyl groups, often regarded as the "seventh major nutrient." They exhibit complex structures and diverse conjugation forms, mainly including phenolic acids, hydroxybenzoic and hydroxycinnamic acid derivatives, flavonoids, anthocyanins, coumarins, and tannins. These compounds possess a wide range of biological activities, such as antioxidant, free radical scavenging, anticancer, antibacterial, and antiviral effects, and have attracted considerable attention in the fields of food and health.
Steam explosion is an emerging pretreatment technology for agricultural raw materials. Its principle involves the brief application of high-temperature, high-pressure steam to the material, followed by an instantaneous pressure release, using the resulting high-pressure differential to disrupt the microstructure of the raw material. This technology integrates multiple effects, including acid-like hydrolysis, thermal degradation, mechanical-like fracture, hydrogen bond disruption, and structural rearrangement, enabling efficient separation of components from hard-textured materials. Due to its advantages of being pollution-free, requiring short pretreatment time, significantly enhancing the extraction yield of bioactive compounds, reducing energy consumption and processing costs, and being easy to industrialize, steam explosion has become an important approach to improve the extraction efficiency of bioactive compounds from plant materials and enhance product added value. This article systematically reviews the impact of steam explosion pretreatment on the polyphenol content and antioxidant activity of extracts from various raw materials, aiming to provide a theoretical basis for the application of this technology in the extraction of different bioactive compounds.
1. Major Physicochemical Changes during Steam Explosion
During steam explosion, the rapid vaporization of hot steam subjects the cells of the material to an instantaneous and substantial internal-external pressure differential, causing a sudden rise in internal pressure and volume expansion that exceeds the structural limits of the cells, leading to "mechanical-like rupture." This intense physical effect further promotes the breakage of hydrogen bonds, decomposition of amorphous and partial crystalline regions, and induces structural reorganization of components. Studies have shown that steam explosion can reduce the polymerization degree of starch molecules in sweet potato and potato, causing starch degradation, and can decrease both the quantity and molecular weight of pectin molecules. For fiber-rich materials such as coconut shells, steam explosion partially degrades cellulose and hemicellulose and hydrolyzes lignin to some extent, breaking molecular chains, reducing particle size, and decreasing long-chain components while increasing short-chain components in water-insoluble dietary fibers. Similar results have been observed for walnut shells, buckwheat bran, and Ephedra.
Mechanistic studies further indicate that steam explosion can partially degrade lignin and hemicellulose, generating low-molecular-weight phenolics and water-soluble sugars. However, when treatment pressure is too high or the duration too long, released phenolic compounds may undergo degradation or polymerization. Additionally, steam explosion may be accompanied by starch gelatinization and Maillard reactions, promoting covalent binding between proteins and sugars; the extent of Maillard reaction correlates with the intensity of the steam explosion, as supported by studies on Amaranthus caudatus seeds.
Microscopic observations show that steam explosion significantly alters the material structure, changing the surface from smooth and dense to wrinkled and porous. For example, the surface of konjac flour becomes wrinkled and develops numerous micropits after treatment, increasing the specific surface area. Germinated wheat exhibits fragmented polysaccharide lamellar structures, forming porous structures with minor fibrous branches. Yak femur bones become loose and porous with significant structural damage to the femoral head, while feathers develop a sponge-like structure with cracks and holes. These microstructural changes favor the penetration and contact of extraction solvents, facilitating the release of bioactive compounds.
2. Effects of Steam Explosion on Polyphenol Extraction
2.1 Effects on Cereal and Oilseed Raw Materials
Cereal and oilseed raw materials are rich in phenolic compounds, which can be classified structurally into phenolic acids, flavonoids, and proanthocyanidins, and further categorized by form into free phenolics and bound phenolics (e.g., protein-bound, glycosidically bound, ester-bound). In cereal bran, most phenolics are bound tightly to cell walls, resulting in low extraction efficiency by conventional solvents. Steam explosion effectively disrupts the cortex structure and hydrolyzes ether and ester bonds, promoting the release of bound polyphenols.
Studies have shown that steam explosion of wheat bran at 2.5 MPa for 30 s increases the vanillic acid content in free phenolics by approximately 50%, while p-coumaric acid and ferulic acid reach 36- and 11-fold higher than controls, respectively. Bound phenolics (e.g., glucuronic acid, p-coumaric acid, ferulic acid) also peak, with ferulic acid increasing nearly 25-fold, and total phenolic content reaching 28 mg/g-about nine times higher than the untreated group. Similar findings have been reported for other cereal brans. Applying steam explosion above 7.4 × 10⁵ Pa to chickpeas damages cell membranes and walls, facilitating the release of bound phenolics and compensating for potential thermal degradation of free phenolics. Steam explosion also enhances phenolic extraction from soybean seed coats and increases isoflavone content in okara with increasing explosion intensity, peaking at 2.0 MPa for 30 s. Due to differences in raw material physicochemical properties and research objectives, the optimal explosion conditions and effects on active components vary.
It is noteworthy that the intensity of steam explosion must be controlled within an appropriate range. Moderate intensity enhances polyphenol extraction, while excessive intensity can have the opposite effect. For example, wheat bran treated at excessively high intensity (e.g., 2.5 MPa for 90 s) produces organic acids such as formic and acetic acid, promoting phenolic degradation or polymerization and reducing content. Amaranthus seeds treated at 0.6 MPa for 60 s reach total phenolics of 7.798 mg/g-5.3 times higher than the control-but further increases in pressure or duration reduce phenolic content, with 120 s treatment even lower than the control. Similarly, treating red beans at 0.25–0.75 MPa for 30–90 s increases phenolic yield, but 1.0 MPa or longer duration causes partial degradation, polymerization, or carbonization. High-intensity steam explosion also decomposes purple sweet potato anthocyanins. Multiple studies support the general principle that steam explosion exhibits an optimal processing window for phenolic content in cereal and oilseed materials.
2.2 Effects on Fruit and Vegetable Raw Materials
Fruit and vegetable raw materials are likewise rich in phenolics, including phenolic acids, anthocyanins, and flavonoids. Steam explosion disrupts the structural integrity of fruits and vegetables, reducing mass transfer resistance and facilitating polyphenol release. Studies on citrus and its pomace, garlic and garlic skins, passion fruit residues, lacquer tree fruits, Tunisian red dates, sugarcane bagasse, pineapple peel residues, and Elaeagnus fruits all indicate that moderate steam explosion improves polyphenol extraction.
2.3 Effects on Other Raw Materials
Beyond cereals, oilseeds, fruits, and vegetables, steam explosion has been applied to other plant materials. For example, after 1.5 MPa steam explosion for 60 s, pine needles yielded 50.8 mg/g of flavonoids, 2.54 times higher than the control, maintaining good stability even at 1.5–2.0 MPa for 60 s. For tea residues and oak, phenolic content initially increases with intensity but declines at higher intensities. Total flavonoid yield in fig leaves, kudzu root, and Ginkgo leaves follows a similar trend. Declines in extraction may result from partial flavonoid degradation or re-polymerization into insoluble forms. Studies on olive branches and leaves, Eucommia leaves, Japanese knotweed, Sasa palmata leaves, and Populus leaves confirm that steam explosion effectively promotes polyphenol release.
The enhancement of polyphenol yield by steam explosion is mainly based on two mechanisms: (1) formation of new phenolics-high-temperature, high-pressure, acidic conditions cause thermal degradation of cellulose, hemicellulose, and lignin, generating phenolic acids and volatile phenols such as guaiacol, eugenol, vanillin, and syringaldehyde; (2) promotion of bound phenolic release-the explosion loosens the material structure, increasing contact surface area, while hydrolyzing ether and ester bonds between cell wall components (polyphenols, polysaccharides, lignin), converting bound phenolics to free phenolics and increasing solubility.
3. Effects of Steam Explosion on Antioxidant Activity of Extracts
3.1 Effects on In Vitro Antioxidant Activity
The in vitro antioxidant activity of plant polyphenols is commonly evaluated by DPPH· radical scavenging, ABTS⁺· radical scavenging, and FRAP (ferric reducing antioxidant power). Steam explosion significantly enhances the in vitro antioxidant capacity of polyphenol extracts. For instance, EC₅₀ values of DPPH· scavenging by garlic skin extracts decrease markedly with increasing pressure and time, stabilizing above 3.0 MPa, and show higher activity than black garlic. This is attributed to enhanced release of bound phenolics and degradation of lignin and hemicellulose, producing low-molecular-weight phenolics and water-soluble sugars (e.g., 5-hydroxymethylfurfural). Similar studies on cereal bran, tea residues, Amaranthus seeds, purple sweet potato, okra seeds, kudzu root, Sasa palmata leaves, and bamboo shoots consistently show that steam explosion not only increases phenolic content but also enhances antioxidant activity. For example, after steam explosion, quercetin-3-O-rhamnoside in lacquer tree fruit undergoes deglycosylation, converting to quercetin with stronger antioxidant activity.
However, the increase in antioxidant activity also depends on suitable explosion conditions. Excessive intensity may decrease activity. High pressure or prolonged treatment can cause ferulic and p-coumaric acids to undergo decarboxylation, polymerize into dimers via free radical intermediates, or degrade into small molecules such as methylguaiacol and vanillin, weakening antioxidant capacity. Studies show that DPPH scavenging and FRAP of wheat bran extracts peak at 1.5 MPa for 90 s, declining with further intensity increase. Similar findings are observed for Amaranthus seeds, purple sweet potato, and tea residues.
Furthermore, antioxidant activity is closely correlated with polyphenol content. Red bean extracts after steam explosion exhibit significant positive correlations between polyphenol concentration and DPPH·, ABTS⁺· scavenging, and FRAP values. Okra seed extracts show positive correlations between total polyphenol content and FRAP, DPPH·, and superoxide radical (O₂⁻·) scavenging, but negative correlations with total flavonoids, suggesting that some non-phenolic compounds (e.g., reducing sugars) generated during explosion may contribute to total antioxidant activity.
3.2 Effects on Intracellular Antioxidant Activity
Cell-level studies further confirm the beneficial effects of steam explosion. In human liver cancer (HepG2) cells, wheat bran extracts from steam explosion-treated material show significantly higher cellular antioxidant activity (lower EC₅₀) than untreated extracts, indicating stronger antioxidant effects. Another study demonstrates that, regardless of PBS washing, wheat bran extracts treated at 2.5 MPa for 30 s exhibit higher cellular antioxidant activity than untreated extracts, mainly due to increased soluble phenolics, particularly soluble ferulic acid.
For bitter buckwheat bran, steam explosion increases free phenolic extract antioxidant activity by 215% in HepG2 cells, with EC₅₀ roughly twice that of untreated samples, while also significantly enhancing inhibition of human colon cancer (Caco-2) cell proliferation. Simulated gastrointestinal digestion experiments indicate that untreated bran digestion products exhibit minimal cellular antioxidant activity, whereas treated bran products show significant activity. Similar findings are reported for Populus leaf extracts.
4. Prospects
As an efficient raw material pretreatment technology, steam explosion shows great potential for improving the extraction yield of polyphenols and other bioactive compounds, as well as enhancing their antioxidant activity. The effectiveness of this technology is influenced by intrinsic factors such as material type, composition, moisture content, pre-soaking, and particle size, and extrinsic factors such as steam pressure (temperature) and holding time. Future research could focus on:
Integrating in vivo and in vitro experiments to elucidate the mechanisms by which steam explosion affects polyphenol content and activity;
Investigating the structure-effect relationship between raw material matrix properties and steam explosion efficacy, and constructing models to optimize explosion parameters;
Clarifying the release mechanisms and transformation dynamics of bioactive compounds during steam explosion;
Systematically evaluating the impact of steam explosion on various nutrients and their interactions in raw materials.
With continued research and technological optimization, steam explosion is expected to play an increasingly important role in the efficient extraction of plant bioactive compounds, the development of functional foods, and green manufacturing in the food industry.
