Abstract: As the saying goes, “Good raw materials yield good medicinal products.” Drying is an indispensable and critical step in the post-harvest processing of Chinese medicinal materials at the place of origin; however, because drying is highly sensitive to temperature, it can readily induce changes in the material’s appearance, active constituents, and biological activity, thereby directly affecting its therapeutic and economic value. Consequently, gaining a thorough understanding of the mechanisms underlying quality changes during drying and comprehensively analyzing the dynamic patterns of variation in the physical properties and chemical composition of Chinese medicinal materials throughout the drying process are of great significance for guiding optimal drying practices. Taking the patterns of physicochemical property changes during drying as the analytical framework, this review synthesizes relevant literature to elucidate the causes and underlying mechanisms by which drying processes alter the physicochemical characteristics of Chinese medicinal materials. The aim is to provide a reference for selecting appropriate drying methods and thus ensure the quality of these materials, while also laying a foundation for further research, design, and innovation in drying technologies and equipment for Chinese medicinal materials.
Traditional Chinese medicinal materials, as a natural resource in which China enjoys original advantages, play an indispensable role in disease prevention and treatment, safeguarding public health, and ensuring the reproduction of the nation. Due to their high moisture content (60%–90%), fresh medicinal materials are prone to mechanical damage and microbial contamination during storage, leading to decay, spoilage, and loss of both medicinal and economic value. Drying removes substantial amounts of internal moisture, inhibits enzymatic and microbial activity, extends shelf life, and reduces quality losses during storage. However, various drying parameters—such as temperature, pressure, oxidizing agents, and drying media—can exert varying degrees of influence on the physicochemical properties and bioactivity of these materials. [1] such as morphological shrinkage, browning, and loss of active constituents, which lead to quality deterioration and consequently compromise therapeutic efficacy. Through extensive research and practical experience, it has been observed that the drying process exerts specific influences on the quality of crude medicinal materials, and the underlying mechanisms have been investigated and analyzed (Figure 1). Accordingly, this paper provides a comprehensive review and analysis of the patterns and mechanisms governing changes in both the physical properties (color, odor, morphology, and microstructure) and chemical properties (polysaccharides, flavonoids, polyphenols, volatile oils, and other constituents) of crude medicinal materials during drying, with the aim of offering reference guidelines for selecting appropriate drying processes and ensuring the quality of these materials.
1 Changes in physical properties
1.1 Color
The impact of drying on the color of Chinese medicinal materials primarily manifests in two ways: first, the drying process can induce the formation of brown polymeric compounds from certain constituents within the materials, resulting in darkening of their color—commonly referred to as browning; second, pigment components present in the materials may degrade during drying, leading to a loss of their inherent color. Therefore, changes in the color of Chinese medicinal materials are closely linked to the chemical transformations occurring within their intrinsic constituents. Our research indicates that the primary chemical changes induced by drying include enzymatic browning, non-enzymatic browning, and pigment degradation.
1.1.1 Enzymatic Browning Enzymatic browning in Chinese medicinal materials occurs when phenolic compounds, under the action of enzymes (polyphenol oxidase, peroxidase, and oxidase), are converted into quinones. These quinones then undergo self-polymerization to form melanoidins, resulting in browning of the dried products. Three conditions are required for enzymatic browning to occur: the presence of polyphenolic compounds, the presence of enzymes, and the availability of oxygen. [2] In most freshly harvested, normally growing Chinese medicinal materials, the enzymes and polyphenolic compounds are spatially segregated within a series of membrane systems, which effectively prevents non-enzymatic browning. Consequently, enzymatic browning primarily occurs during processing steps such as drying. The main factors influencing enzymatic browning during drying can be summarized as follows: (1) Degree of tissue and cell disruption: Most drying methods employ high temperatures or other physical treatments—such as ultrasonic vibration or cyclic pressure oscillation—to compromise the permeability of cell membranes, thereby accelerating water loss from the medicinal materials. This process gives rise to three key effects that lead to the activation of proenzymes and subsequent formation of brown pigments through a cascade of reactions: first, substantial oxygen influx into the tissue cells; second, the leakage of certain intracellular phenolic compounds into the external environment; and third, the disruption of the spatial segregation of enzymes and their substrates. (2) Temperature: Drying temperature exerts a significant influence on enzyme activity, resulting in varying degrees of browning in the medicinal materials. During the early stages of drying, when exposure to heat is brief and temperatures remain relatively low, enzyme activity is high, leading to vigorous browning. In contrast, during the later stages of drying, elevated temperatures combined with reduced water activity suppress enzyme activity, thereby mitigating the extent of browning. [3] , enzymatic browning reactions virtually do not occur. Studies have shown that when the drying temperature is around 55°C, polyphenol oxidase activity remains at a high level over the long term, whereas at temperatures above 75°C only moderate activity is observed. [4] (3) Oxygen factor: Oxygen is one of the indispensable conditions for enzymatic browning. Therefore, drying processes that use oxygen as the drying medium are more prone to enzymatic browning, resulting in darkening of the color of Chinese medicinal materials. Techniques such as vacuum drying and vacuum pulsating drying can significantly remove oxygen, thereby effectively controlling enzymatic browning during the drying process of Chinese medicinal materials.
1.1.2 Non-enzymatic browning Traditional Chinese medicinal materials are particularly rich in chemical constituents, such as ascorbic acid, carbohydrates, and amino acids. These components can undergo non-enzymatic browning reactions, leading to deterioration of the color of the medicinal materials. Non-enzymatic browning is a reaction that does not require biological enzymes as catalysts and mainly includes ascorbic acid browning, caramelization, and the Maillard reaction. Ascorbic acid browning primarily occurs during the processing of fruits and vegetables and fruit juices, whereas caramelization requires high temperatures (140–170°C) to heat carbohydrate substances above their melting points, resulting in charring and blackening of the medicinal materials. Therefore, the aforementioned non-enzymatic browning reactions rarely occur during conventional drying processes. The Maillard reaction involves the formation of sugar–amino compound interactions between carbonyl groups and amino groups, followed by a series of rearrangements, dehydration, condensation, and polymerization steps that produce dark brown to black-colored compounds. [5] is the primary reaction responsible for non-enzymatic browning of Chinese medicinal materials during drying. During the drying process, both temperature and the moisture content of the materials influence the occurrence of the Maillard reaction. Higher temperatures are more conducive to triggering the Maillard reaction. Relevant studies have shown that the Maillard reaction is most vigorous at a drying temperature of 80°C. [6] When the moisture content of Chinese medicinal materials drops to 10%–15%, the Maillard reaction is likely to occur; however, completely dried Chinese medicinal materials hardly undergo this reaction. In addition, both the pH value and the types of sugars present in the medicinal materials can influence the Maillard reaction. Specifically, the reaction is inconspicuous when the pH is below 7.0, but it accelerates markedly as the pH rises above 7.0; yet, as the pH continues to increase to 11.0, the Maillard reaction begins to weaken again. Wu Huiling et al. [7] It was found that the reactivity of sucrose, xylose, galactose, glucose, and fructose in the Maillard reaction follows the order: xylose > galactose > glucose > fructose, with sucrose exhibiting no detectable reactivity. Although these two influencing parameters are not closely related to the drying process, they can nonetheless provide useful guidance for the development of novel drying technologies.
1.1.3 Pigment Degradation Most traditional Chinese medicinal materials contain pigment components, such as anthocyanins, chlorophyll, carotenoids, and curcuminoids. The degradation of these pigments leads to the fading of the original coloration of the medicinal materials. Drying affects pigment degradation primarily through the following mechanisms: (1) Thermal degradation: β-carotene undergoes an oxidative–degradative reaction at high temperatures, resulting in a decrease in its content. High temperatures also readily induce deglycosylation or ring-opening of anthocyanins, ultimately leading to their degradation into phenolic acids and aldehydes. (2) Photodegradation: During natural sun-drying, exposure to light easily causes the degradation of certain pigment components in medicinal materials, including β-carotene, anthocyanins, and cyanidins. Among these, anthocyanins can be photodegraded to form C 4 An intermediate product of the hydroxyl group; this substance is in C 2 At the position of water, ring opening occurs to form chalcone, which then further degrades into benzoic acid and other products. [8] (3) Sugar degradation, which is primarily associated with furfural compounds or their derivatives formed via the Maillard reaction. These compounds can rapidly bind to anthocyanins or anthocyanidins through electrophilic interactions, ultimately leading to pigment degradation. [9] (4) Enzymatic degradation: During drying, an appropriate temperature can activate the activity of relevant enzymes, leading to enzymatic breakdown of pigments through various pathways. Chlorophyll, under the action of chlorophyllase and pheophytinase, undergoes a series of reactions to produce non-fluorescent compounds. Phenolic compounds, catalyzed by peroxidase, are oxidized to form free radicals, which promote lipid peroxidation in the membrane, destabilize chloroplasts, and thereby induce chlorophyll degradation. [10] Polyphenol oxidase can oxidize anthocyanins directly or indirectly, converting their catechol structure into a quinone structure; the resulting quinone then further oxidizes the anthocyanins, leading to the formation of colorless compounds. [11] In addition, polyphenol oxidase can promote the formation of polyphenols. O - Quinone compounds; this product then rapidly reacts with anthocyanins to form anthocyanin- O - Quinone condensation products [12] In summary, pigment components in traditional Chinese medicinal materials undergo numerous degradation pathways during the drying process, making their color stability highly susceptible to deterioration due to factors such as high temperature, light exposure, enzymatic activity, oxygen, and tissue cell damage. Therefore, non-thermal drying technologies—such as freeze-drying, radio-frequency drying, and high-voltage pulsed electric field drying—can mitigate the adverse effects associated with thermal heating. By developing novel, safe non-thermal drying methods, it is possible to effectively control and preserve the color of these materials; further investigation into the underlying mechanisms of color preservation will help maintain their original coloration.
1.2 Smell
Odor plays a crucial role in determining the quality of dried Chinese medicinal materials as well as consumer preference and acceptance. The key odor-contributing compounds in Chinese medicinal materials include esters, aldehydes, lactones, terpenes, alcohols, carbonyl compounds, and sulfur- and nitrogen-containing compounds. [13] Under different drying processes and conditions, the aforementioned constituents—or other constituents in the crude herbal materials—undergo varying qualitative and quantitative changes, thereby exerting differing degrees of influence on the aroma of the herbs. Specifically, these effects manifest in three ways: first, the aromatic fragrance becomes more intense; second, the aroma weakens or even dissipates entirely; and third, an unpleasant odor develops.
During the drying process, components such as proteins, lipids, and carbohydrates in traditional Chinese medicinal materials undergo the following chemical reactions, yielding compounds like lactones, saturated and unsaturated aldehydes, and fatty acids, thereby forming a complex aroma profile. (1) Thermal peroxidation: Fatty acids and lipids are prone to thermal peroxidation during drying, leading to the formation of a series of characteristic aroma compounds. For example, long-chain fatty acids undergo a series of thermal oxidation reactions to produce γ-lactones. [14] Under dry conditions at 70°C, lipids are prone to thermal oxidation, yielding γ-octalactone and γ-heptalactone. (2) Thermal decomposition or thermal degradation: Unsaturated fatty acid compounds and fats undergo thermal degradation or thermal decomposition upon heating, respectively producing aromatic small-molecule carboxylic acids (such as acetic acid) and alcohols. Studies have shown that as the drying temperature increases, the relative mass fraction of acidic substances in red dates gradually rises. [14] (3) Enzyme-mediated synthesis: Certain amino acids and unsaturated fatty acids, under the action of bioenzymes such as lipoxygenase and aldehyde reductase, are directly converted into aromatic alcohols. Non-volatile straight-chain esters, valine, and isoleucine can, upon exposure to high temperatures and enzymatic catalysis, give rise to volatile branched-chain esters, thereby enhancing the aroma of traditional Chinese medicinal materials. (4) Maillard reaction: The products of the Maillard reaction typically exhibit distinct aromas. For example, nitrogen-containing heterocyclic compounds (such as pyrazines and pyrroles) impart a nutty aroma; cyclic enol-ketone structures (such as maltol) confer a caramel-like fragrance; monocarbonyl compounds produce ketonic-alcoholic notes; and carbonyl compounds yield a roasted aroma.
During the drying process, the reduction in the concentration of highly volatile aroma compounds is the primary reason for the fading or loss of fragrance in traditional Chinese medicinal materials. The main influencing factors include: (1) high temperature; excessively high temperatures can induce thermal degradation of monoterpenes and oxygenated terpenoids, such as d - Limonene, β-caryophyllene, borneol, linalool, bornyl acetate, and others [15] (2) Light exposure: Under natural sun-drying conditions, polyene compounds are prone to photodegradation. (3) Oxygen and structural damage: Drying can disrupt the tissue structure of Chinese medicinal materials, leading to the volatilization of certain volatile constituents or to oxidative reactions; such reactions are particularly pronounced in drying processes that use an oxygen-containing drying medium.
When appropriate drying conditions are carefully controlled, the deterioration of aroma in Chinese medicinal materials is relatively rare. However, when such deterioration does occur, it is primarily attributable to improperly set drying temperatures. This issue manifests in three specific ways: (1) excessively high drying temperatures intensify the Maillard reaction, leading to the formation of compounds that impart a burnt or scorched odor. Xu Wanxiu et al. [16] Studies have shown that when the drying temperature is 70 or 80°C, the intensity of the ginger’s original flavor and aroma is at its highest, while the retention of the original aroma is minimal; at the same time, the burnt odor is most pronounced. (2) For oil-rich Chinese medicinal materials, such as peach kernels and hemp seeds, heating during the drying process readily leads to oil migration, accompanied by a rancid odor. (3) Lipids and fatty acids are prone to oxidative degradation upon heating, resulting in an increase in acid value, the onset of rancidity, and the development of off-odors. In addition, various chemical reactions occurring during drying can generate compounds with unpleasant odors, such as α-terpineol, thereby causing odor deterioration and ultimately compromising the quality of the medicinal materials.
1.3 Volume shrinkage
The structure of traditional Chinese medicinal materials can be regarded as a three-dimensional solid-network or matrix composed of aqueous solutions and biopolymer components. The specific structural features and the mechanical properties of these constituents in equilibrium determine the sample’s volume and define its size and shape. During drying, the molecular structure of the medicinal materials and the dynamic equilibrium among their various constituents are disrupted, leading to deformation—namely, volumetric shrinkage. This manifests in several ways: as drying proceeds, the spaces previously occupied by internal moisture are progressively evacuated and filled with air, causing the tissue of the medicinal material to lose its structural integrity. This results in an imbalance between internal and external pressures, establishing a moisture gradient that generates shrinkage stresses and ultimately induces deformation. [17] ; Drying can cause the epidermal structure of Chinese medicinal materials to collapse; it reduces or even eliminates turgor pressure within cells, leading to the collapse of cellular structures due to internal thermal stresses. Therefore, thermal stress and shrinkage stress are the primary causes of collapse and wrinkling in Chinese medicinal materials during drying. Among these, stresses arising from moisture gradients persist throughout almost the entire drying process and are the main driver of wrinkling. During drying, the key parameters influencing the degree of wrinkling in Chinese medicinal materials include drying temperature, air velocity, and relative humidity of the drying air. Drying temperature plays a crucial role in accelerating the drying rate and ultimately affects the extent of wrinkling in these materials. [18] Under low-temperature conditions, the diffusion rate of moisture from the interior to the exterior of Chinese medicinal materials is equal to the surface evaporation rate, preventing the formation of a sharp moisture gradient; as a result, uniform shrinkage occurs only in the final stage of drying, with a relatively small volumetric shrinkage ratio. In contrast, at higher drying temperatures, the moisture gradient throughout the material is more pronounced, leading to greater thermal and shrinkage stresses and consequently more severe volumetric shrinkage. However, elevated temperatures can also promote the formation of a hard crust on the surface of the medicinal materials, which provides structural support to the material’s internal framework and can delay or inhibit further shrinkage. Wind speed is another critical factor influencing the shrinkage of Chinese medicinal materials during drying; generally, the shrinkage rate decreases as wind speed increases. [19-20] This may be attributed to the changes in mass transfer from the interior to the exterior at different stages of drying. In drying processes where energy transfer is externally controlled, mass transfer involves both internal diffusion and external convection. At low air velocities, surface resistance predominates, resulting in a relatively flat moisture content profile within the sample and consequently lower internal stresses. Consequently, traditional Chinese medicinal materials exhibit uniform shrinkage under low air velocity conditions. In some cases, the relative humidity of the air can also regulate the degree of wrinkling in these materials. When the relative humidity is low, the Biot number—a dimensionless parameter used in heat-transfer calculations—increases, ultimately limiting the extent of wrinkling in the medicinal materials. [20] At lower relative air humidity, surface hardening significantly affects the shrinkage rate of Chinese medicinal materials.
1.4 Microstructure
The drying process involves mass transfer and heat transfer, which can induce deformation and increase internal stresses in Chinese medicinal materials, thereby disrupting and damaging their internal tissue structure and leading to changes in their microstructure. These microstructural changes are primarily manifested in the size and uniformity of porosity, and they directly affect the rehydration properties, hardness, crispness, and other functional characteristics of dried Chinese medicinal products. The variation in porosity during drying is mainly determined by two factors: (1) the direction and rate of moisture migration. Generally speaking, if the drying process causes moisture to evaporate from the exterior toward the interior, the temperature gradient and the humidity gradient will be oriented in opposite directions during evaporation, resulting in a slower drying rate, longer drying time, and greater damage to the tissue structure, which in turn leads to non-uniform pore sizes. For example, in hot-air drying, the surface temperature of the medicinal material is higher than the internal temperature, causing moisture to evaporate more rapidly from the surface than from the interior. Consequently, after drying, the surface of the product exhibits severe hardening, the internal tissue structure becomes tightly compacted, the pore distribution is uneven, and there is significant interparticle bonding, leading to a large number of dead-end pores that impede fluid flow. [21] . During microwave or infrared drying, the temperature gradient and the moisture gradient in traditional Chinese medicinal materials migrate in the same direction, and the rapid migration of moisture readily leads to the formation of a porous and loose structure. Koç et al. [22] It is evident that freeze-dried materials exhibit the highest porosity. This is because the sublimation of ice during freeze-drying generates a large number of uniform pores, which can largely preserve the original microstructure of the material’s tissue and cells. (2) The degree of structural disruption, as well as the content and type of intrinsic chemical constituents, respectively influence the mechanical strength of the solid matrix, capillary pressure, and glass transition temperature, thereby exerting a certain impact on porosity. During drying, structural damage and exposure to high-temperature conditions may induce the conversion of insoluble pectin in traditional Chinese medicinal materials into soluble pectin, which reduces the mechanical strength of the solid matrix, leading to decreased hardness and collapse of cell folds. He Yuqian et al. [23] According to the report, cellulose undergoes deformation and even fracture as moisture evaporates, with significant tissue shrinkage that leads to a reduction or complete disappearance of pore size. Lewicki et al. [24] According to the report, the amorphous fractions of polysaccharides and proteins, along with other components of the cell sap, may form amorphous structures. The glass transition temperature of these structures increases sharply as the water content in traditional Chinese medicinal materials decreases, eventually transitioning into a glassy state during the final stage of drying and thereby significantly enhancing the mechanical strength of the materials. Under these conditions, shrinkage of the medicinal materials is impeded, leading to the formation of pores.
The effects of drying on the physical properties of Chinese medicinal materials are shown in Table 1.
2 Changes in chemical properties
2.1 Polysaccharide
Polysaccharide compounds exhibit antitumor, immunomodulatory, gut microbiota–regulating, and antioxidant activities, making them among the important bioactive constituents of traditional Chinese medicinal materials. [25] During the drying process, various saturated and unsaturated glycosidic bonds, monosaccharide residues, and spatial conformations present in polysaccharides may be disrupted, thereby compromising their bioactivity and leading to a reduction in polysaccharide content. The primary reasons for the decline in polysaccharide content during drying include the following: (1) respiration; the intensity of respiration directly influences the extent of polysaccharide consumption. In the vacuum and low-temperature conditions of the drying process, enzyme activity is diminished, resulting in reduced respiratory activity in the medicinal materials and thus minimizing polysaccharide loss. Conversely, when appropriate processing conditions—such as temperature and oxygen levels—are employed, respiratory activity may be enhanced, increasing polysaccharide consumption. For example, in conventional hot-air drying, the prolonged drying time, elevated temperature, and lack of air isolation maintain relatively vigorous respiration in Dendrobium nobile leaves, thereby accelerating polysaccharide degradation. In contrast, freeze-drying conducted at −60°C suppresses enzyme activity and isolates the material from oxygen, weakening respiratory activity and consequently reducing the degradation of polysaccharide components. [26] (2) Enzymatic degradation: At a certain drying temperature, the activities of polysaccharide-lytic enzymes and polysaccharide-hydrolyzing enzymes are relatively high, leading to the breakdown of polysaccharides into smaller sugar molecules or other substances and consequently reducing the polysaccharide content. (3) High-temperature degradation: High temperatures reduce polysaccharide content primarily in the following three ways: ① High temperatures readily disrupt the aggregation and shear forces of polysaccharides, causing them to degrade into smaller molecular substances. Yin Shaowen et al. [27] The report indicates that drying at temperatures exceeding 80°C can induce thermal degradation of Cyclocarya paliurus polysaccharides. Meng Yingxia et al. [28] The study found that microwave drying resulted in the lowest total polysaccharide content, presumably because localized overheating during the drying process led to Maillard or caramelization reactions, converting some polysaccharides into oligosaccharides or caramel. ② Certain polysaccharides are highly susceptible to oxidative degradation under high-temperature conditions. [29] For example, uronic acids are highly susceptible to oxidative degradation at elevated temperatures, which reduces the antioxidant capacity of polysaccharides. ③ Under high-temperature conditions, proteins readily undergo Maillard reactions with polysaccharides, yielding products with both high and moderate molecular weights. [30] (4) Other factors include the oxidative degradation of polysaccharides caused by drying processes that use air as the drying medium. Spray drying and ultrasonic drying generate shear forces that can break the side chains and other structural elements of polysaccharides (such as chitosan), leading to their degradation into other substances. In addition, drying may also result in an increase in polysaccharide content. Xin Ming et al. [31] Studies have shown that under hot-air drying conditions (60°C), the metabolic balance of carbohydrates in Dendrobium officinale is disrupted, leading to the extensive formation of high-molecular-weight compounds such as cellulose and, consequently, an increase in polysaccharide content; however, the underlying mechanisms remain to be further investigated.
2.2 Flavonoids
Flavonoids constitute the largest class of plant phenolics, primarily including flavanols, flavones, flavanones, flavanols, and anthocyanins. Due to their intrinsic properties and varying drying process parameters, these compounds may undergo both qualitative and quantitative changes. Notably, the drying process often leads to a reduction in flavonoid content—for instance, alterations in anthocyanin levels—ultimately resulting in a decline in the quality of traditional Chinese medicinal materials. The primary reasons for the reduction of other constituents during drying are as follows: (1) high-temperature degradation; certain flavonoids, such as genistein, puerarin, genistin, and daidzein, are highly temperature-sensitive and suffer substantial losses during drying. (2) oxidation; the disruption of tissue structure during drying increases the exposure of flavonoids to oxygen, so drying methods that use air as the drying medium can lead to the oxidative degradation of flavonoids, such as daidzin. Liu Yaoru et al. [32] The study found that microwave vacuum drying and vacuum drying can significantly remove oxygen with relatively low losses of flavonoids, whereas forced-air drying leads to a substantial reduction in flavonoid content. Jiang Gan et al. [33] Low-temperature adsorption drying is employed for the drying of Smilax glabra; due to the minimal damage inflicted on the tissue structure of Smilax glabra under low-temperature conditions, the loss rate of flavonoids is significantly lower than that observed in hot-air drying. (3) Destruction of flavonoid-synthesizing enzymes: High temperatures can disrupt the structure and activity of enzymes involved in the biosynthesis of flavonoids, thereby preventing the synthesis of certain flavonoid compounds. (4) Photodegradation: Light-sensitive flavonoid compounds, such as rutin and luteolin glycosides, suffer substantial losses during natural sun-drying and shade-drying.
The drying process may also induce an increase in the content of flavonoid compounds, for the following reasons: (1) Flavonoids can interconvert; under certain drying conditions, naringin chalcone can be converted into naringin. Kamiloglu et al. [34] It was also found that oven drying and sun drying can lead to the formation of luteolin-7- in dried figs. O - Glucosides. (2) Enhancing the extraction yield of flavonoids: infrared and microwave radiation possess penetrating power that can penetrate cell interiors, breaking covalent bonds between macromolecules, thereby facilitating the release and extraction of flavonoids and other bioactive compounds; consequently, the total flavonoid content in dried Chinese medicinal materials is found to be higher. (3) In other respects, certain constituents—such as proteins, carbohydrates, and cellulose—may influence the formation and degradation of flavonoids during the drying process; however, research on this aspect remains limited. Therefore, a comprehensive understanding of the dynamic changes in flavonoid composition in Chinese medicinal materials under different drying conditions and processes still requires further exploration and investigation.
2.3 Phenolic acid
Among phenolic compounds, phenolic acids have garnered considerable attention in recent years due to their potential health benefits, including antioxidant, antibacterial, antiviral, anticancer, anti-inflammatory, and vasodilatory activities. However, the presence of numerous phenolic hydroxyl substituents in their structures renders them chemically unstable; during drying, they are readily susceptible to environmental factors such as moisture, temperature, light, and enzymatic activity, leading to structural alterations. This instability represents a major challenge that limits the extraction and application of phenolic acids. In this review, we summarize the effects of various drying techniques on the content of phenolic acids in different traditional Chinese medicinal materials, demonstrating that their levels may either decrease or increase. Specifically, drying typically reduces the content of phenolic acids for the following reasons: (1) enzymatic degradation—during drying, the activation of polyphenol oxidase and peroxidase can catalyze the oxidation of phenolic acids into other compounds, with chemical constituents featuring ortho-diphenolic hydroxyl groups, such as chlorogenic acid, caffeic acid, and rosmarinic acid, being particularly vulnerable to these enzymes. [35] . Activation of other enzymes, such as phenylalanine ammonia-lyase, also leads to the degradation of relevant phenolic acid compounds. (2) Thermal degradation: water-soluble phenolic acid compounds are inherently unstable and readily undergo thermal decomposition, oxidation, and dehydroxylation upon heating. [36] The representative constituents are danshensu B and chlorogenic acid, whose content decreases sharply under drying conditions above 60°C. Flavanol-type polyphenols undergo isomerization and autoxidation upon exposure to heat. [37] , thereby transforming into other substances. In addition, phenolic compounds may undergo chemical structural changes or bind to proteins during the drying process, leading to a reduction in their content.
The increase in the content of phenolic acids during the drying process can be attributed to the following factors: (1) enzyme induction—under the action of key enzymes such as phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate-CoA ligase, chlorogenic acid compounds are synthesized. A high-voltage electrostatic field may induce enhanced activity of leucoanthocyanidin reductase, thereby increasing the total content of flavan-3-ols. Ossipov et al. [38] Studies have also shown that the activation of shikimate dehydrogenase during drying can oxidize shikimate to dehydroshikimate, which is then further converted into gallic acid. (2) Drying-induced changes and mutual conversions among constituents: salvianolic acid B is a product of the drying process rather than an originally accumulated constituent in the plant during the cultivation period, and other salvianolic acid derivatives are likewise formed during drying concomitantly with the substantial production of salvianolic acid B. Peng Jiuman et al. [39] Studies have shown that shikonin and salvianolic acid B undergo co-degradation to yield salvianolic acid A, which can further undergo cyclization to form isosalvianolic acid C and salvianolic acid C. When the drying temperature exceeds 60°C, the content of salvianolic acid B decreases substantially, while the levels of danshensu, protocatechuic aldehyde, and salvianolic acid A increase accordingly. [40] , ultimately resulting in no significant change—or even an increase—in the content of phenolic acids during the drying process. (3) The extraction yield of phenolic acids is enhanced, a phenomenon similar to that observed for flavonoids. During drying, covalent bonds among macromolecules within cells may break, facilitating the release and extraction of phenolic acids and other bioactive compounds; consequently, the total phenolic acid content in the dried medicinal materials increases. (4) In other respects, studies have shown that the content of gallic acid increases with rising temperature, as the presence of oxygen promotes the biosynthesis of gallic acid. [41-42] Moreover, high temperatures may promote the release of bound phenolic acids, thereby indirectly increasing the levels of gallic acid, ferulic acid, and other constituents. Ding Shenghua et al. [43] A similar viewpoint was also expressed. In addition, a high-voltage electrostatic field can effectively maintain a high level of soluble sugar content, which in turn induces the accumulation of polyphenolic compounds and ultimately helps to balance the levels of flavanol-type polyphenolic acids. [44] 。
2.4 Essential oil
The impact of drying on the volatile oil constituents of Chinese medicinal materials has been extensively studied. During the drying process, volatile oils may undergo various chemical transformations, resulting in increases or decreases in their content. Szumny et al. [45] Microwave vacuum drying was employed to dry rosemary, revealing that the levels of monoterpenes in the essential oil increased, whereas those of sesquiterpenes decreased. Loss of essential oil constituents during drying is a widespread and unavoidable phenomenon. This is primarily attributable to: (1) disruption of tissue structure and oxidation; in fresh medicinal herbs, a portion of the essential oil is stored within glandular trichomes. Drying causes rupture of the cuticle of these trichomes, allowing external air to penetrate into the tissue cells or volatile components to volatilize into the surrounding environment, thereby leading to oxidation and evaporation of the essential oil. (2) Thermal degradation; certain volatile compounds, such as monoterpenes and caryophyllene, are thermally sensitive and readily undergo thermal degradation during the drying process. Xue Min et al. [46] Studies have shown that after oven-drying and sun-drying, the monoterpenoid constituents of Pogostemon cablin decrease by 20% and 26%, respectively. (3) Photodegradation: polyene compounds such as elemene are photosensitive, and sun-drying can readily lead to the degradation of these constituents. Calín-Sánchez et al. [47] The study found that the total volatile compound concentration in marjoram obtained via microwave-assisted drying was not significantly different from that of the fresh material.
The phenomenon of increased volatile oil content in Chinese medicinal materials due to drying is also frequently reported. This may be attributed to: (1) the formation of new volatile compounds through chemical reactions such as high-temperature induction, oxidation, and glycosylation-induced hydrolysis. [48] . High-temperature treatment may cause the decomposition of sesquiterpenoids, leading to their conversion into monoterpenoids. [49] . Tang Wenwen et al. [50] Studies have also shown that a moderate increase in temperature can significantly enhance the relative contents of terpenoid compounds (β-pinene, myrcene, and ocimene). Schuh et al. [51] It is pointed out that drying processes involving heat treatment may lead to the hydrolysis of glycosylated compounds, resulting in the formation of new volatile constituents. (2) Different drying conditions can disrupt tissue structure and alter the morphology and density of oil cells, which may facilitate the extraction of essential oils and thereby increase their content. (3) Higher temperatures may induce the formation of a hard cuticular layer on the surface of crude medicinal materials, thereby impeding the diffusion of high-molecular-weight volatile compounds and causing the loss of other volatile constituents. This phenomenon is particularly common in leaf-type herbs. According to reports, lemon-scented peach leaves [52] and betel leaves [53] This phenomenon is observed throughout the drying process. Furthermore, the hydrophobic nature of certain volatile compounds may inhibit their degradation during drying, whereas hydrophilic compounds are more prone to loss; further research is needed to elucidate this relationship. Therefore, for medicinal materials containing volatile oils, low-temperature, anaerobic, or non-thermal drying techniques should be prioritized to preserve their pharmacological properties, such as anti-inflammatory, antibacterial, and antiviral activities.
2.5 Other ingredients
Polysaccharides, flavonoids, phenolic acids, and volatile oils are prone to degradation and transformation during drying due to their unstable structures and reactive functional groups, thereby affecting the chemical characteristics of crude medicinal materials. In addition, alkaloids, coumarins, and steroidal saponins are also major bioactive constituents in certain crude medicinal materials; although these compounds are relatively stable, they can still undergo varying degrees of degradation and transformation during the drying process. Therefore, understanding and elucidating the patterns and mechanisms of these changes is of great significance for further ensuring the quality of crude medicinal materials.
2.5.1 Alkaloids Alkaloids are a class of naturally occurring nitrogen-containing basic organic compounds with complex cyclic structures. Certain alkaloids exhibit high structural stability, and their content remains essentially unchanged during drying; for example, leonurine hydrochloride. [54] , Dendrobine [38] Meanwhile, other alkaloid constituents undergo oxidation, degradation, and transformation under the high temperature and oxidative conditions during drying. The primary reason for the decline in alkaloid content during drying is thermal degradation. A typical example is arecoline, which is highly volatile upon heat treatment, thereby adversely affecting the quality and flavor of betel nuts. Consequently, low-temperature drying is generally recommended for betel nuts. When the drying temperature exceeds 150°C, the cyclic structures of berberine, coptisine, and palmatine are prone to cleavage, leading to a significant reduction in their contents. Bai Yongliang et al. [55] Studies have shown that drying mulberry leaves at 105°C leads to a significant reduction in their jujubine content. The primary reason for the observed increase in alkaloid content during drying is the transformation of alkaloid constituents, specifically through reactions such as hydrolysis and oxidation. [54] For example, matrine is prone to oxidation into oxymatrine at higher drying temperatures, leading to a decrease in the matrine content and a corresponding increase in the oxymatrine content. In contrast, certain alkaloid components exist in free or galactoside forms. [56] Under appropriate drying conditions, hydrolysis may occur, converting the compound back to its parent alkaloid and thereby increasing the content of the parent alkaloid; however, the mechanisms and extent of this transformation remain to be investigated in greater detail.
2.5.2 Coumarins: Coumarins are a class of organic heterocyclic compounds containing a benzopyranone structure and exhibit excellent thermodynamic and photochemical stability. Consequently, there are relatively few reports on the effects of drying on these chemical constituents. According to existing studies, drying temperature is the primary factor influencing the reduction in the content of certain coumarins. When the temperature exceeds 50°C, the contents of imperatorin and isoimperatorin decrease due to degradation and transformation, thereby failing to meet pharmacopoeial requirements. [57] Therefore, Ningqianhu should be dried at temperatures no higher than 50°C whenever possible. For simple phenylpropanoid compounds with low relative molecular weights, their volatile nature makes them particularly prone to loss during the drying process. Coumarin compounds also undergo interconversion during drying, leading to an increase in coumarin content. Zhang Zhimei et al. [58] According to the report, drying at 35°C increases the contents of imperatorin and isoimperatorin in Angelica dahurica roots, whereas drying at 70°C leads to an increase in imperatorin content but a significant decrease in isoimperatorin content; the underlying mechanisms behind these changes require further investigation.
2.5.3 Steroidal saponins are a class of steroidal glycosides formed by the conjugation of spirostanoid compounds with sugars; consequently, the drying process is likely to induce cleavage of the glycosidic bonds, thereby altering their content. According to existing research, the main reasons for the decline in steroidal saponin content during drying include: (1) high-temperature degradation—certain saponins, such as ginsenosides, neomangiferin, diosgenin, stigmasterol saponins, and monk fruit saponins, are prone to degradation at elevated drying temperatures. Therefore, Chinese medicinal materials containing these constituents should preferably be dried using low-temperature or non-thermal drying techniques. Xie Qiliang et al. [59] Studies have also shown that low-temperature drying is preferable for Phytolacca to ensure the retention of total saponin content. (2) Enzymatic degradation: at appropriate drying temperatures, enzyme activity remains high, leading to the cleavage of glycosidic bonds in certain steroidal saponins and consequently reducing their content. Li Xian et al. [60] Similar reports have also been published. Therefore, drying techniques such as high-temperature drying (80°C) and microwave drying, by disrupting or inhibiting enzyme activity, may effectively prevent the loss of total saponin content in certain traditional Chinese medicinal materials. However, the cleavage of certain glycosidic bonds can generate new secondary glycosides, leading to an increase in steroidal saponin content. For example, ginsenoside Re undergoes glycosidic bond cleavage under high temperatures, thereby yielding ginsenoside Rh. 1 . Guo Xiaoye et al. [61] This indicates that the glycosidic bond in neo-mangiferin is thermally unstable and tends to cleave during heating and drying, converting into mangiferin and thereby increasing the mangiferin content. In addition, cleavage of the acyl bond can also yield new steroidal saponins. Studies have shown that the acyl bond in malonyl ginsenosides is unstable; under high-temperature drying conditions, hydrolysis removes the malonic acid group, yielding the corresponding ginsenosides and leading to an increase in ginsenoside content. [62] Therefore, both high-temperature and low-temperature drying conditions are suitable for the drying of Chinese medicinal materials containing steroidal saponins; the specific drying process and conditions should be selected based on their relevant constituents and properties.
The effects of the drying process on the chemical properties of Chinese medicinal materials are shown in Table 2.
3 Conclusion and Outlook
Drying has a significant impact on the physicochemical properties of traditional Chinese medicinal materials, inducing changes from external attributes such as color, odor, and morphology to internal characteristics like microstructure and active constituents, ultimately leading to a decline in product quality. In summary, drying primarily affects the physicochemical properties of these materials by altering their temperature and moisture content. During the drying process, although an increase in temperature can accelerate the drying rate, it also intensifies enzymatic, oxidative, and hydrolytic reactions, resulting in rapid degradation or transformation of active ingredients and consequent alterations in the material’s physical properties. The reduction in moisture content brought about by drying disrupts the dynamic equilibrium among chemical components and molecular structures within the material, thereby triggering changes in its form, microstructure, and active constituents. Given that most traditional Chinese medicinal materials are plant-based and their active ingredients generally exhibit thermosensitivity and susceptibility to oxidation, ensuring high-quality drying requires adherence, in the selection and development of drying technologies and equipment, to the principles of low temperature, rapid processing, and anaerobic conditions. This entails the judicious application of vacuum-assisted drying and enhanced drying techniques—such as forced convective heating, electric-field enhancement, ultrasonic assistance, and temperature/pressure pulsing—and the integration of conventional thermal drying with non-thermal methods to leverage the strengths of each approach. Such an integrated strategy not only safeguards the quality of dried materials but also minimizes drying time, conserves energy, and reduces labor requirements—key objectives for sustainable development, energy conservation, emission reduction, and green, pollution-free production, all of which warrant vigorous promotion. Moreover, due to the vast diversity in the properties and structures of medicinal materials, the varying characteristics of drying technologies, and the current limitations in analytical and testing infrastructure, establishing a comprehensive systematic framework to describe the patterns of physicochemical changes remains challenging. Therefore, future research should increasingly focus on modeling and design to identify critical thresholds for changes in various physicochemical properties—for example, models for color change, volume shrinkage, and mass degradation—so as to gain a holistic understanding of the underlying mechanisms governing these transformations during drying. Such models will provide a solid basis for optimizing process parameters at different drying stages, gradually advancing the standardization and regularization of traditional Chinese medicinal material drying in China. Furthermore, full utilization of cutting-edge domestic and international testing and drying equipment is essential to achieve precise data collection throughout the production process, enhance automation and intelligence, and ultimately realize green, efficient, and high-quality drying in medicinal material processing.
