Lipidomic Analysis and Assessment of Quality Changes of Phallus impudicus During Hot Air Drying

TL;DR

This study explores how hot air drying affects the quality and flavor of Phallus impudicus mushrooms by analyzing changes in their lipid composition.

Contribution

The study provides new insights into the lipidomic changes during hot air drying and their impact on mushroom quality and flavor.

Findings

Drying significantly altered lipid metabolism, with glycerophospholipids increasing and glycolipids and fatty acids decreasing.

Hydrolysis of structural lipids led to cellular structure collapse and increased levels of specific hydrolyzed lipids.

Lipid unsaturation and oxidation intensified browning and flavor compound formation during drying.

Abstract

Hot air drying is widely used in edible mushroom processing, but often leads to quality changes, including browning and flavor changes. This study focused on Phallus impudicus (P. impudicus), combining dynamic monitoring of browning-related indicators with lipidomics technology to systematically investigate the mechanism by which lipid changes influence quality during hot air drying. The results showed that drying significantly altered lipid metabolism. Encompassing 28 subclasses, five major lipid categories were identified: glycerophospholipids (GP), glycolipids (GL), sphingolipids (SP), isoprenylglycolipids (PR), and fatty acids (FA). From among these, the total content of GP remained the highest and increased significantly after drying, whereas the contents of GL and FA decreased markedly. Hydrolysis of structural lipids led to the collapse of cellular structure, and the levels of hydrolyzed lipids phosphatidic acid (PA), lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), and lysophosphatidic acid (LPA) increased significantly after drying, which may adversely affect long-term storage. Furthermore, increased lipid unsaturation intensified browning, and lipid oxidation also promoted the formation of volatile flavor compounds. Overall, this lipidomic research demonstrated that hot air drying determines the final quality and flavor profile of dried P. impudicus through coordinated mechanisms involving membrane lipid oxidation, structural membrane damage, browning, and flavor generation. These findings provided a new insight into the mechanism of quality changes and a theoretical basis for quality improvement and process regulation for dried edible mushroom products.

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1. Introduction

Phallus impudicus (P. impudicus) is a kind of precious edible mushroom. Its fruiting body is white and lustrous, rich in various nutrients and bioactive substances, and has been reported to possess antibacterial, anticancer, and antitumor functions [1]. It plays a positive role in promoting human health and disease prevention and treatment, and thus has attracted much attention. At present, P. impudicus is mainly sold in the form of mature fruiting bodies. However, fresh fruiting bodies are prone to spoilage and deterioration during transportation, a condition which is not conducive to maintaining their commercial value. Therefore, dried products have become their main form of sale.

Hot air drying is regarded as one of the most convenient and mature drying methods at present due to its simple process, stable operation and wide applicability. It has been widely used in the processing of various edible mushroom products [2]. However, under hot air-drying conditions, the original tissue structure and chemical composition of edible mushrooms inevitably undergo changes. In particular, the disruption of cell structure and the transformation of nutritional components will further affect the texture and apparent color of the products. From the perspective of cell physiology, lipids are an important structural component of the cell membrane, playing a crucial role in maintaining cell integrity and regulating the exchange of substances between the intracellular and extracellular environments [3]. Under the influence of external stress factors such as heat and oxygen, membrane lipids are highly prone to undergo irreversible changes such as oxidation and degradation, which in turn leads to damage to the cell membrane structure and subsequently alters the overall morphology and physicochemical properties [4]. Furthermore, lipid oxidation can cause browning of the dried products [5,6,7], while the hydrolysis of lipids has an impact on the flavor [8]. The results of our previous metabolomics study [9] demonstrated that lipids represent a major class of metabolites in fresh P. impudicus, accounting for approximately 26.5% of the total detected metabolites, and are closely associated with membrane integrity, oxidative stability, and flavor precursor formation. During hot air drying, both the diversity and abundance of lipid metabolites changed markedly, a development which was accompanied by pronounced quality alterations, particularly browning development. These observations suggest that lipid composition, especially with respect to fatty acid-related components, is highly sensitive to drying conditions and may play a pivotal role in determining the final quality of dried products. Therefore, monitoring lipid changes during drying is essential not only to minimize unfavorable oxidative degradation but also to identify potentially beneficial transformations that contribute to desirable color and flavor attributes. However, determining the specific roles of different lipid changes, as well as the optimal drying conditions that balance lipid preservation and quality enhancement, still requires further systematic investigation.

Based on this, this work aimed to define appropriate drying conditions by clarifying which lipid changes should be limited during the hot air drying of P. impudicus. At the same time, we also dynamically monitored browning-related indicators, including color change, browning index, total phenols, free amino acids, and so on, to evaluate quality evolution during drying. By integrating lipidomic analysis, this study systematically investigated the roles of lipids. The results showed that the lipid metabolism underwent significant changes. A total of five major lipid categories were identified: glycerophospholipids (GP), glycolipids (GL), sphingolipids (SP), isoprenylglycolipids (PR), and fatty acids (FA). These categories encompassed 28 subcategories of lipids. Among them, the total amount of GP remained the highest and significantly increased after drying, while the contents of GL and FA significantly decreased. Meanwhile, the hydrolysis of structural lipids leads to the collapse of cell structure, and the unsaturation levels of phosphatidic acid (PA), lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), and lysophosphatidic acid (LPA), which were hydrolyzed lipids, significantly increased after drying. We speculated that excessive hydrolysis and oxidation of structural lipids, when accompanied by increased lipid unsaturation, represent unfavorable changes due to their association with cell structure collapse, accelerated browning, and reduced storage stability, whereas moderate lipid oxidation may contribute positively to flavor development through the formation of volatile compounds such as aldehydes. Through comprehensive lipid classification and compositional analysis, this work seeks to establish a mechanistic basis for balancing lipid preservation and desirable quality attributes, thereby providing guidance for optimizing drying conditions. The drying experiments were performed under controlled laboratory conditions to ensure reproducibility, and the findings are intended to serve as a reference framework for optimizing practical drying processes in industrial applications, achieving effective dehydration while minimizing unfavorable lipid-related changes.

2. Materials and Methods

2.1. Materials and Reagents

Mature fruiting body of P. impudicus was obtained (Yunnan Xingmin Agricultural Technology Co., Ltd., Kunming, China). Acetic acid, zinc acetate, ferrocyanide potassium and potassium tartrate sodium were obtained (AR, Shanghai Titan Technology Co., Ltd., Shanghai, China). Potassium sulfate, glucose, potassium hydroxide and anhydrous copper sulfate were obtained (Grade BC, Shanghai Beyotime Biotechnology Co., Ltd., Shanghai, China). The malondialdehyde (MDA) assay kit (TBA method), plant total phenol test kit (colorimetric method), and total free amino acids assay kit (colorimetric method) were purchased from Nanjing Jiancheng Bioengineering Institute (Nanjing, China). An electrothermal blowing dry box was obtained (BGZ-240, Shanghai Boxun Medical Biological Instrument Corp., Shanghai, China). The scanning electron microscope (SEM) (Apreo 2C) and micro-ultraviolet spectrophotometer (Nanodrop 2000) used were both from Thermo Fisher Scientific Inc., Waltham, MA, USA. An electrical conductivity tester was obtained (DDS-11C, Leici Technology Co., Ltd., Shanghai, China). A color difference meter was obtained (NR200, Guangzhou 3nh Technology Co., Ltd., Guangzhou, China).

2.2. Sample Preparation

Mature P. impudicus of the same size and shape (75 ± 5 g) were selected for the experiment. The red mushroom cap and the brown cap were removed, and the skirt and the stalk retained. Then, the samples were placed in an electrically heated and ventilated drying oven and dried under a hot air flow at 60 °C for a total duration of 6 h. During the drying process, samples were taken at 0 min, 90 min, 180 min, 270 min, and 360 min, respectively, and the corresponding samples were numbered as HD-0, HD-90, HD-180, HD-270, and HD-360. The entire drying experiment was repeated three times, and in each experiment, ten fruit bodies of P. impudicus were randomly selected as samples. Some of the samples were rapidly frozen in liquid nitrogen and ground into powder, which was then stored at −80 °C for subsequent analysis.

2.3. Browning Index Analysis

The L* (lightness), a* (reddish–greenish) and b* (yellowish–blueish) values of the dried samples were measured using a color difference meter. Each sample was measured six times. The browning index was calculated using the following formula [10]:

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2.4. SEM Microstructure Analysis

Samples HD-0, HD-180 and HD-360 were cut into uniformly sized blocks (10 × 10 × 3 mm), and their surfaces were gold-coated. Then, the external surface micro-structural features were observed under SEM.

The porosity was measured through SEM images [11]. The porosity of P. impudicus was calculated as the ratio of intercellular space area (A_p_) to the total tissue area (A). Both parameters were measured from microstructural images using the software Image J 1.54f [12].

2.5. Relative Conductivity

For each treatment group, five samples of P. impudicus were selected. Samples were taken from the stems using a puncher, and circular slices with a diameter of 1 cm were cut from each sample. Three slices were taken from each sample. The obtained slices were grouped in three sets for repetition, with each repetition containing 3 slices. After the samples were rinsed with distilled water, they were placed in 40 mL of deionized water and the initial conductivity was immediately measured using an electrical conductivity meter. Then, at room temperature, the samples were slowly shaken in a shaker for 2 h and the conductivity was immediately measured. After that, the samples were boiled for 10 min, cooled to room temperature, and then 40 mL of deionized water was added to make the total volume 40 mL. The conductivity was measured again. The relative conductivity was calculated using the following formula:

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In the formula, P_0_ represents the initial conductivity; P_1_ represents the conductivity of the sample after slow shaking of the shaker for 2 h; P_2_ represents the conductivity measured after the sample was boiled for 10 min.

2.6. Total Phenol

The total phenol content was determined using a total phenols assay kit (A143-1-1, Nanjing Jiancheng Bioengineering Institute, Nanjing, China). Under alkaline conditions, phenolic substances reduce tungstic molybdic acid to produce a blue compound, which has a characteristic absorption peak at 760 nm; this is proportional to the total phenol concentration. Dried samples were ground into powder and sieved through a 40-mesh screen. Approximately 0.10 g of dried sample was extracted with 2.0 mL of 60% (v/v) ethanol solution by ultrasonic-assisted extraction (300 W, 5 s on/8 s off); this was followed by incubation at 60 °C for 30 min. After centrifugation at 4000 rpm for 10 min, the supernatant was collected as the sample extract. The working reagents were prepared according to the manufacturer’s instructions. Briefly, 50 μL of sample extract or standard solution was mixed with the reaction reagent; this was followed by color development under alkaline conditions. After incubation at room temperature for 10 min in the dark, the absorbance was measured at 760 nm using a UV–Vis spectrophotometer; thus, the total phenol content was determined.

2.7. Lipidomic Analysis

Take 20 mg of the freeze-dried powder sample and place it in a 2 mL centrifuge tube. Add a steel ball with a diameter of approximately 4 mm and 1 mL of lipid extraction solution (methyl tert-butyl ether: methanol = 3:1, containing an internal standard mixture). Vibrate for 30 min. Then, add 300 μL of ultrapure water and vibrate for 1 min. Let it stand at 4 °C for 10 min. Centrifuge at 4 °C, 12,000 r/min for 3 min, and transfer 400 μL of the supernatant to a 1.5 mL centrifuge tube. Concentrate at 20 °C for 2 h until all the water is completely evaporated. Add 200 μL of the reconstitution solution (acetonitrile: isopropanol = 1:1), vortex for 3 min and centrifuge again at 4 °C, 12,000 r/min for 10 min. Finally, take 120 μL of the supernatant and transfer it to the inner sleeve tube of the sample vial for LC-MS/MS analysis.

Lipid metabolites were analyzed and data were collected using the LC-MS/MS system. The samples were separated by a chromatographic column (Thermo Accucore^TM^ C30, 2.6 μm, 2.1 mm × 100 mm, Thermo Fisher Scientific Inc., Waltham, MA, USA). The mobile phase A was acetonitrile–water (60:40, v/v, containing 0.1% formic acid and 10 mmol/L formic acid ammonium), and the mobile phase B was acetonitrile–isopropanol (10:90, v/v, containing 0.1% formic acid and 10 mmol/L formic acid ammonium). The flow rate was set at 0.35 mL/min, the column temperature was 45 °C, and the injection volume was 2 μL. The gradient elution procedure is shown in Table S1.

The mass spectrometry analysis was conducted using an electrospray ionization (ESI) source, with the ion source temperature set at 500 °C. The spray voltage in positive ion mode was 5500 V, and in negative ion mode, it was 4500 V. The ion source gas parameters were set as follows: Gas 1 (GS1) 45 psi, Gas 2 (GS2) 55 psi, curtain gas (CUR) 35 psi. In the triple quadrupole detection, each ion pair was scanned and detected based on the optimized declustering voltage (DP) and collision energy (CE).

2.8. Statistical Analysis

The retention time and ion current intensity data for the lipid detection were exported using Analyst 1.6.3 software. The lipid components in the samples were qualitatively analyzed by mass spectrometry through comparison with the lipid database (METLIN database). Subsequently, the peak intensities of each substance in the different samples were corrected. Utilizing the orthogonal partial least-squares discriminant analysis (OPLS-DA) model in multivariate analysis, lipids with significant differences between groups were selected based on variable importance (VIP) in projection (VIP ≥ 1) and fold change (FC) (FC ≥ 2 or FC ≤ 0.5).

All the repeated experimental results were expressed as the mean ± standard deviation (Standard deviation, SD). The significance test was performed using analysis of variance (ANOVA), and a value of p < 0.05 was used as the criterion for statistically significant differences.

3. Results and Discussion

3.1. Effects of Hot Air Drying on Physicochemical Indicators

During the hot air-drying process, the appearance quality and microstructure of P. impudicus showed significant changes (Figure 1A), and its mass also declined significantly (Figure 1B). At the initial stage of drying (0 min), the P. impudicus was light yellow in color, with the cell walls expanding and the supporting structure intact. As a result, the whole structure presented a loose and porous network form with clear and full pores, and had good elasticity and extensibility. When the drying time reached 180 min, from the macroscopic structure perspective, as the water continued to evaporate, a significant moisture gradient was formed between the interior and the surface. The internal moisture migrated toward the surface, constituting the moisture gradient, which was the core mechanism responsible for the pronounced macroscopic contraction of P. impudicus tissue at this stage [13]; this directly led to the formation of a hardened surface crust [14]. SEM showed that the cell wall gradually lost water and underwent deformation, the originally uniform network structure was destroyed, the pore size began to be irregular, the wrinkled and collapsed areas increased significantly, and the space between tissues was further compressed. When dried for 360 min, there was no further significant volume shrinkage or overall deformation, compared with samples examined at 180 min. This indicated that the structural collapse and volume shrinkage of P. impudicus had basically stabilized from 180 min to 360 min. Within this duration, the main change has shifted to the densification of the microstructure. The results showed that the microstructure of P. impudicus further collapsed, and the original honeycomb pores closed almost completely, forming a dense, hardened surface layer with a highly contracted, collapsed, and tightly cohesive morphology. This mainly stems from the effects of hot air drying on polysaccharides in the cell wall. P. impudicus contains large amounts of polysaccharides, which are key structural substances associated with the cell wall and they play an important role in maintaining cell integrity and providing support. Under hot air-drying conditions, these polysaccharides tend to flow due to the loss of expansion pressure [15], which gradually weakens the stability of the cell wall, and then causes the shrinkage and collapse of the microstructure [16]. The porosity at different drying stages was calculated based on the SEM images. The evolution of porosity corresponds to the contraction of the P. impudicus [17]. At the very beginning of the hot air-drying process, the pore area fraction was 4.54 ± 0.47%. After drying for 180 min, the pore area fraction decreased to 2.52 ± 0.18%. When drying was completed, the pore area fraction was only 1.37 ± 0.29%. This decline indicated that a large proportion of initially open pores collapsed or closed. The reduction in porosity indicated progressive pore collapse and an increase in solid volume fraction, which structurally supported the shrinkage of the tissue during drying [11]. The results indicated that prolonged heat treatment led to an irreversible collapse of the cell wall. The consequent structural closure and volume contraction concentrated pigments and other chromophores associated with browning in localized regions, which in turn resulted in more severe browning during the hot air-drying process. Thus, structural damage not only accelerated moisture migration and compromised tissue elasticity, but also was closely associated with negative changes in macroscopic quality, including increased browning and deepened color. The value of L* decreased after 180 min compared to at the beginning of drying, and showed a general downward trend as the drying time prolonged, indicating that the sample gradually darkened (Figure 1C). On the contrary, a* and b* continued to increase during the drying process, and were significantly higher than those of the fresh sample at 180 min and later stages, showing a color shift from light to reddish and yellowish tones. The browning index calculated based on the L*, a*, and b* values significantly increased with the drying time, further quantitatively indicating that the browning degree of P. impudicus deepened during the hot air-drying process (Figure 1D).

Relative conductivity and malondialdehyde content are key indicators of cell membrane integrity and lipid oxidation. During hot-air drying, moisture evaporation and the consequent cellular shrinkage generate mechanical stress within the tissue. Furthermore, dehydration leads to changes in the lipid bilayer structure that weaken membrane integrity and affect cellular activity [18]. Thus, membrane lipid status can indicate the extent of cell viability impairment. The experimental results indicated that the relative electrical conductivity of P. impudicus significantly increased after hot air-drying treatment (Figure 1E), with an increase of 56.12%. This result indicated the leakage of the electrolyte. Throughout the drying process, its content was nearly twice the original level (Figure 1F). The early stage of hot air drying caused significant damage to the cell membrane structure of P. impudicus, leading to a rapid increase in membrane permeability and a large amount of intracellular electrolytes leaking out, which resulted in a quick rise in relative conductivity. As the drying process continued, the degree of cell membrane damage intensified, and the lipid peroxidation reaction gradually accumulated, as indicated by the continuous increase in malondialdehyde content, reflecting the deepening of oxidative damage to the cell membrane. Previous studies have confirmed that membrane lipid oxidation is a key factor inducing browning [19,20]. Not only does it promote the enzymatic browning process, but its oxidation products also directly contribute to color change, ultimately affecting the appearance-based quality of the dried edible mushrooms. Endogenous enzymatic browning was considered one of the main pathways for browning in dried P. impudicus. In previous studies, several key metabolites potentially involved in the browning reaction, such as phenylalanine and tyrosine, were identified. These substances could generate a large number of phenolic derivatives during their metabolic processes, providing sufficient substrates for enzymatic browning reactions and being closely related to the degree of browning in P. impudicus [9]. The total phenolic content in P. impudicus samples showed a significant upward trend throughout the hot air-drying process, increasing from 40.98 μmol/g to 52.10 μmol/g (Figure 1G). Notably, the increase in total phenolic content was mainly concentrated in the initial 90 min of drying and the last 90 min before the end of drying, demonstrating a distinctly phased characteristic.

The content of polyphenol oxidase (PPO) showed a significant declining trend (Figure 1H). Although the content of polyphenol oxidase decreased significantly, it still remained at 45.33 U/g after drying. Combining the results from the relative conductivity, it could be seen that the cell membrane structure of P. impudicus was significantly damaged in the drying process, and the integrity of the vacuoles was impaired, resulting in the large release of phenolic substances stored in the vacuoles, which led to a rapid increase in total phenolic content. As the drying continued, the internal temperature of the sample gradually increased, and the tissue moisture content significantly decreased. The further release of phenolic substances was limited, causing the growth rate of total phenolic content to slow down. Furthermore, continuous catalysis by PPO may result in a slow increase in the total content of polyphenols. The total contents of free amino acids showed trends of first decreasing and then increasing: from 404.2 μmol/g protein at the beginning of the 90 min period to 317.54 μmol/g protein, and then continuing to rise to 571.9 μmol/mg protein, with a cumulative increase of 167.7 μmol/g protein (Figure 1I). When exposed to oxygen, heat, or the action of enzymes, protein hydrolysis, lipolysis, or enzyme oxidation may occur. Proteins are hydrolyzed by proteases to generate large peptides, which are then further hydrolyzed into small peptides and free amino acids. Therefore, when the amount of amino acids produced by proteins is greater than the amount of amino acids consumed in the Maillard reaction, the final total free amino acids concentration will be higher than that of the fresh sample. Therefore, throughout the entire hot air-drying process, the key substrates for the Maillard reaction were adequately supplied, which provided an important chemical basis for the significant browning of P. impudicus under heat treatment conditions.

3.2. Changes in Lipid Composition During Hot Air Drying of P. impudicus

Samples of P. impudicus with drying times of 0 min (HD-0), 180 min (HD-360), and 360 min (HD-360) were selected for lipid profiling. In the samples associated with different drying stages of P. impudicus, a total of five major lipid classes were identified, namely, GP, GL, SP, PR, and FA, covering 28 subcategories (Figure 2A). Arranged by the number of detections, they included 191 glycerol triesters (TG), 53 phosphatidylcholines (PC), 50 phosphatidylethanolamines (PPE), 47 glycerol diesters (DG), 27 LPC, 27 free fatty acids (FFA), 26 phosphatidylglycerols (PG), 24 phosphatidylinositol (PPI), 24 dimethylmethylsphingosine diacylglycerols (DGTS), 22 phosphatidylserines (PS), 20 ceramides (Cert), 16 monosaccharide glycerol diesters (MGDG), 16 sphingomyelins (Cer), 14 LPE, 13 PA, 10 glycosphingolipids (HexCer), eight LPA, eight lysophosphatidylglycerol tri-methylsphingosine (LDGTS), eight sphingosine (SPH), five lysophosphatidylinositol (LPI), five phosphatidylalcohol (PMeOH), five monoglycerides (MG), three lysophosphatidylglycerols (LPG), three coenzyme Q (CoQ), one lysophosphatidylserine (LPS), one dimethylsphingosine-3-O-carboxymethylcholine (DGCC), one dimethylsphingosine glucuronide (DGGA), and one thioglycosylglyceraldehyde (SQDG). A total of 629 lipid species were identified. During the drying process, significant differences were observed, indicating that the lipid composition of P. impudicus was rich and diverse, with GL and GP being particularly prominent. Figure 2B uses stacked column diagrams to visually display the changes in the contents of various lipid subclasses at different hot air-drying times (HD-0, HD-180, and HD-360). The total lipid contents in HD-0, HD-180, and HD-360 were 1.75 × 10^4^ nmol/g, 2.11 × 10^4^ nmol/g, and 1.90 × 10^4^ nmol/g, respectively. At the initial stage of drying (0 min), the relative contents of GP, GL, SP, PR, and FA were 51.9%, 32.2%, 1.02%, <1%, and 14.39%, respectively. As the drying time increased, the content of GP gradually increased, while the contents of GL and FA decreased accordingly. By the 180th min of drying, the proportion of GP had increased by 24.3%, the proportion of GL had decreased by 16.44%, and the proportion of FA had dropped to 6.72%. At the end of drying (360 min), the content of GP had further risen to 73.69%, while the proportions of GL and FA were 16.97% and 7.97%, respectively; the proportions of SP and PR were both less than 1%. The results showed that GP remained the dominant lipid throughout the drying process, and its relative content increased significantly as the drying progressed. Meanwhile, the relative contents of GL and FA decreased significantly. Figure 2C used a heat map to visually analyze the differences in lipid subcategories of P. impudicus samples during different hot air-drying stages. The results indicated that in HD-0, the contents of FFA, DG, MG, SQDG, TG, PC, PI, CoQ, and SPH were relatively high. In HD-180, the contents of DGTS, PG, PMeOH, and Cer were relatively high. MGDG was more abundant in HD-360.

3.3. Differences in Lipid Composition During Hot Air-Drying Process

The principal component analysis (PCA) score plot (Figure 3A) shows that the three groups of samples, HD-0, HD-180, and HD-360, were significantly separated. The first principal component (PC1) contributed 45.66% of the variance, while the second principal component (PC2) contributed 14.9%, resulting in a cumulative variance of 60.56%. Spatially, HD-0 is distributed on one side of the PC1 axis, while HD-180 and HD-360 are on the other side, indicating significant differences in lipid composition. HD-180 and HD-360 are relatively close in the plot, suggesting smaller differences between them. Volcano plots were used to visualize the lipid distribution (Figure 3B–D), with each point representing a lipid molecule. Upregulated (red) and downregulated (green) lipids are highlighted, and gray dots represent lipids with no significant differences. The sizes of the spots in the category plot correspond to the VIP value. The significance threshold was set at p < 0.05 and |log_2_FC| ≥ 1 for the analysis of differentially expressed lipids in the early drying stage (HD-0 vs. HD-180), the late drying stage (HD-180 vs. HD-360), and the entire drying process (HD-0 vs. HD-360). In terms of lipid quantity changes, the lipid alterations during the entire drying process were very significant. In the early drying stage (Figure 3B), 21 lipids were downregulated and 123 lipids were increased. In the late drying stage (Figure 3C), the number of lipid changes tended to stabilize, with only eight lipids downregulated and seven lipids upregulated. However, the lipids changes measured were significantly different from those in the early drying stage. From the entire drying process (Figure 3D), a total of 27 lipids were downregulated and 124 lipids were upregulated. These differentially expressed lipid metabolites belonged to 14 subcategories: TG, DG, PE, PS, PC, PA, LPA, LPC, LPE, LPI, LPG, Cer, HexCer, and LDGTS. This indicated the diversity and complexity of lipids in P. impudicus at different drying stages.

3.4. Changes in GL During the Hot Air-Drying Process

Phospholipid degradation and the accumulation of phosphatidic acid have been associated with internal browning [21]. Therefore, exploring lipid changes will help to elucidate the mechanisms underlying both browning and quality deterioration in P. impudicus during hot air drying. Indeed, the subclasses of GL that showed the most significant changes during drying were DG and TG (see Table S2 for full lipid list). The lipids of DG showed significant downregulation (Figure 4A), including DG (18:2_18:2), DG (18:1_18:2), DG (18:0_18:2), DG (16:1_18:2), and DG (18:1_18:3). The decrease in DG directly indicated that the cell membrane structure underwent hydrolysis and rupture during hot air drying. The loss of membrane integrity led to the disruption of cellular compartmentalization, allowing phenol oxidase and its phenolic substrates to come into closer contact. At the same time, in most TG species, such as TG (16:0_18:2_18:3), TG (16:1_18:2_18:2), and TG (16:0_16:2_18:2), their contents increased rapidly in the early stage of drying, while TG (16:0_18:2_18:2) exhibited a different trend (Figure 4B). The observed increase in most TG species, accompanied by a decrease in others, might be due to the lipid hydrolysis of the cell membrane during hot air drying and lipase activation consuming some TG. At the same time, the cell might undergo lipid remodeling, re-esterifying some polyunsaturated fatty acids and storing them in a relatively stable TG form to cope with oxidative stress [22].

3.5. Changes in GP During the Hot Air-Drying Process

In addition, five GP lipid subclasses exhibiting significant changes during the drying process of P. impudicus were selected, namely, PA, PE, LPE, LPC, and LPA (see Table S3 for full lipid list). Among them, PA (18:2_18:1), PA (16:0_18:2), PA (16:0_16:1), PA (16:1_16:1), and PA (18:2_18:3) showed significant increases throughout the drying process, with PA (18:2_18:1) reaching up to 13 times the initial content (Figure 5A). PE (18:2_18:2), PE (18:2_16:0), PE (18:1_18:2), PE (16:1_18:2) and PE (16:1_16:0) decreased significantly, mainly in the later drying stage (Figure 5B). This decline was attributed to both the drying-induced hydrolysis of PE and a Maillard-type reaction between its amino groups and carbonyl compounds derived from lipid oxidation [23]. LPE (18:2), LPE (16:1), LPE (18:1), LPE (16:0), and LPE (16:2) showed significant increases in the early stage of drying and then slightly decreased in the later stage, but their contents throughout the drying process still showed significant increases (Figure 5C). The abundances of LPC (18:2), LPC (16:1), LPC (18:1), LPC (19:1), and LPC (17:1) basically continued to increase throughout the drying process (Figure 5D). Similarly, LPA exhibited an overall upward trend, consistent with those of LPC and LPE (Figure 5E).

3.6. Changes in FFA During Hot Air Drying

Hot air drying induced marked declines in key FFAs (see Table S4 for full lipid list), specifically, FFA (18:1), FFA (16:0), FFA (18:2), and FFA (18:0) (Figure 6). This reduction phenomenon might not only be caused by the re-esterification process, but it also might be related to the oxidation process. In the early stage of hot air drying, FFAs were rapidly oxidized and consumed under the action of heat treatment and oxygen to form peroxides. These resulting peroxides then degraded into volatile flavor compounds, such as aldehydes, ketones, acids, and others [24], aligning with the significant increase in MDA observed.

3.7. Changes in Unsaturated Lipids During Hot Air Drying

The unsaturation of lipids is related to non-enzymatic browning [25]. In P. impudicus, the unsaturated double bonds in certain lipids, including FFA, PA, and LPA, were statistically compared among different drying stages (Figure 7, Table S5). Among these lipids, the proportion containing one or more double bonds was above 80%. The proportion of lipids with two double bonds in LPA was notably high (67.5–82.8%). Additionally, as hot air drying progressed, the content of unsaturated double bonds in PA and LPA significantly increased. This further increase in lipid unsaturation may facilitate browning through carbonyl-amine reactions involving lipid oxidation products, unsaturated fatty acids, and phospholipid decomposition products, ultimately contributing to color change. Thus, the significant increase in unsaturated double bonds in lipids such as PA and LPA during drying provides a material basis for the participation of lipid oxidation in non-enzymatic browning.

3.8. Potential Mechanisms of Lipid Influence on Quality During Hot Air Drying

Our study has shown that changes in phospholipid content could partially reflect the degree of cell membrane damage [26]. Under heating conditions, a significant increase in PA content and a significant decrease in PE content were observed, which was consistent with previous reports [27]. These changes indicate aggravated membrane degradation, in agreement with the elevated MDA content and increased relative conductivity. During hot air drying, higher temperatures accelerated phospholipid degradation and membrane lipid oxidation, leading to a continuous rise in MDA levels. Consequently, membrane stability declined, permeability increased, intracellular constituents leaked out, and relative conductivity increased. Meanwhile, rapid surface moisture evaporation caused tissue contraction and hardening in P. impudicus. Structural disruption of the membrane further promoted browning and flavor-related reactions. Membrane damage exposed phenolic substrates, and during the early drying stage, rapid water loss led to a sharp increase in phenolic concentration. These phenols were readily oxidized by polyphenol oxidase into quinones, which subsequently underwent non-enzymatic polymerization to form brown pigments. In parallel, intensified lipid oxidation increased the degree of unsaturation, which not only contributed directly to non-enzymatic browning but also potentially initiated or enhanced Maillard reactions [28,29]. Moreover, hot air drying promoted the accumulation of free amino acids in P. impudicus, providing sufficient reactants for Maillard reactions with reducing sugars and thereby influencing both color development and flavor formation [30]. Notably, the intermediates generated from Maillard reactions serve as precursors to aromatic and colored compounds, which can further degrade into smaller molecules and contribute to the overall sensory properties of the final food product [31]. In addition, lipid oxidation products were able to react with PE to generate colored pyrrole compounds, further deepening the color [32]. In our previous studies on the metabolome, furfural methyl ester, a dark substance with a caramel color, was found, and its content increased significantly in the hot air-drying method [9]. Combined with the pronounced decrease in PE content, these results suggest that intensified lipid oxidation and subsequent secondary reactions jointly promoted browning development and contributed to the formation of characteristic thermal processing flavors during hot air drying.

4. Conclusions

This study systematically revealed the quality changes of P. impudicus during hot air drying. Hot air drying caused the collapse of the microscopic structure and led to the continuous accumulation of colored substances, membrane lipid peroxidation and the disruption of membrane system integrity. Five key glycerophospholipids (PA, PE, LPE, LPC, and LPA) underwent dynamic changes: PA content increased continuously and sharply throughout the process, whereas PE decreased significantly in the later stage. Concurrently, lyso-phospholipids such as LPE and LPC showed an overall upward trend, confirming membrane phospholipid degradation. Simultaneously, the accumulation of free amino acids provided key substrates for the Maillard reaction, further promoting the development of the product’s characteristic aroma. From a quality optimization perspective, excessive lysophospholipid formation may impair storage stability, whereas controlled Maillard reactions enhance aroma. Therefore, drying conditions should be regulated to limit lysophospholipid accumulation while promoting moderate flavor development without excessive browning.