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Article

Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage

by
Gabriela Rodriguez
1,2,
Manish Thapaliya
1,
Duyen Bui
1,
Fatemeh Malekian
2,
Achyut Adhikari
1 and
Zhimin Xu
1,*
1
School of Nutrition and Food Science, Louisiana State University, Baton Rouge, LA 70803, USA
2
Agricultural Research and Extension Center, Southern University, Baton Rouge, LA 70813, USA
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(10), 2270; https://doi.org/10.3390/agronomy15102270
Submission received: 26 August 2025 / Revised: 17 September 2025 / Accepted: 22 September 2025 / Published: 25 September 2025
(This article belongs to the Special Issue Phytopathogens and Crop Diseases)
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Abstract

Natural preservatives are gaining attention as chemical-free solutions to extend produce shelf life and prevent microbial spoilage. Therefore, sweet potato peel (SPP) was investigated as a source of antifungal bioactive compounds in this study. We evaluated essential oils and, for the first time, a bound terpene (BT) concentrate extracted from SPP against Aspergillus flavus, using both in vitro and in vivo assays. Murasaki organic Japanese sweet potato (Ipomoea batatas L.) peels, A. flavus AF13, a highly aflatoxigenic fungus, and Creole tomato (Solanum lycopersicum) fruits were used in the study. Essential oils were extracted by hydrodistillation (HD) and vacuum distillation (VD), while the BT fraction was isolated and concentrated. HD and VD yielded 19 and 10 terpenes, respectively, with linalool and α-terpineol dominating and representing more than 50% of total terpenes in both distillates. The BT concentrate demonstrated significant inhibition of A. flavus growth at concentrations starting from 12.5 µL/mL. The strongest effect was observed at 100 µL/mL, with a 26.0 ± 1.0 mm inhibition zone and 55.56 ± 4.53% growth reduction. In contrast, HD and VD distillates showed no antifungal activity in either in vitro or in vivo assays. Consistently, the BT concentrate-treated tomatoes reduced fungal growth and spoilage, with lesion diameters less than 10 mm after 7 days of storage, while the HD and VD distillate treatments had diameters over 20 mm, and the untreated control had diameters over 60 mm. These findings highlight that SPP waste could be an economical and bio-based source for developing natural antifungal ingredients. The success is anticipated to offer a potential alternative to current synthetic fungicides in preventing fungi A. flavus-induced spoilage of nightshade vegetables.

1. Introduction

Fresh produce and crops usually experience significant postharvest losses due to microbial spoilage, rapid respiration, and fragile tissues [1]. Microbial spoilage is responsible for up to 25% of global postharvest losses, preventing fresh produce from reaching consumers in good quality [2,3]. These processes may lead to discoloration, off-odors, biochemical changes, moisture loss, or the formation of toxic compounds, each of which poses a potential risk to human health [4]. Tomatoes are particularly vulnerable due to their short shelf life and susceptibility to both biotic and abiotic deterioration [5]. Fungal pathogens are the principal cause of fruit rotting in tomatoes, with several species identified as key contributors. Fungi including Aspergillus, Alternaria, Fusarium, and Penicillium are frequently associated with postharvest decay and disease in tomatoes [6]. From a food safety perspective, Aspergillus is considered the most hazardous because of its ability to produce aflatoxins, which the International Agency for Research on Cancer (IARC) has classified as carcinogens [7].
Although greenhouse cultivation provides a controlled environment for tomato growth, it often promotes high humidity levels that favor the proliferation of pathogens, ultimately increasing the risk of disease and crop loss [8]. A. flavus has been detected in both fresh tomatoes and tomato-based products [9]. This fungus is known for causing postharvest diseases in stored crops, leading to rotting and spoilage [10,11]. Many growers depend extensively on chemical fungicides, applying them during both preharvest and postharvest stages to control fungal diseases [12]. Fungicides from multiple classes, including anilinopyrimidine, benzimidazole, carboxamide, chloronitrile, strobilurin, and thiazole, effectively control tomato fungal infections [13]. They have different modes of action, such as systemic fungicides, which are absorbed and translocated to infection sites, and protective fungicides, which act as surface barriers [14]. Although chemical fungicides remain the primary approach for controlling tomato diseases, their excessive use can promote pathogen resistance, accumulate harmful residues on crops, and generate environmental and health risks due to the poor biodegradability of many active compounds [12,15]. In response to these challenges, research has focused on natural and safer antifungal solutions that can reduce postharvest losses without compromising consumer safety. Some of the natural compounds that have been proven to have antifungal and antimicrobial properties are phenolics, flavonoids, and essential oils [16,17,18]. Terpenes are the major constituents of essential oils and exhibit broad antifungal activity, but are often volatile and thermolabile, which limits their stability and practical application.
Hydrodistillation remains the most common method for extracting essential oils [19], but its high temperature can degrade thermolabile terpenes or generate unwanted compounds [20]. Vacuum distillation, by contrast, operates under reduced pressure, allowing for terpene recovery at lower temperatures and reducing thermal degradation [21]. In addition to free-form terpenes, bound-form terpenes are also rich in plant tissues but underexplored, as they are odorless, water-soluble, and chemically stable, providing enhanced bioavailability [22]. Exploring bound-form terpenes as a possible postharvest intervention could be a promising and value-added approach for sustainable produce preservation. Our recent investigation has shown that sweet potato peel, a major agro-industrial by-product, contains high levels of bound-form terpenes, primarily bound-form linalool and α-terpineol [23]. Therefore, this study employed hydrodistillation (HD) and vacuum distillation (VD) to extract free terpenes from sweet potato peel (SPP). Then, the compositions of both distillates were analyzed, and their antifungal activities against A. flavus, along with bound-form terpene (BT) concentrates, were evaluated in in vitro assays and an in vivo tomato model. The results of this study suggest a possible method to produce natural antifungal terpenes.

2. Materials and Methods

2.1. Chemical Reagents and Raw Materials

Linalool (99%) and α-terpineol (97%) standards and hydrochloric acid (HCl) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Fresh Murasaki organic Japanese sweet potatoes (Ipomoea batatas) and Creole tomatoes (Solanum lycopersicum) were obtained from a local market in Baton Rouge (LA, USA). Both were firm, uniformly sized, and unblemished. They were stored at room temperature before use. A. flavus strain (AF13), a fungus commonly contaminating agricultural crops, was obtained from Dr. Zhi-Yuan Chen at the Department of Plant Pathology and Crop Physiology, Louisiana State University (Baton Rouge, LA, USA).

2.2. Preparations of Free and Bound Terpenes from Sweet Potato Peel

Sweet potato peels (100 g) were removed and combined with 100 g of distilled water after being thoroughly cleaned. The mixture was blended using a high-speed blender (Nutribullet Pro Plus, Nutribullet, LLC, Los Angeles, CA, USA) until a uniform slurry was achieved. Based on the method of our previous study [23], 2 mL of 12 M HCl was added to the mixed slurry in a sealed flask to carry out acid hydrolysis at 30 °C for 45 min at a speed of 600 rpm using an orbital shaker. The acid hydrolysis broke down the binding between the sugar moiety and terpene to release free-form terpenes. The resulting hydrolysate sample was centrifuged at 5000 rpm for 10 min. The separated supernatant was approximately 100 mL, which was collected for distillation by essential oil hydrodistillation or vacuum distillation. The supernatant collected without acid hydrolysis (approximately 100 mL) was used for preparing BT concentrate by vacuum distillation.
The parameters of the hydrodistillation method were based on a previously established procedure with some modifications [24]. The supernatant was added to the bottom flask of a Clevenger-type apparatus. The flask was heated by a temperature-controlled heating mantle at 97 ± 3 °C and agitated with a magnetic stirrer. A condenser for collecting the vapor from the flask was controlled at −20 °C with a refrigerated circulating bath (Endocal, Neslab Instruments Inc., Portsmouth, NH, USA) to obtain the HD (hydrodistillation) distillate. For vacuum distillation, the experimental conditions were selected based on a previous method [25]. The supernatant was transferred into a rotary evaporator flask in a water bath at 40 °C and under vacuum on a Rotavapor R-300 (Buchi Analytical Inc., New Castle, DE, USA) to obtain the VD (vacuum distillation) distillate. For BT concentrate, the supernatant obtained without undergoing the acid treatment was transferred to a rotary evaporator flask for vacuum distillation at 40 °C. The distillations were completed when the concentration of terpenes no longer increased. The total volume collected was approximately 30 mL from each supernatant.

2.3. Identification of Terpenes Using Solid-Phase Microextraction (SPME) and Gas Chromatography–Mass Spectrometry (GC-MS) Analysis

The free terpenes in HD or VD distillates or bound terpenes in BT concentrate were analyzed using SPME-GC-MS following the procedure described by Rodriguez, Prinyawiwatkul, Aryana, King, and Xu [23]. The distillate (10 mL) was added to a 20 mL vial for headspace solid-phase microextraction (SPME) using a polydimethylsiloxane (PDMS) fiber with a 250 μm coating (Sigma-Aldrich, St. Louis, MO, USA). The vial was sealed and stirred for 35 min at 60 °C. Then, the adsorbed substances on the fiber were desorbed in the GC-MS injection port.
The GC-MS consisted of a DB-5 fused silica capillary column (30 m × 0.25 mm ID × 0.25 µm) (Sigma-Aldrich, St. Louis, MO, USA) and a 5977A mass selective detector MSD (Agilent Technologies, Lexington, MA, USA). The GC was operated with the injection port at 200° C in splitless mode. The oven temperature was at 35 °C for 5 min, ramped to 135 °C at a rate of 4 °C/min, then raised by 10 °C/min to 200 °C and held for 1.5 min. The flow rate of the carrier gas helium was 3 mL/min. The MS detector was set at an ionization voltage of 70 eV and an ion source temperature of 350 °C. The range of MS spectra scans was from 45 to 500 m/z. Compounds were identified using their mass spectra and retention time index in the NIST Library. The concentrations of linalool and α-terpineol were calculated based on their external standard curves. The level of each terpene in the sample was compared by using its percentage of peak area.

2.4. Determination of Minimal Inhibitory Concentration (MIC)

The isolates of A. flavus strain (AF13) were maintained on potato dextrose agar (PDA). The concentration of the spores was adjusted to 5 × 105 CFU/mL using a hemocytometer.
The MIC was determined by a modified agar well diffusion method [26]. The distillates and BT concentrate were measured and dissolved in sterile distilled water at a stock concentration of 100 µL/mL. Then, a series of 2-fold dilutions was made to obtain concentrations from 0.78 to 100 μL/mL (0.78, 1.56, 3.13, 6.25, 12.5, 25, 50, and 100 μL/mL). A 100 μL of suspension containing 5 × 105 CFU/mL of fungal spores was spread on a PDA plate. Discs impregnated with 10 μL of each serial dilution of HD distillate, VD distillate, or BT concentrate were placed in the center of the inoculated plate. The inoculated plates were incubated at 37 ± 2 °C for 72 h. The lowest concentration of each solution showing a clear zone of inhibition was considered the MIC.

2.5. Observation of Mycelial Growth

Based on the MIC results, the BT concentrate was tested at four concentrations: 100, 50, 25, and 12.5 µL/mL, while the HD and VD distillates were tested at 100 µL/mL. PDA (20 mL) mixed with the measured volumes of the distillate or concentrate was poured into a sterilized 90 mm Petri dish. To ensure uniform dispersion of the distillate or concentrate in the medium, Tween 80 was added to the PDA at a final concentration of 0.05% (v/v). Plates prepared with the same volume of sterile distilled water instead of the distillate or concentrate served as controls. All treatment and control groups were tested on three plates per replicate, and the experiment was repeated three times.
For the assay, a 5 mm diameter mycelial disc of A. flavus was removed from a 7-day-old culture and placed approximately 3 mm from the edge of each prepared Petri dish. All the plates were incubated at 22 ± 2 °C for 7 days. Then, the colony diameters in the Petri dishes were measured. Each treatment was performed in triplicate. The percentage of mycelial growth inhibition (MGI) was calculated according to the following formula: MGI (%) = [(dc − dt)/dc] × 100, where dc (cm) is the mean colony diameter for the controls and dt (cm) is the mean colony diameter for the treatments. Controls were prepared with the same volume of sterile distilled water.

2.6. Determination of Terpene Release During Proliferation of Aspergillus Flavus

To evaluate the ability of A. flavus to release free terpenes from bound terpenes, the BT concentrate was incubated with fungal cultures under controlled conditions. An aliquot of 15 μL of A. flavus spore suspension (5 × 106 spores/mL) was inoculated into a flask containing the BT concentrate. The incubated cultures were collected at 3 and 7 days post-inoculation to monitor terpene release over time. The terpenes were determined by the GC-MS method described above.

2.7. Study of Antifungal Activity Using Tomato as an In Vivo Model

Tomatoes were washed in warm water, air-dried, and externally disinfected using 80% ethanol. After three wounds were made on each fruit using a sterile pipette tip (8 mm in diameter), 30 μL of each test solution or sterile distilled water (control) was applied to each wound. Based on the in vitro results, for BT concentrate, concentrations of 100, 50, 25, and 12.5 μL/mL were tested, whereas for HD and VD distillates, only the highest concentration (100 μL/mL) was evaluated. All tomatoes were maintained under a laboratory hood at room temperature. Two hours later, each wound was inoculated with 15 μL of A. flavus spore suspension (5 × 106 cells/mL). All tomatoes were stored for 7 days in perforated plastic containers inside a controlled environment chamber maintained at 21–22 °C and 85% relative humidity under ambient laboratory lighting with a 12 h light/12 h dark cycle. Lesion diameters caused by A. flavus infection, characterized by skin rupture, discoloration, softening, and exudation, were measured on each tomato wound using a ruler. Each group contained three tomatoes, and the study was conducted in triplicate.

2.8. Statistical Analysis

Statistical analyses were conducted by using SAS software, version 9.4 (SAS Institute Inc., Cary, NC, USA). Data are presented as mean ± standard deviation (SD) from three independent replicates. Statistical significance was assessed using one-way analysis of variance (ANOVA), followed by Tukey’s post hoc test for multiple comparisons. Differences between groups were considered statistically significant at p < 0.05.

3. Results and Discussion

3.1. Volatile Compositions in Distillates from Sweet Potato Peel (SPP) Using Essential Oil Hydrodistillation and Vacuum Distillation

Hydrodistillation is one of the most common techniques for isolating semi-volatile compounds, especially for making essential oils, which are essentially terpene concentrates. Despite its widespread use, this technique demands prolonged processing time and high energy consumption [27]. These limitations have driven the adoption of greener, more efficient extraction methods that operate at lower temperatures and use no or fewer solvents [28]. Vacuum distillation allows low-temperature operation through reduced system pressures, which can preserve thermolabile compounds and enhance yields [29]. The chromatograms of the GC-MS analysis for both distillates are shown in Figure 1. A total of 26 and 10 volatile terpenes were obtained in HD and VD distillates, respectively (Table 1). Most terpenes are semi-volatile compounds with relatively high boiling points. For example, humulene has a boiling point of about 125 °C. Usually, the dominant ions for most terpenes are 69, 93, 121, and 136 m/z [30]. The percentage of the ion peak area closely reflects the level of the terpenes in the distillates. HD yielded a terpene profile dominated by α-terpineol (44.2%), linalool (8.0%), two ocimenols together (~14%), and p-mentha-1-en-9-al (8.5%), with smaller amounts of β-damascenone (3.8%). In contrast to the HD distillate, fewer terpenes were found in the VD distillate. The dominant terpenes in VD were linalool (51.6%) and α-terpineol (22.5%), which together accounted for over 70% of the extract, followed by β-myrcene (7.0%), guaiol (5.2%), and terpinen-4-ol (2.4%) (Table 1). The results are in agreement with our previous study, which found that linalool and α-terpineol were the two major bound terpenes comprising over 50% of total terpenes in sweet potato peel [23]. Unlike free terpenes, bound-form terpenes exist in glycosidically bound form, making them non-volatile and therefore undetectable by GC–MS until enzymatic or acid hydrolysis releases free-form terpenes. The concentrations of bound linalool and α-terpineol in the BT concentrate increased by approximately 12-fold and 14-fold, respectively, compared with the original extraction solution.
Meanwhile, the higher diversity of terpenes in the HD distillate than in the VD distillate suggested that the elevated temperatures in HD may promote the conversion of new terpenes and the formation of additional compounds. These reactions can occur through oxidative reactions, C–C bond cleavage, elimination, hydrolysis, or thermal rearrangements [31]. Additionally, water as a polar solvent can accelerate such reactions [32,33]. Given that linalool and α-terpineol have boiling points of 198 °C and 217–219 °C, respectively, the relatively lower recovery of linalool in HD may be attributed not only to its relatively higher volatility but also to its greater thermal vulnerability compared to α-terpineol. Furthermore, previous work has suggested that compound recovery during distillation is influenced by polarity as well as volatility, adding another factor that may affect selective extraction of these compounds [34]. A similar finding was observed during HD of linalool-rich chemotypes from Taiwan cinnamon, where linalool degradation led to the formation of β-myrcene and ocimene isomers at high temperatures [35]. In vacuum distillation, compounds with the highest vapor pressures evaporate first. Linalool has a much higher vapor pressure at 20 °C (0.115 Torr) than α-terpineol (0.0301 Torr), which explains the more efficient recovery of linalool under the applied vacuum conditions [36].

3.2. MIC of Antifungal Activity and Inhibition of Mycelial Growth

The results of the disk diffusion assay provided valuable information on the antifungal characteristics of the distillates and concentrate (Table 2). The results showed that the BT concentrate exhibited greater antifungal activity compared to the distillates. Notably, neither HD nor VD distillates obtained in this study inhibited A. flavus at the highest concentration tested (Table 2). This finding clearly demonstrates that the antifungal activity of terpenes depends not only on the functional groups of the compounds but also on the interaction capability between terpenes and fungi, which may be significantly enhanced by sugar moieties in bound terpene glycosides.
Terpenes generally possess moderate antifungal activity [37], with previous studies identifying carvone and thymol as the two main terpenes that exhibited potential against various pathogenic fungi [38]. Similarly, eugenol and menthol serve as effective alternatives to synthetic fungicides for controlling food-decaying fungi in the food industry [39]. For instance, the antifungal activity against Aspergillus species depends mainly on hydrocarbon monoterpenes rather than oxygenated hydrocarbons like linalool and α-terpineol [24]. Despite their antifungal potential, some terpenes require concentrations as high as 3000 ppm to achieve microbial growth inhibition [40].
Fungi possess a unique cell wall composed of chitin, glycans, and glycoproteins, which forms a complex and dynamic protective barrier that defends against physical damage and osmotic stress [41]. Although these properties are protective, the fungal cell wall represents a target for developing effective antifungal agents. One of the key characteristics of an effective antifungal agent is its ability to combine water solubility with lipophilicity, allowing it to disperse in both aqueous environments and lipid-rich cell membranes while penetrating the fungal cell wall [41,42]. Thus, the BT concentrate might facilitate cellular penetration; upon hydrolysis, the released linalool and α-terpineol can permeate the plasma membrane and cause structural disruption. Additionally, there may also be synergistic or antagonistic interactions between different terpene compounds [43]. However, contributions from other minor constituents present in the BT concentrate cannot be excluded from the antifungal activity.
The results of mycelial growth varied significantly among the samples tested (Figure 2 and Table 3). In Table 3, a positive percentage value indicates a decrease or inhibition of mycelial growth, and a negative percentage indicates a promotion in mycelial size compared to the initial measurement. At a concentration of 100 μL/mL, both HD and VD distillates showed no inhibitory effect on mycelial growth. In contrast, the BT concentrate, at tested concentrations of 100, 50, 25, and 12.5 μL/mL, significantly reduced mycelial growth (p < 0.05). The observed color changes are indicative of fungal colonization and tissue breakdown. Monoterpenes inhibit mycelial growth by promoting lipid peroxide formation, leading to cell death [44]. Moreover, many studies have shown that essential oils have greater efficacy in inhibiting mycelial growth when applied in the vapor phase rather than by direct contact. Higher toxicity against B. cinerea in tomatoes was reported when using a volatile phase, with similar results observed in strawberries [45,46]. This may be another reason why HD or VD distillates did not show antifungal activity under the direct-contact assay in this study.
The effect of the proliferation of A. flavus on the release of bound terpenes was observed in this study (Figure 3). Upon inoculation with A. flavus, the released linalool concentration increased significantly over time, indicating active hydrolysis of bound linalool glycosides. In contrast, the released α-terpineol concentrations remained stable after 7 days, indicating that its bound forms were less susceptible to fungal hydrolysis. The dependence of microorganisms on enzymatic systems for energy production is well documented. Aspergillus species rely on proteolytic, lipolytic, and amylase enzyme systems to support key metabolic processes [47], and are also well known for their natural ability to produce organic acids during growth, which are associated with their biological processes [48]. These fungi also produce enzymes that can break down molecules into smaller products as a source of nutrients for growth, such as sugars [49]. The release of bound terpenes into free forms could result from both the acidic conditions and fungal enzymes. Although α-terpineol and linalool are both monoterpenes with the same molecular formula (C10H18O), the ring structure of α-terpineol makes it more chemically stable than the acyclic linalool. Evidence indicates that enzymes prefer acyclic monoterpene glycosides over cyclic ones [50], making the less stable linalool more enzymatically accessible. On the other hand, bound α-terpineol may be less likely to react with acids compared with bound linalool, which could be explained by the fact that α-terpineol has greater thermodynamic stability, so its bound form is less prone to undergoing rearrangements [51]. Further studies, such as microscopy or cell permeability assays, could be applied to fully explore the proposed role of bound-form terpenes in facilitating cellular penetration.

3.3. Inhibition of Fungal Spoilage in Tomato In Vivo Model

Several natural approaches have been investigated to control postharvest fungal spoilage in tomatoes. Combining refrigeration with essential oils preserved tomato quality against pathogenic fungi [52]. Similarly, pomegranate peel extract inhibits fungal growth in tomatoes, but high concentrations can cause allelopathic effects on the plants [53]. Biological antagonists have also proven to be effective antifungal agents in tomatoes. L. plantarum and L. fermentum inhibited several tomato pathogens in vitro [54], while selected yeast strains have shown similar activity in inhibiting the growth of A. flavus in tomatoes [55].
As shown in Figure 4G,H, after 7 days of inoculation with A. flavus, the tomatoes in the control group exhibited signs of fungal spoilage, including mold colonies with greenish coloration, skin rupture, softening, exudation, and discoloration around the infected sites. As expected, this group presented the most severe skin damage, characterized by lesions measuring 61.0 mm in diameter. The tomatoes treated with HD and VD distillates showed pronounced deterioration, particularly softening, exudation, and discoloration, with mean lesion diameters of 23.9 mm and 29.9 mm, respectively (Figure 4E,F). However, the tomatoes treated with BT concentrate had fewer signs of fungal spoilage and retained better firmness throughout the evaluation period (Figure 4A–D). In the BT-treated tomatoes, lesions ranged from superficial mold or minor skin rupture to no visible deterioration. The highest concentration (100 µL/mL) of the BT concentrate provided the strongest inhibition against A. flavus and maintained the qualitative characteristics of the tomatoes. A single application at this concentration effectively prevented skin rupture, softening, and liquid exudation.
As mentioned above, A. flavus is capable of hydrolyzing glycosidic bonds associated with bound linalool but not α-terpineol. Given the limited release of α-terpineol and the stronger antifungal effect observed in the BT concentrate, it is possible to infer that linalool is the primary contributor to the antifungal activity of the concentrate. Antifungal efficacy is also heavily influenced by both the concentration of active compounds and the duration of exposure [56]. It is important to highlight that some in vivo studies have suggested that essential oils may need to be applied repeatedly to remain effective [57]. Free terpenes and essential oils usually have strong scents, which limit their application in food products. The odorless bound terpenes offer significant advantages for applications that require neutral and water-soluble food preservatives, providing antimicrobial benefits without compromising sensory properties. This study was limited to one fungal strain, one crop, and short-term storage. Future work should test multiple pathogens, commodities, and longer storage periods. Additionally, future studies should be designed to fully confirm the mechanism across other fungal classes and produce types and to apply current chemical fungicides as positive controls to compare antifungal efficacy.

4. Conclusions

The BT concentrate from sweet potato peel was more effective in antifungal activity than its free-form terpenes in HD and VD distillates. BT concentrate inhibited A. flavus growth in both in vitro assays and an in vivo tomato model. The minimal inhibitory concentration was as low as 12.5 μL/mL. At 100 μL/mL, the BT concentrate had the largest inhibition zone (26.0 ± 1.0 mm), whereas HD and VD showed no activity at the same concentration. In the in vivo tomato model, the BT concentrate treatment lowered tomato lesion diameters (less than 10 mm) across all tested concentrations, while tomatoes treated with HD and VD developed extensive deterioration with large lesion diameters (higher than 20 mm). The sugar moiety and terpene component might contribute to the hydrophilic and lipophilic characteristics required for effective membrane penetration, leading to fungal cell destruction. Overall, these findings provide a value-added use of sweet potato peel waste as a natural source to produce a promising and chemical-free alternative for controlling fungal spoilage. However, the results of this study remain confined to one type of agricultural by-product and laboratory conditions. Furthermore, assessments of safety aspects such as toxicity and allergenicity, sensory properties after treatment, and the cost of manufacture are necessary.

Author Contributions

G.R.: Investigation, Data analysis, Methodology, and Writing; M.T.: Investigation and Writing; D.B.: Investigation; F.M.: Review; A.A.: Methodology, Resources, and Review; Z.X.: Conceptualization, Funding Acquisition, Resources, Supervision, and Review. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the partial funding and contributions of the LSU AgCenter Hatch Project no. LAB94563.

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors thank Zhi-Yuan Chen at the Department of Plant Pathology and Crop Physiology, Louisiana State University, and the LSU AgCenter for providing the fungus species.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 15 02270 g001
Figure 1. Total ion chromatograms of hydrodistillation (HD) distillate (A) and vacuum distillation (VD) distillate (B), with main compounds listed in order of retention time.
Figure 1. Total ion chromatograms of hydrodistillation (HD) distillate (A) and vacuum distillation (VD) distillate (B), with main compounds listed in order of retention time.
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Agronomy 15 02270 g002
Figure 2. Effects of HD (hydrodistillation) distillate, VD (vacuum distillation) distillate, and BT (bound-form terpene) concentrate on A. flavus mycelial growth after 7 days of incubation. Treatments: (A) BT concentrate (100 µL/mL); (B) BT concentrate (50 µL/mL); (C) BT concentrate (25 µL/mL); (D) HD distillate (100 µL/mL); (E) VD distillate (100 µL/mL); (F) sterile distilled water (control).
Figure 2. Effects of HD (hydrodistillation) distillate, VD (vacuum distillation) distillate, and BT (bound-form terpene) concentrate on A. flavus mycelial growth after 7 days of incubation. Treatments: (A) BT concentrate (100 µL/mL); (B) BT concentrate (50 µL/mL); (C) BT concentrate (25 µL/mL); (D) HD distillate (100 µL/mL); (E) VD distillate (100 µL/mL); (F) sterile distilled water (control).
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Agronomy 15 02270 g003
Figure 3. Release of free terpenes from bound terpenes during A. flavus proliferation after 7 days of incubation. Different letters indicate significant differences between means (p < 0.05).
Figure 3. Release of free terpenes from bound terpenes during A. flavus proliferation after 7 days of incubation. Different letters indicate significant differences between means (p < 0.05).
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Agronomy 15 02270 g004
Figure 4. Wounded tomatoes inoculated with Aspergillus flavus and incubated at 22 °C for 7 days under controlled conditions. Treatments: (A) bound-form terpene (BT) concentrate (100 µL/mL); (B) BT concentrate (50 µL/mL); (C) BT concentrate (25 µL/mL); (D) BT concentrate (12.5 µL/mL); (E) hydrodistillation (HD) distillate (100 µL/mL); (F) vacuum distillation (VD) distillate (100 µL/mL); and (G,H) controls: sterile distilled water. * Mean lesion diameter (MLD) values for skin rupture, discoloration, softening, and exudation are displayed above each image.
Figure 4. Wounded tomatoes inoculated with Aspergillus flavus and incubated at 22 °C for 7 days under controlled conditions. Treatments: (A) bound-form terpene (BT) concentrate (100 µL/mL); (B) BT concentrate (50 µL/mL); (C) BT concentrate (25 µL/mL); (D) BT concentrate (12.5 µL/mL); (E) hydrodistillation (HD) distillate (100 µL/mL); (F) vacuum distillation (VD) distillate (100 µL/mL); and (G,H) controls: sterile distilled water. * Mean lesion diameter (MLD) values for skin rupture, discoloration, softening, and exudation are displayed above each image.
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Table 1. Volatile compositions in HD (hydrodistillation) distillate and VD (vacuum distillation) distillate.
Table 1. Volatile compositions in HD (hydrodistillation) distillate and VD (vacuum distillation) distillate.
No.CompoundRI a% rel b HD% rel b VDIdentification Methods c
1β-Myrcene995-6.99MS, RI
2p-Cymene10871.96-MS, RI
3Sylvestrene1028-2.99MS, RI
4β-Trans-ocimene1041-2.18MS, RI
5Chrysanthenol10691.53-MS, RI
6Linalool10998.0451.56MS, RI
7Myrcenol11201.27-MS, RI
8β-Terpineol11421.22-MS, RI
9α-Ocimene1051-2.86MS, RI
10(Z)-Ocimenol11535.06-MS, RI
11Є-Ocimenol11648.99-MS, RI
12Piperitone11720.94-MS, RI
13Terpinen-4-ol11751.722.36MS, RI
14α-Terpineol118944.1822.45MS, RI
15γ-Terpineol11953.19-MS, RI
16p-Menth-2-en-7-ol, cis12053.42-MS, RI
18p-Menth-1-en-9-al12168.49-MS, RI
19(R)-Lavandulyl acetate1230-1.53MS, RI
20p-Mentha-1(7),8(10)-dien-9-ol12481.6-MS, RI
21Myrtanol12561.71-MS, RI
22Guaiol1257-5.22MS, RI
23Phellandral12730.7-MS, RI
24Menthofuran12971.08-MS, RI
25p-Mentha-1, 4-dien-7-ol13291.13-MS, RI
26β-Damascenone13853.781.85MS, RI
Compounds are listed by retention time (RT). RI ᵃ: Retention index calculated using a homologous series of n-alkanes. % rel ᵇ: Relative abundance expressed as the percentage of each peak area relative to the total peak area from hydrodistillation (HD) or vacuum distillation (VD) distillate. Identification c: Based on mass spectra (MS) comparison with the NIST database and matching RI values with literature data.
Table 2. Minimal inhibitory concentrations (MICs) of hydrodistillation (HD) distillate, vacuum distillation (VD) distillate, and bound terpene (BT) concentrate against A. flavus.
Table 2. Minimal inhibitory concentrations (MICs) of hydrodistillation (HD) distillate, vacuum distillation (VD) distillate, and bound terpene (BT) concentrate against A. flavus.
Concentration (µL/mL)Zone of Inhibition (mm)
HD distillate100ND
50ND
25ND
12.5ND
6.25ND
VD distillate100ND
50ND
25ND
12.5ND
6.25ND
BT concentrate10026.0 ± 1.0 a
5021.0 ± 1.0 b
2518.0 ± 2.0 c
12.515 ± 1.0 d
6.25ND
Different letters within a column indicate significant differences between means (p < 0.05). ND: not detected.
Table 3. Effects of hydrodistillation (HD) distillate, vacuum distillation (VD) distillate, and bound-form terpene (BT) concentrate at different concentrations on A. flavus mycelial growth.
Table 3. Effects of hydrodistillation (HD) distillate, vacuum distillation (VD) distillate, and bound-form terpene (BT) concentrate at different concentrations on A. flavus mycelial growth.
Extraction MethodConcentration (µL/mL)Mycelial Growth Inhibition (%)
HD distillate100−14.80 ± 9.44 a
VD distillate1000.00 ± 0.00 b
10055.56 ± 4.53 d
BT concentrate 5042.59 ± 10.48 c
2542.59 ± 6.92 c
12.539.63 ± 3.66 c
Different letters indicate a significant difference between data (p < 0.05).
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MDPI and ACS Style

Rodriguez, G.; Thapaliya, M.; Bui, D.; Malekian, F.; Adhikari, A.; Xu, Z. Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage. Agronomy 2025, 15, 2270. https://doi.org/10.3390/agronomy15102270

AMA Style

Rodriguez G, Thapaliya M, Bui D, Malekian F, Adhikari A, Xu Z. Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage. Agronomy. 2025; 15(10):2270. https://doi.org/10.3390/agronomy15102270

Chicago/Turabian Style

Rodriguez, Gabriela, Manish Thapaliya, Duyen Bui, Fatemeh Malekian, Achyut Adhikari, and Zhimin Xu. 2025. "Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage" Agronomy 15, no. 10: 2270. https://doi.org/10.3390/agronomy15102270

APA Style

Rodriguez, G., Thapaliya, M., Bui, D., Malekian, F., Adhikari, A., & Xu, Z. (2025). Free- and Bound-Form Terpenes in Sweet Potato Peel and Their Antifungal Activity Against Aspergillus flavus-Induced Tomato Spoilage. Agronomy, 15(10), 2270. https://doi.org/10.3390/agronomy15102270

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