2024 Volume 93 Issue 3 Pages 263-272
Unpleasant odors are crucial in terms of consumer acceptance. However, the volatile compounds responsible for the unpleasant rubbery odor in green papaya have not been thoroughly documented. Therefore, the primary objective of this study was to identify these key volatile compounds and examine how they are influenced by different papaya cultivars, harvest seasons, and fruit maturity stages. Using gas chromatography-olfactometry (GC-O), five odorants were identified as having a rubbery odor, with benzyl isothiocyanate and 2-cyclohexen-1-one showing a significant positive correlation with rubber-like odor scores (r > 0.8). In the ‘Khaek Nuan’ cultivar, known for its strong rubber odor, the intensity of the rubber odor and the concentrations of (E)-1,2-cyclohexanediol were higher in cool season fruits and at the immature stage. Conversely, the ‘Yellow Flesh’ cultivar, which has a mild rubber odor, exhibited no significant effect of season or maturity on the unpleasant rubber odor or the presence of benzyl isothiocyanate and (E)-1,2-cyclohexanediol. By specifically targeting these odorants, effective strategies can be developed to mitigate or minimize the unpleasant smell through approaches such as genetic modification, management practices, or postharvest interventions.
Green or unripe papaya (Carica papaya L.) is well-known for its health benefits, making it a popular choice among health-conscious individuals. It contains abundant enzymes, such as papain, that facilitate digestion and possess anti-inflammatory properties (De Oliveira and Vitória, 2011; Saeed et al., 2014). Moreover, it is a good source of fiber, vitamins A and C, and antioxidants (Saeed et al., 2014). With its low-calorie content of only 0.43 kcal·g−1 (USDA, 2019), it is frequently consumed as a healthy alternative to rice noodles and serves as the primary ingredient in the iconic Thai green papaya salad. The global production of papaya has doubled from 7.3 MT in 2000 to 13.8 MT in 2022 (FAOSTAT, 2022). The strategic practice of harvesting green papaya while still immature, taking into account its shorter cultivation period, makes green papaya an appealing choice for both small-scale and large-scale farmers. Papaya breeders face challenges to enhance existing cultivars to meet the specific demands and expectations of the supply chain.
Breeding programs for green papaya cultivars primarily aim to improve shelf-life and flavor attributes, whereby texture and aroma serve as the two key factors influencing the overall taste experience of green papaya. Notably, while the evaluation of shelf-life and texture can be easily accomplished through date monitoring and a texture analyzer, respectively, aroma assessment is more subjective, as it requires the identification of active odor compounds to accurately evaluate this trait. Precise trait evaluation plays a significant role in the process of selective breeding, as it enables the targeted enhancement of desired attributes for crop improvement.
The unpleasant odor from the flesh of green papaya has been acknowledged by retailers and consumers, and has an influence on consumer acceptance and future purchasing intentions (Ulrich and Wijaya, 2010; Wijaya and Feng, 2013; Zhou et al., 2022). Similar to other fruits containing latex, such as mango, odor-active compounds in the latex contribute to the overall aroma of the flesh, as they share common volatile organic compounds (VOCs). However, the specific aromatic components of papaya latex and their relationship to the olfactory sensory characteristics of papaya flesh have not been documented. More than 400 VOCs have been identified in papaya flesh; however, only some are recognized as the flavor contributors (Wijaya and Feng, 2013; Zhou et al., 2022). The aroma of papaya fruit is influenced by factors such as harvesting stage and cultivar (Fuggate et al., 2010; Nakamura et al., 2007; Tang, 1971; Zhou et al., 2022). At immature and mature green stages, 2-ethyl-1-hexanol with a characteristic sweet, oily and weak rose-like odor is predominant, while in fruit from the quarter-ripe stage onwards, abundant compounds are ethanol, ethyl butanoate, ethyl hexanoate, and ethyl benzoate with characteristic sweet, ripe fruit notes, banana-like aromas, and a fruity odor (Fuggate et al., 2010). Benzyl isothiocyanate, which contributes a pungent, mustard-oil-aroma and linalool, with its floral odor characteristics, have been reported as the key impact odorants in ripe and unripe papayas (Fuggate et al., 2010; Jirovetz et al., 2003; Pino, 2014; Zhou et al., 2022).
Papaya latex is produced in almost all parts of a tree, including the fruit skin, and remains isolated in latex vessels or laticifers. The latex contains water, carbohydrates, proteins (especially papain, peroxidase, and peptidase), salts, lipids, ash, and biomolecules such as glutathione (El Moussaoui et al., 2001; Macalood et al., 2013). It has been widely studied for its benefits and potential uses in the agricultural and pharmaceutical industries (Saeed et al., 2014). However, to the best of our knowledge, no comprehensive study has been undertaken to elucidate the active volatile compounds present in papaya latex.
This research aimed to identify the key volatile compounds that contribute to the unpleasant rubber odor of papaya flesh. Additionally, we investigated the influence of papaya cultivars, harvest seasons, and fruit maturity stages on the differential expression of candidate VOCs. The identification of these key aroma compounds provides invaluable information that can guide breeding efforts to improve the flavor of green papaya.
Four papaya cultivars, including ‘Khaek Nuan’, ‘Khaek Dam Kaset’, ‘Khaek Dam Damnoen’, and ‘Yellow Flesh’ (Fig. 1A), were cultivated in experimental fields situated in Kamphaeng Saen district, Nakhon Pathom Province, Thailand (14°02'07.0"N, 99°58'58.2"E). The harvest took place in February 2022, the dry season, characterized by an average temperature of 27 ± 5°C, RH of 72 ± 17%, and no rainfall. Additionally, ‘Khaek Nuan’ and ‘Yellow Flesh’ were also harvested in September 2022 during the wet season, which experienced an average temperature of 29 ± 3°C, RH of 77 ± 9%, and 185 mm of rainfall.
Four cultivars including ‘Khaek Nuan’, ‘Khaek Dam Kaset’, ‘Khaek Dam Damnoen’, and ‘Yellow Flesh’ (A) and two distinct stages of maturity including the immature green stage and the breaker stage (pictures of ‘Khaek Nuan’) (B) were investigated. The latex was collected after making four longitudinal scratches at a depth of 3 mm with a stainless-steel knife (C).
The fruits were harvested from eight-month old trees at two distinct stages of maturity. The first stage, known as the immature green stage, occurred approximately 75 days after anthesis (DAA). At this stage, the fruits exhibited a dark green peel, white flesh, and contained seeds. The second stage, referred to as the breaker stage, occurred at approximately 150 DAA. During this stage, the fruits displayed two to three yellow stripes at the blossom end, along with black seeds and colored flesh (Fig. 1B). On the experimental day, fruit were harvested at 8:30 am, then placed in moist boxes and transported to a laboratory within 3 h. On arrival, fruit were washed and sorted according to maturity, size, and absence of defects. Unless otherwise stated, all experiments were carried out in six biological replicates. One fruit per tree of each cultivar was used as a biological replicate.
Collection of fresh latexFresh latex was collected following the method described by Macalood et al. (2013) with some modifications. Initially, four longitudinal scratches at a depth of 3 mm (to avoid contamination of volatiles from flesh tissue) were carefully made on the surface from the stylar end to the blossom end of a fruit using a stainless-steel knife. Latex was allowed to drip into a collecting cup for 5 min and was used for further extraction of volatile profiling within 10 min after tapping (Fig. 1C).
Extraction of VOCs by simultaneous distillation extraction (SDE)VOCs in the papaya flesh and latex were extracted by SDE following the method adapted from Pino (2014) and Wieczorek et al. (2020). Papaya flesh (approximately 0.5 × 0.5 × 0.5 cm) was prepared by longitudinally slicing the middle part of a peeled papaya fruit. The experiment was carried out in four replicates, in which each replicate was a pooled sample from three fruits. Samples were placed into two layers of a high-density polyethylene bag, then frozen in liquid nitrogen for 5 min. The frozen samples were homogenized for 1 min using a household blender (Otto Kingglass, Taiwan). Then, either 10 g of papaya flesh or 4 g of papaya latex was introduced into a flask and 10 mL of saturated sodium chloride, 4 μL of 3.8 mM 2-octanol (internal standard) (Lieb et al., 2018), and 20 mL dichloromethane as an extraction solvent were added. The mixture was then stirred at 200 rpm in an ice bath (~4°C) for 30 min before the volatile aqueous phase was separated using a separating funnel. The sample was then re-extracted twice using 20 mL dichloromethane for each extraction to obtain a 60 mL total volume. The volatile extract was dehydrated over anhydrous sodium sulfate and concentrated to 1 mL using a Vigreux column in a water bath at 45°C and then gently concentrated using a nitrogen stream to 200 μL. The concentrated extract was passed through a 0.45 μm nylon syringe filter into a 1.5 mL vial with an insert and sealed with a Teflon-line screw cap before storing at −20°C until analysis.
Analysis of VOCs by gas chromatography-mass spectrometry (GC-MS) on DB wax and HP-5MS columnsThe extracts were identified for VOCs by a GC-MS system. The gas chromatography system consisted of an Agilent 7890B Chromatograph (Agilent Technologies, Wilmington, DE, USA), equipped with a polar DB wax column of 30 m × 0.25 mm i.d. × 0.25 μm film thickness (J&W Scientific, Folsom, CA, USA) and a mass spectrometry detector (Agilent 5977B MSD; Agilent Technologies). Two μL of each sample were injected into the hot splitless mode (220°C, 1.2 min valve delay). The oven temperature was programmed from 40 to 220°C at a rate of 5°C·min−1 with initial and final holding times of 2 and 5 min, respectively. The ultra high purity helium carrier gas flow rate was 2 mL·min−1. The mass spectrometry (MS) conditions were programmed as follows: transfer line temperature, 250°C; ionization voltage, 70 eV; mass range (scan mode) 35–350 amu. Compounds were identified by comparing their mass spectra with the National Institute of Standards and Technology (NIST 17) database library and retention indices (RI) of each compound calculated against n-alkanes (C8–C40) reference standards. Quantitative analysis was performed using the response factor and area ratio of each volatile compound against the internal standard (2-octanol).
To identify more VOCs, the extracts were analyzed by a GC-MS (GC 2010 QP; Shimadzu, Tokyo, Japan) on a nonpolar HP-5MS column of 30 m × 0.25 nm i.d. × 0.25 μm film thickness (Agilent Technologies). One mL of each sample was injected into the hot splitless mode (220°C). The oven temperature was programed as previously described with initial and final holding times of 5 and 5 min, respectively. Helium was used as the carrier gas at a flow rate of 2 mL·min−1. The MS was operated as follows: ionizing energy 70 eV and mass range (scan mode) 25–500 amu. Identification of compounds was based on a comparison of their mass spectra with the NIST 107.LIB database library and RI of each compound calculated against n-alkanes (C8–C40) reference standards. Quantitative analysis was performed using the response factor and area ratio of each volatile compound against the internal standard (2-octanol).
Analysis of odorants by a GC-olfactometry (GC-O) technique on a DB wax columnVolatile aroma compounds (odorants) in papaya flesh and latex were determined using the GC-O technique and extract without adding an internal standard. The extracted samples (2 μL) were introduced into a GC-O system that was equipped with a DB wax column and analyzed following the method described by Pino (2014). The compounds were determined by three experienced panelists using a sniffing port (OPD3, Gerstel Inc., Mulheim, Germany) attached to the GC system to obtain the odor characteristics.
A training session was performed prior to the experimental session for the panelists to be familiar with the descriptions of aroma compounds in the samples (Noble et al., 1984; Talavera-Bianchi et al., 2010). During sniffing, the panelists were asked to record detection time, odor description and odor intensity using a four-point intensity scale; 1) weak, 2) moderate, 3) strong and 4) very strong. Each sample was sniffed in triplicate by each panelist, and aroma compounds were reported only if detected by at least two panelists. To confirm the sniffing results, the RI values of each compound, which were calculated from the olfactometry runs, were compared with RI data from the mass spectrometer. The odor activity value (OAV) of each compound was calculated by dividing the volatile concentration by its odor threshold obtained from previous literature reviews (van Gemert, 2011).
Analysis of rubber odor scores by sensory evaluationThe rubber odor scores of papaya flesh were evaluated by a group of five trained panelists. Initially, each panelist compared the papaya flesh aroma with a reference sample of latex from papaya fruit. They then assigned scores on a 5 cm line-scale, adapted from Aliani et al. (2011) and Reinbach et al. (2011), where 0 represented no rubber odor and 5 indicated a very strong rubber odor. Subsequently, the correlation between the rubber odor scores and the active volatile compounds resembling rubber odor, identified through GC-MS analysis of the papaya flesh samples, was calculated using Pearson’s correlation.
Statistical analysisThe latex content experiment was conducted using a completely randomized design (CRD). Statistically significant differences were calculated using SPSS software program version 15 for Windows (SPSS Inc., IBM Company, Chicago, IL, USA) with analysis of variance (ANOVA) followed by Tukey’s multiple range test at P < 0.05. Pearson’s correlation coefficient was also analyzed by SPSS software.
The ‘Kheak Nuan’ papaya cultivar is widely used in Thai green papaya salad, but it emits a strong and unpleasant rubber-like odor. Therefore, this cultivar was chosen to identify the key aroma compound responsible for the rubber odor. Volatile compounds were collected from both the flesh and the papaya’s latex to ensure the capture of all compounds associated with the rubber odor. GC-MS analysis was performed after SDE using DB wax and HP-5MS columns to expand the range of compounds that could be analyzed. Subsequently, GC-O was used to characterize the key odor compounds.
A total of 43 volatile compounds, representing several chemical classes including volatile carboxylic acids, alcohols, aldehydes, aliphatic ketones, aromatic hydrocarbons, phenol derivatives, cyclic derivatives containing nitrogen or sulphur, furan, and fatty acid esters, were identified (Table 1). Among these compounds, 23 compounds were found to be present in both flesh and latex (Table 1), while the specific odor descriptions of only 19 compounds could be characterized by GC-O (Table 2). A variety of odor characteristics was detected including natural rubber, herb, green, musty, fecal, bitter, latex, nutty, pungent, meaty, soup, bitter, ferment, sour, salty, metal, and floral aromas. The active compounds responsible for the aroma of papaya were those with high odor activity values (OAV). An OAV exceeding 1 signifies the potential for humans to perceive the compounds as having a notable aromatic impact. Among the identified odorants, five, namely 2-cyclohexen-1-one, 2-cyclohexen-1-ol, cyclohexylbenzene, (E)-1,2-cyclohexanediol, and benzyl isothiocyanate, were found to contribute to the rubber odor characteristics and were consistently observed in both the flesh and latex (Table 2). Notably, benzyl isothiocyanate exhibited the highest OAV (625) (Table 2) and the highest positive correlation with rubber smell scores (r = 0.837) (Table 3), as evaluated by the panelists. Additionally, 2-cyclohexen-1-one and (E)-1,2-cyclohexanediol showed a significant correlation with rubber smell scores in green papaya flesh (r = 0.598 and 0.583, respectively) (Table 3). In contrast, 2-cyclohexen-1-ol and cyclohexylbenzene emitted a rubber odor, but their correlation with rubber smell scores was not significant (r = 0.225 and 0.364, respectively) (Tables 2 and 3).
Volatile compounds of ‘Khaek Nuan’ papaya latex and flesh sampled at immature stages. VOCs identified by GC-MS analysis on DB wax and HP-5MS columns after SDE.
Odor active VOCs in ‘Khaek Nuan’ papaya latex and papaya flesh identified by GC-O on a DB wax column after simultaneous distillation extraction (SDE).
Pearson’s correlation coefficient (r) and probability (P-value) of rubber odor scores from sensory evaluation and rubber odor-like active volatile compounds identified by GC-MS of papaya flesh samples.
Decanal, benzyl nitrile, and indole were also identified in the papaya with characteristic green, pungent, and musty or fecal odors, which could potentially contribute to the unpleasant odors in the fruit. However, it is worth noting that these compounds exhibited high aroma thresholds (ranging from 40–1,200) and were absent from the papaya flesh (Table 2). Interestingly, some unknown compounds with distinct odors such as chocolate, raw coffee, meaty soup, metal, sweet, and fruity were detected in both papaya flesh and latex (Table 2). These compounds may also play a role in the unpleasant odor in papaya flesh; however, further investigation is necessary to fully understand their contribution.
Validation of odors of rubber-like compounds in papaya fruit conducted considering various factors including different papaya cultivars, seasons, and stages of maturityTo confirm the contribution of benzyl isothiocyanate, 2-cyclohexen-1-one, and (E)-1,2-cyclohexanediol to the unpleasant rubber-like odor, a series of investigations was conducted to explore various scenarios that could influence the intensity of the rubber smell. These specific rubber-like compounds were closely observed and analyzed. Among the different papaya cultivars examined, ‘Khaek Nuan’ fruit exhibited the highest scores for rubber-like odor, which coincided with the highest concentration of 2-cyclohexen-1-one, (E)-1,2-cyclohexanediol, and benzyl isothiocyanate (Table 4). Similarly, ‘Khaek Dam Kaset’ had high rubber-like odor scores, characterized by elevated levels of 2-cyclohexen-1-one and (E)-1,2-cyclohexanediol, albeit with a lower concentration of benzyl isothiocyanate. Comparatively, ‘Khaek Dam Damnoen’ did not significantly differ from ‘Khaek Dam Kaset’ in terms of odor attributes, except for a relatively lower concentration of 2-cyclohexen-1-one. On the other hand, ‘Yellow Flesh’ papaya exhibited the lowest rubber-like odor, which corresponded to the lowest concentration of 2-cyclohexen-1-one, (E)-1,2-cyclohexanediol, and benzyl isothiocyanate (Table 4).
The effect of different harvest periods, maturity stages and cultivars on latex content, rubber odor scores from sensory evaluation and rubber odor-like active volatile compounds identified by GC-MS.
In the second scenario, the influence of maturation on the odor profiles was investigated. It was observed that as ‘Kheak Nuan’ fruit reached the breaker stage, the rubber odor decreased, along with a reduction in 2-cyclohexane-1-one and benzyl isothiocyanate levels. In contrast, during maturation, the ‘Yellow Flesh’ cultivar exhibited an unpleasant smell accompanied by the presence of malodorous compounds. Furthermore, two repeated experiments were conducted, varying the harvesting periods, to assess the impact of seasons on the unpleasant smell and specific malodorous compounds. The results revealed that seasons did not significantly affect the unpleasant smell or the levels of 2-cyclohexen-1-one and benzyl isothiocyanate. However, they did have an impact on the accumulation of (E)-1,2-cyclohexanediol (Table 4). Higher levels of (E)-1,2-cyclohexanediol were observed in the ‘Kheak Nuan’ cultivar when harvested at the immature stage in February, while no such effect was observed in the ‘Yellow Flesh’ cultivar (Table 4). Moreover, significant positive correlations between rubber odor scores and rubber odor-like active volatile compounds, namely 2-cyclohexen-1-one (r = 0.844) and benzyl isothiocyanate (r = 0.805), confirmed that these two compounds caused the unpleasant rubbery odor in papaya flesh (Table 5).
Pearson’s correlation coefficient (r) and probability (P-value) were calculated to assess the relationship between rubber odor scores obtained from sensory evaluation and the presence of rubber odor-like active volatile compounds identified by GC-MS in papaya flesh samples.
To address the cause of the unpleasant rubber odor in green papaya fruit, it is crucial to identify the key volatile compound responsible for this specific odor. The current study successfully identified benzyl isothiocyanate and 2-cyclohexen-1-one as the predominant compounds associated with the unpleasant rubber-like odor in papaya. This conclusion was supported by its high concentration, OAV, and strong positive correlation with the outcomes derived from sensory panelist assessments conducted under various scenarios encompassing diverse papaya cultivars, seasons, and maturity stages.
While benzyl isothiocyanate has previously been recognized as an important odorant contributing to the aroma of both green and ripe papaya flesh (Jirovetz et al., 2003; Pino, 2014; Tang, 1971; Wijaya and Feng, 2013; Zhou et al., 2022), its role as an unpleasant rubber-like odorant in rubber-bearing plants has not been previously documented. Thus, this study serves as the first report establishing the involvement of benzyl isothiocyanate in both papaya flesh and latex, thus highlighting its contribution to the undesirable rubber odor in papaya flesh. Benzyl isothiocyanate is synthesized through enzymatic hydrolysis of glucosinolates, a process triggered by tissue disruption. When plant tissues are damaged, the enzyme myrosinase is activated, initiating the conversion of glucosinolates into various compounds, including benzyl isothiocyanate. This enzymatic reaction occurs rapidly upon tissue damage and is often accompanied by the release of pungent-smelling volatile compounds (Rossetto et al., 2008). Apart from its distinctive aroma, benzyl isothiocyanate has multiple biological functions, serving as a defense mechanism against herbivores, pathogens, and competing plants (Kissen et al., 2016). This compound exhibits antimicrobial, antifungal, and insecticidal properties, which contribute to its role in plant protection (Jioe et al., 2022).
2-Cyclohexen-1-one was also identified as a key compound responsible for the rubber-like odor, and its presence in papaya flesh is reported here for the first time. Discrepancies in identification of volatile compounds across previous reports could potentially be attributed to variations in the studied cultivars under investigation or variances in the analytical techniques employed (Jirovetz et al., 2003; Lieb et al., 2018). Notably, previous studies have reported the presence of 2-cyclohexen-1-one in rosemary (Rosmarinus officinalis) and fennel (Foeniculum vulgare) (Asadi, 2022; Gulec et al., 2013). This compound was detected in papaya flesh and latex, demonstrating a significant correlation with the studied odor and thereby supporting its role as a key odorant contributing to the unpleasant rubber-like odor in green papaya. To further ascertain the significance of the above two compounds in contributing to the rubber-like odor in papaya, additional research with a specific focus on determining their odor thresholds will be necessary.
Decanal, benzyl nitrile, and indole have been identified as contributors to the unpleasant odors in papaya. Notably, these compounds were exclusively detected in the latex and were absent from the flesh, rendering them important indicators of odor in papaya latex. Indole, widely recognized for its strong fecal and musty odor, is commonly found in emissions emanating from natural rubber (Kamarulzaman et al., 2019) and livestock waste (Ranau and Steinhart, 2005). In contrast, decanal and benzyl nitrile possess relatively high odor thresholds, quantified at 245 and 1,200, respectively, with OAV values below 1. However, GC-O analysis revealed the existence of a floral scent attributed to decanal and a pungent odor associated with benzyl nitrile. It is worth mentioning that the odor thresholds documented in this study may diverge from those reported in other publications due to disparities in the analytical methodologies employed (van Gemert, 2011). Different approaches and techniques used for odor threshold determination can yield variations in the recorded values. Therefore, for accurate interpretation and assessment of the results, it is crucial to consider the specific methodologies used to compare odor thresholds across different studies.
Unidentified compounds contributing to green, herbaceous, and potato-like odors were detected using GC-O. Interestingly, no distinct peaks above the baseline were observed in the GC-MS results. This observation can be attributed to the possibility that these compounds have extremely low odor thresholds. Consequently, even at trace concentrations where they remain unidentified by GC-MS, they can still be perceived by GC-O. Notably, certain compounds such as pyrazines (RI = 1231) (Ulrich et al., 1998) or methional (RI = 1465) (Mahattanatawee et al., 2007) are recognized for imparting a characteristic potato-like odor in plant extracts. Compounds belonging to the pyrazine or methional groups often exhibit markedly low odor thresholds, enabling their detection by GC-O, despite their inability to be identified by GC-MS due to their limited concentrations.
A comparative analysis of the VOC profiles associated with the rubber-like odor in papaya, as investigated in this study, and previous research on rubber-producing plants such as Hevea brasiliensis (Juntarachat et al., 2013; Kamarulzaman et al., 2019), revealed notable differences and some similarities between the two crops. Several compounds were commonly detected in both crops, including eugenol, hexanal, decanal, 2-chlorocyclohexanol, benzyl nitrile, and indole. However, specific compounds unique to papaya, such as benzyl isothiocyanate, 2-cyclohexen-1-one, (E)-1,2-cyclohexanediol, 2-cyclohexen-1-ol, and cyclohexylbenzene were not observed in H. brasiliensis. Conversely, major components associated with the characteristic odor of natural rubber such as, acetic acid, phenylacetic acid, valeric acid, and butanoic acid, were not detected in papaya during the course of this investigation. The absence of these volatile compounds may be due to differences in sample preparation. For rubber studied by Juntarachat et al. (2013), the natural rubber odor analysis involved processing naturally coagulated rubber obtained through drying at 115°C for 4 h or using acetic acid with a neutral hydroxylamine sulfate solution. The presence of carboxylic acids could be due to residual amounts from the coagulation treatment and carbohydrate fermentation from microbial activity in the latex during drying treatment. In contrast, our study focused on identifying volatile compounds in fresh papaya flesh and latex without any processing steps.
Odor rubber-like compounds in papaya fruit across different papaya cultivars, seasons, and maturity stagesThe presence of unpleasant smells and two aroma-active compounds responsible for the rubber odor varied among the four cultivars studied. The variation in rubber-like odor among cultivars may be attributed to differences in latex contents. Papaya latex, stored under pressure within laticifers, is released abundantly when laticifers are injured (El Moussaoui et al., 2001). Laticifer cells are predominantly located in the cortex near the papaya peel, while they are loosely arranged in the flesh (unpublished data). Therefore, the different rubber smells among cultivars may be due to varying numbers of laticifer vessels or amounts of latex contents. Another possible explanation involves the production of glucosinolate, which serves as the precursor of benzyl isothiocyanate, or the presence of different glucosinolate degrading enzymes (myrosinase) that break down glucosinolate into benzyl isothiocyanate.
Benzyl isothiocyanate was reported to have a role in the defense mechanism against Hawaiian fruit flies and Phytophthora parasitica (Seo et al., 1983; Tang, 1971). Thus, it is speculated that the ‘Khaek Nuan’ cultivar could be more tolerant to insects and diseases since it had high benzyl isothiocyanate contents. Unfortunately, there is no information available on these aspects of the studied cultivars, and further investigation is warranted. Since benzyl isothiocyanate contributes to an unpleasant odor, the balance between improving pest resistance and consumption qualities poses a challenge for breeding programs.
The impact of seasons on rubber odor and VOCs was also found to be genotype-specific. ‘Khaek Nuan’ exhibited higher levels of rubber odor and (E)-1,2-cyclohexanediol in fruits harvested during the dry season in February compared to those harvested during the wet season in September. However, this response was not observed in the ‘Yellow Flesh’ cultivar. In contrast, benzyl isothiocyanate content of ‘Kheak Nuan’ and ‘Yellow Flesh’ showed no significantly different between dry and wet season. Previous studies have reported a close correlation between the dynamic change in glucosinolate and myrosinase with temperature and seasonal variation (Kissen et al., 2016). However, it is worth noting that different genotypes can elicit distinct responses in terms of glucosinolate and myrosinase activity to temperature variations. For instance, Jioe et al. (2022) observed no significant difference in the benzyl isothiocyanate content of ‘Tainung No. 2’ papaya between warm and cool seasons. In contrast, Guo et al. (2014) demonstrated enhanced myrosinase activity in broccoli under high temperatures ranging from 25–30°C.
The rubber odor and VOCs of the cultivar with a strong rubber smell (‘Khaek Nuan’) decreased as the fruit matured, while the odor of the cultivar with a mild rubber smell (‘Yellow Flesh’) remained relatively constant throughout both the seasons and maturity stages. Similar findings were reported by Almora et al. (2004), Flath et al. (1990), and Fuggate et al. (2010), indicating a decrease in benzyl isothiocyanate as ripening progressed. Papaya is a climacteric fruit, displaying a burst in respiration and ethylene biosynthesis at the onset of ripening. Although the present study did not check the methionine and glucosinolate contents, the accumulated methionine may be utilized for ethylene synthesis in ripening fruits, leading to reduced availability for glucosinolate biosynthesis, which serves as a precursor for benzyl isothiocyanate.
ConclusionsThe most odor-active compounds responsible for the unpleasant rubber-like odor in papaya flesh were benzyl isothiocyanate and 2-cyclohexen-1-one. The rubber-like odor varied depending on papaya cultivars. In ‘Khaek Nuan’ cultivar, a stronger rubber odor was noted in the immature stage and in the cool season. On the other hand, the maturity and seasons had no effect on the unpleasant rubber odor or the presence of benzyl isothiocyanate and 2-cyclohexen-1-one in ‘Yellow Flesh’. Identifying and characterizing the specific compound responsible for the rubber-like odor in papaya flesh opens avenues for further research aimed at manipulating scent profiles by selectively adding or removing specific odorants. By targeting the specific odorant, effective strategies can be devised to mitigate or minimize the unpleasant smell, whether through genetic modification, management practices, or postharvest interventions.
The authors are indebted to Ms. Uthaiwan Doung-ngern from the Tropical Vegetable Research Center, Faculty of Agriculture at Kamphaeng Saen, Kasetsart University, Kampheang Sean Campus for help with papaya fruit material. This research was supported by the Postdoctoral Fellowship at Kasetsart University and the Postharvest Technology Innovation Center, Science, Research and Innovation Promotion and Utilization Division, Office of the Ministry of Higher Education, Science, Research and Innovation, Bangkok 10400, Thailand.
