From Waste to a Potential Food Resource: Evaluation of Papaya Trunk Xylem Rays in Temperate Cultivation Systems

Abstract

The use of underutilized biomass improves resource-use efficiency and reduces agricultural waste, particularly in temperate systems cultivating tropical crops. Papaya (Carica papaya L.), grown as an annual crop in these systems, produces substantial trunk biomass that is typically discarded after harvest. This study evaluated the potential of papaya trunk xylem rays as an edible resource through compositional, sensory, and functional analyses. Trunks were harvested at the end of the fruiting period (December) and after exposure to a cold wave (January) and were classified by organ types and maturity level. Xylem rays showed moisture and carbohydrate contents comparable to those of green papaya fruit, and were judged as edible by all panelists (100%) in December-harvested samples. However, exposure to a cold wave reduced sweetness and increased bitterness, resulting in decreased overall acceptability. Nevertheless, boiling effectively reduced bitterness and improved palatability even in cold-exposed samples. In addition, xylem rays exhibited higher total polyphenol content than green papaya fruit, while showing comparable DPPH radical scavenging activity. These results suggest that xylem rays have potential as an edible plant resource with antioxidant-related properties, contributing to resource-use efficiency and potentially providing opportunities for biomass valorization in temperate production systems.

1. Introduction

In recent years, the global agricultural workforce has been steadily declining owing to multiple factors, including structural changes in the industry, aging of farmers, and inherently demanding working conditions in agriculture [1]. To address labor shortages, various countermeasures have been implemented worldwide, such as farmland consolidation, the introduction of large-scale mechanization, and the application of information and communication technologies (ICT/IoT) to improve work efficiency [1]. However, in Asian regions, including Japan, where flat arable land is scarce and small-scale farms predominate, these measures have often proven ineffective [2,3]. For these smallholders, increasing labor efficiency is difficult; therefore, strategies to sustain agricultural operations increasingly depend on enhancing farm income rather than reducing labor requirements. Such strategies include improving crop yields, increasing the unit price of harvested produce, and diversifying outputs from the same unit area of farmland [3,4].

Among these strategies, diversification strategies present unique challenges for fruit -tree production systems, which are typically perennial crops, and their primary harvest is limited to fruit production. Although multi-cropping systems, such as intercropping vegetables or legumes between fruit trees, have been attempted [5,6], they frequently require additional labor inputs and are not always feasible for small-scale farms to implement. As an alternative strategy, the utilization of underutilized plant parts of certain fruit species to generate added value has been explored. For example, banana flower buds and pseudo stems are used in salads, stir-fries, and soups [7]. Furthermore, leaves are sometimes harvested for use as plates and wrappers [8]. Citrus and date palm by-products (e.g., leaves, peels, and stems) have been used to produce functional foods, dietary supplements, and other value-added products [9,10]. These approaches increase the proportion of marketable biomass and provide farmers with additional income sources.

Papaya (Carica papaya L.), a fast-growing herbaceous perennial fruit tree belonging to the family Caricaceae, is widely cultivated in tropical and subtropical regions, including India, Indonesia, the Dominican Republic, and Mexico [11]. Recently, papaya cultivation has been introduced into temperate regions, such as Japan, where the crop is managed as an annual crop because of its sensitivity to low temperatures [12]. In temperate climates where winter minima fall below 6 °C, papaya cannot survive outdoors, and open-field cultivation cannot meet the thermal requirements for producing ripe yellow fruit (approximately 1293–1488 growing degree days from flowering to harvest) [13]. Consequently, papaya cultivation in these regions is primarily limited to the harvesting of immature fruits (“green papaya”), which are consumed as vegetables [12].

A major issue with this cultivation system is the large quantity of trunk biomass discarded after harvest at the onset of winter. Although papaya leaves have been processed into herbal teas and the roots have been used in traditional medicine [14], there are no systematic reports on the utilization of papaya trunks. Interestingly, Folharini et al. reported that the medullary parenchyma, a part of the trunk of Vasconcellea quercifolia A. St-Hil, a wild papaya relative native to Brazil, contains substantial amounts of ash, protein, carbohydrates, fiber, and carotenoids, suggesting its potential as a nutritious food source for humans [15]. Papaya trunks, which do not lignify due to their herbaceous nature, may possess favorable properties for edible consumption.

If papaya trunks, which are currently discarded as waste, can be effectively utilized as food, this may increase the utilization of plant biomass in temperate papaya production systems. In addition, the use of papaya trunk tissues as edible resources may provide additional options for the utilization of plant materials harvested after fruit production. Therefore, this study aimed to preliminarily investigate the potential use of papaya trunk tissues, especially xylem-ray tissues, as a food resource through sensory evaluation and compositional analysis. In addition, suitable xylem-ray sections, harvest timing, and appropriate pre-treatment methods for food use were examined.

2. Materials and Methods

2.1. Plant Materials and Cultivation Conditions

All plant samples used in the present study were collected from a commercial papaya (Carica papaya L.) orchard located in Matsuzaki Town, Shizuoka, Japan (34°45′31″ N, 138°48′44″ E). The average, highest, and lowest temperatures in this town in 2023 were 17.5 °C, 35.0 °C, and −2.3 °C, respectively, with an annual precipitation of 1756.5 mm [16]. Papaya has been cultivated annually in this orchard since 2016. Approximately 20 cm tall papaya seedlings planted in 20 cm diameter pots were transplanted (2.0 m × 2.0 m) into the orchard every May, and immature green fruits were harvested from September to December every year. Irrigation was applied only at the time of transplantation, and subsequent cultivation depended on natural rainfall. Fertilization was conducted according to the grower’s conventional management practices.

This study consisted of two experimental phases conducted during different cultivation seasons. In Phase 1 (2020 season), two ‘Fruit Tower’ trees were randomly selected on 4 December 2020, during the final harvest period of green papaya before severe winter cold. These samples were used for preliminary screening of papaya trunk organs potentially suitable for food use based on proximate composition analysis.

In Phase 2 (2022–2023 season), three ‘Grande’ trees were randomly selected on 8 December 2022 and 24 January 2023 for detailed sensory and chemical analyses focusing on xylem-ray organs. This phase also evaluated the effects of the harvest period. Samples were collected in December 2022 under conditions similar to those in December 2020, whereas samples were collected in January 2023 after cold-wave exposure, when leaf necrosis occurred.

In both seasons, the trees were harvested by cutting the trunk approximately 10 cm above the ground. In addition, three green papaya fruits used as reference samples were randomly harvested during the commercial harvest season (November).

Because commercial green papaya cultivation in Japan is still developing, different cultivars were used only in Phase 1 (proximate composition analysis) due to limited cultivar availability. However, Phase 1 was conducted only as a preliminary screening of trunk organs potentially suitable for food use, whereas Phase 2 constituted the main experimental phase of this study. Therefore, although cultivar-related effects should be considered when interpreting the Phase 1 proximate composition results, they were not considered critical to the main objectives of the study.

2.2. Measurement of Biomass Distribution

After cutting down the trees on 8 December 2022, the weights of the trunks, leaves, and petioles were measured using a digital scale. Data are presented as the mean of three trees (n = 3).

2.3. Sampling Design and Tissue Classification

For the analysis of proximate composition, two trees collected in 2020 were processed as described below. Leaves, petioles, and fruits were removed, except for the trunk portion. The trunks were divided into three equal sections along the vertical axis of the trunk and classified as young, middle, and mature according to the degree of lignification from the apical to the basal region (Figure 1A). Each section was further separated into three organs: bark, fiber sheath, and xylem ray, based on visible structural characteristics with reference to the anatomical descriptions of papaya stems reported by Jiménez et al. [17] (Figure 1B). Organ fractions from each maturity-level section obtained from the two trees were pooled as described below (n = 1).

Similarly, trunks obtained from three trees collected on 8 December 2022 and 24 January 2023 were processed in the same manner, and only xylem-ray organs were used for subsequent analyses. Each tree was treated as an independent biological replicate (n = 3). On December 8, the weights of each maturity-level section (young, middle, and mature) of the trunk were measured individually. To calculate the total fresh weight of the xylem ray within each maturity-level section, the central 20 cm portion of each trunk was measured. The proportion of xylem rays in the measured 20 cm portion was multiplied by the total weight to estimate the total weight of the xylem rays in each maturity-level section.

For both the 2020 and 2022–2023 seasons, three green papaya fruits harvested during the commercial harvest season were peeled, and only the flesh was used (n = 3).

2.4. Sample Preparation for Proximate/Compositional, Sensory, and Chemical Analyses

For the proximate composition analysis, organ fractions from each maturity-level section were cut into approximately 0.5 cm squares. Samples obtained from the two trees were thoroughly pooled, adjusted to 100 g, and immediately stored at −60 °C. The frozen samples were subsequently transported to the Japan Food Research Laboratories (Fukuoka, Japan) at −15 °C.

For sensory evaluation, thin rectangles (2 cm × 1 cm × 0.3 cm) were sliced and used as the samples. Samples were randomly collected from three trees at each time. Raw samples were immediately stored at −60 °C, whereas pre-treated samples were stored at −60 °C after each pre-treatment described below. Before sensory evaluation, the frozen samples were thawed at 4 °C for 12 h and subsequently at 25 °C for 2 h.

For chemical composition analyses, including total polyphenol, water-soluble polyphenol, and nitrate nitrogen analyses, xylem-ray samples were cut into 0.5 cm squares, pre-frozen at −60 °C, and freeze-dried using a freeze-dryer (FDU series, EYELA, Tokyo, Japan) at −40 °C for 24 h under reduced pressure. The dried samples were then ground into powders using a mill. In contrast, the samples used for glucosinolate analysis were continuously stored at −60 °C without freeze-drying. For all analyses, samples were prepared separately for each tree.

All compositional data are expressed as g or mg per 100 g fresh weight.

2.5. Proximate Composition Analysis

The proximate composition (moisture, protein, lipid, carbohydrate, salt equivalent, sodium, ash, and energy content) was analyzed using methods consistent with Notification No. 139 of the Consumer Affairs Agency of Japan (“Food Labeling Standards”) [18]. Moisture content was determined using the vacuum-drying method. Protein content was determined using the combustion method, and the nitrogen content was converted to protein using a conversion factor of 6.25. Lipid content was determined using the acid hydrolysis method. Sodium was analyzed using atomic absorption spectrophotometry, and salt was calculated from the sodium content. Ash content was determined using the direct ashing method. Carbohydrate content was calculated as 100 minus the sum of moisture, protein, lipid, and ash. Energy was calculated using conversion factors of 4 kcal/g for protein, 9 kcal/g for lipids, and 4 kcal/g for carbohydrates. All measurements were conducted in duplicate as technical replicates, and mean values were used for data analysis.

2.6. Sensory Analysis

Sensory evaluation was conducted by eight untrained panelists from the Faculty of Agriculture, Shizuoka University. This study involved a simple sensory evaluation of edible horticultural products conducted with healthy adult participants. The study was non-invasive, involved minimal risk, and did not include the collection of medical data or biological specimens. All data were collected anonymously. According to the institutional guidelines for research involving human participants at Shizuoka University [19], studies of this nature—characterized by minimal risk and no collection of personally identifiable or medical information—are generally considered outside the scope of formal institutional review board approval.

The study was conducted in accordance with the principles of the Declaration of Helsinki (2013 revision). Prior to participation, all participants were fully informed about the purpose, procedures, potential risks, and data handling procedures, and informed consent was obtained. Participation was voluntary, and participants retained the right to withdraw at any time without penalty.

The sensory evaluation was conducted in the following order: (1) taste characteristics and edibility evaluation for raw samples; (2) comparison of taste among different maturity-level sections; (3) comparison of taste between harvest periods; and (4) evaluation after pre-treatment for late-harvest samples.

For the taste characteristics and edibility evaluation of the raw samples, two pieces of samples prepared from the mature section, harvested in December and January, respectively, were served on a plastic dish. Evaluations were conducted using a questionnaire, and five basic tastes (sweetness, sourness, saltiness, bitterness, and umami) were evaluated using a three-point scale (absence, slightly perceived, and strongly perceived). Edibility was evaluated based on the panelists’ subjective judgment as either edible or inedible.

The taste of different maturity levels was compared following the method described by Kondo et al. [20]. The evaluation was independently conducted for samples harvested in December and January, comparing the five basic tastes and overall acceptability across three sections: young, middle, and mature. A five-point relative scale was used, with a score of three representing the reference section. The categories for the five basic tastes were weak, slightly weak, same, slightly strong, and strong, whereas the overall acceptability was categorized as bad, slightly bad, same, slightly good, and good. Each section was evaluated relative to one of the other sections used as the reference sample. The final score for each sample was calculated as the mean value of the eight panelists’ ratings.

Similarly, taste comparisons between harvest periods were conducted using mature-section samples harvested in December and January. The evaluation was performed using the same five-point relative scale described previously. Reciprocal evaluations were conducted such that each harvest-period sample served alternately as the reference sample with a score of three.

Evaluation after pre-treatment for late-harvest samples was conducted only for bitterness, sweetness, and overall acceptability. Five pre-treatments were applied to xylem ray samples before storage at −60 °C (

Table 1. Five pre-treatment methods applied to late-harvest xylem rays.

Treatment Method	Solution z
Soaking	Water
	1% saline solution
Boiling	Water
	5% rice bran suspension
	0.2% sodium bicarbonate solution

^z All solutions were prepared using tap water.

). Samples were placed in plastic nets and soaked or boiled in treatment solutions at ten times the sample weight for 10 min. The soaking and boiling treatment temperatures were 25 and 100 °C, respectively. Pre-treated samples were evaluated using raw samples as references, and conversely, raw samples were evaluated using each pre-treated sample as a reference. After these evaluations, the panelists determined whether each sample was edible or inedible.

All evaluations were conducted without informing the panelists about the sample types, and the order of the samples served was randomized within each evaluation. The panelists rinsed their mouths with water after each sample to avoid carryover effects.

2.7. Chemical Composition Analysis

Xylem ray sections harvested at different periods were used to evaluate the total polyphenol, water-soluble polyphenol, nitrate nitrogen, and glucosinolate (derived isothiocyanate) contents, as well as antioxidant activity (DPPH radical scavenging activity). Green papaya fruit was also analyzed as a control. All analyses were performed on independent biological samples (n = 3), and technical replication was not conducted based on preliminary tests confirming low analytical variability.

For total polyphenol analysis, 0.1 g of freeze-dried samples were homogenized (12,000 rpm, 1 min) with 15 mL of 80% ethanol in a microcentrifuge tube using a homogenizer (PT33M, I.S.O. Co., Ltd., Kanagawa, Japan) and centrifuged at 10,000 rpm for 10 min using a microcentrifuge (MCF-1350, Sakuma, Tokyo, Japan). The total polyphenol content was measured using the Folin–Ciocalteu method [21] with the obtained supernatant. In accordance with established procedures, 1 mL of the extract was mixed with 1 mL of Folin–Ciocalteu reagent, and after 3 min, 1 mL of 10% sodium carbonate solution was added to terminate the reaction. The mixture was incubated at 25 °C for 1 h, and the absorbance was measured at 750 nm using a spectrophotometer (U-1900, Hitachi, Tokyo, Japan). The results were expressed as gallic acid equivalents (mg/100 g) based on a gallic acid calibration curve (0–50 μg/mL).

Water-soluble polyphenols and nitrate nitrogen were also extracted using the same method used to extract total polyphenols, although the extraction solvent was changed to distilled water. Water-soluble polyphenol content was measured using the Folin–Ciocalteu method, as described above. Nitrate nitrogen content was determined using an RQflex (plus 10, Merck, Darmstadt, Germany) according to the manufacturer’s protocol, with nitrate values converted to nitrate nitrogen using a factor of 0.226.

To estimate glucosinolate compounds, 1 g of frozen sample was placed in a 15 mL conical tube and manually crushed with 1 mL of distilled water using a plastic stick without homogenization. The mixture was then incubated at 30 °C for 30 min to allow endogenous myrosinase-mediated hydrolysis of glucosinolates into isothiocyanates. After incubation, the samples were centrifuged (5500 rpm, 5 min), and the supernatant was used for subsequent analyses. The resulting isothiocyanates were quantified using the Grote method, as described by Esaki and Onozaki (1980) [22]. Because the method determines isothiocyanates generated after glucosinolate hydrolysis, the obtained values were expressed as isothiocyanate equivalents rather than direct glucosinolate concentrations. Absorbance at 600 nm was measured and expressed as allylthiourea equivalents (mg/100 g) based on a calibration curve (0–150 μg/mL).

The extraction procedure for antioxidant activity analysis was similar to that used for total polyphenol analysis, except that 50 mg of freeze-dried sample was extracted with 1 mL of 80% ethanol. After centrifugation, the supernatant was collected and appropriately diluted to four concentrations based on preliminary experiments. Antioxidant activity was evaluated using a DPPH radical scavenging assay kit (D678, Dojindo Laboratories, Kumamoto, Japan). Aliquots (20 μL) of each diluted extract were incubated with DPPH solution for 30 min in the dark at room temperature, and the absorbance was measured at 517 nm using a microplate reader (MPR-A100; AS ONE, Osaka, Japan). IC₅₀ values were calculated, and the results were expressed as Trolox-equivalent antioxidant capacity (TEAC, µg TE/mL).

2.8. Statistical Analysis

Tukey–Kramer’s HSD test was used to evaluate differences among xylem ray sections of different maturity levels and green papaya fruit samples, and Student’s t-test was used to evaluate differences in the harvest period. The experimental unit consisted of samples collected from three independent plants (n = 3). No technical replication was performed for chemical analyses; each sample from an individual plant was treated as a biological replicate. No pooled samples were used for statistical analyses. Prior to statistical testing, normality and homogeneity of variance were confirmed. All analyses were conducted using JMPIN software (version 5.0.1; SAS Institute, Cary, NC, USA), and statistical significance was set at 5% or 1%.

For paired sensory comparisons, samples were evaluated bidirectionally by switching the reference sample. The reference value of the evaluation scale was set at 3. Differences were considered meaningful when one sample scored ≥ 3.5 while the reciprocal sample scored ≤ 2.5. These paired evaluations were treated as dependent comparisons and were not analyzed as independent replicates for statistical testing.

3. Results

3.1. Distribution of Papaya Biomass Excluding Fruit at the End of Fruit Harvesting

Table 2. Distribution of papaya biomass, excluding fruit, at the end of fruit harvesting.

Biomass Type	Biomass Amount (kg/tree)
Trunk	19.2 ± 2.5 z
Petiole	6.1 ± 0.5
Leaf	5.5 ± 0.6

^z Average fresh weight calculated from 3 trees (n = 3).

shows the biomass quantities of the trunk, petioles, and leaves at the end of the papaya fruit harvest. The trunk accounted for 62% of the unused biomass and tended to be the largest component among the measured tissues after fruit harvest.

3.2. Differences in the Proximate Composition of Each Trunk Organ Based on Maturity, and a Comparison with Green Papaya

Moisture was the most abundant component in each trunk organ and in the fruit flesh (

Table 3. Proximate composition of papaya trunk organs at different maturity levels and comparison with green papaya fruit.

Plant Part	Organ	Maturity Level	Moisture	Carbohydrates	Protein	Lipid	Salt	Sodium	Ash	Energy
			g/100 g	g/100 g	g/100 g	g/100 g	g/100 g	mg/100 g	g/100 g	kcal
Trunk	Xylem rays	Young	89.8 z	7.5	1.1	0.1	0.03	11	1.5	35
		Middle	90.1	7.0	0.9	0.1	0.03	11	1.9	33
		Mature	92.8	4.7	0.9	0.2	0.02	6	1.4	24
	Fiber sheath	Young	83.7	13.5	1.2	0.1	0.03	10	1.5	60
		Middle	80.9	16.6	0.9	0.1	0.02	7	1.5	71
		Mature	81.4	16.1	1.1	0.1	0.01	5	1.3	70
	Bark	Young	83.5	12.0	2.0	0.1	0.00	3	2.4	57
		Middle	83.0	12.1	2.0	0.2	0.00	2	2.7	58
		Mature	81.6	13.7	1.7	0.2	0.00	2	2.8	63
Fruit	Fresh	Green papaya	91.5	7.1	0.8	0.2	0.00	2	0.4	33

^z Values represent the mean of duplicate technical measurements. Trunk and fruit samples were collected from two trees or three fruits, respectively, and pooled before analysis (no biological replication).

). Within the trunk, regardless of maturity level, the xylem rays had the highest moisture content (89.8–92.8 g/100 g), comparable to that of the fruit flesh (91.5 g/100 g). Carbohydrates were the second most abundant component. Within the trunk, regardless of maturity level, the fiber sheath contained the highest amount of carbohydrates (13.5–16.6 g/100 g), whereas the xylem rays contained the lowest amount (4.7–7.5 g/100 g). Within the xylem rays, the mature section contained slightly fewer carbohydrates, but the content in this organ was very similar to that in the fruit flesh (7.1 g/100 g). Protein and lipid contents were low in both the trunk and fruit flesh, although the protein content was slightly higher in the bark than in the fruit flesh and other trunk organs. Salt content was scarcely detected in the fruit flesh and bark, whereas approximately 0.03 g/100 g was detected in the xylem rays. Sodium exhibited a similar distribution to salt, with low concentrations in the fruit flesh and bark, while approximately 6–11 mg/100 g was detected in the xylem rays. Ash content was highest in the bark and lowest in the flesh. The other trunk organs showed intermediate ash values. Energy content was the highest in the fiber sheath and bark, whereas the xylem rays exhibited low values, similar to those of the fruit flesh.

3.3. Variations in the Proportion of Xylem Rays Due to Differences in Trunk Maturity

The trunk was divided into three sections based on length; therefore, the fresh weight of the lower mature section with the larger diameter was greater than that of the other sections (Figure 2). The proportion of xylem rays relative to the trunk at each maturation level increased with increasing trunk maturation. The proportion of xylem rays in the young section was 18.8%, in contrast to 60.9% in the mature section.

3.4. Independent Investigation of Edibility and Sensory Characteristics of Xylem Rays at Each Harvest Period

To evaluate the taste characteristics of xylem rays, we investigated the presence or absence of taste perception for the five basic tastes (sweetness, bitterness, umami, sourness, and saltiness) and examined their edibility. The evaluation was conducted independently for the December- and January-harvested samples. Regardless of the harvest period, sweetness, bitterness, and umami were perceived by more than 50% of the panelists, whereas sourness and saltiness were perceived by less than half (Figure 3). The results for the five basic tastes fluctuated by harvest period, but while edibility was 100% in December, it dropped to 62.5% in January.

3.5. Direct Comparison of Acceptability and Sensory Characteristics of Xylem Rays at Each Harvest Period

Table 4. Comparison of strength of the five basic tastes and acceptability among different harvest periods of the papaya xylem rays.

Sensory Characteristics					Comparison Between Two Harvest Periods z
					December				January
Sweetness					3.8 ± 0.3 x				2.3 ± 0.4 w
Bitterness					2.1 ± 0.2				3.6 ± 0.4
Umami					3.4 ± 0.2				3.0 ± 0.2
Saltiness					3.0 ± 0.2				3.5 ± 0.2
Sourness					3.0 ± 0.0				2.9 ± 0.1
Overall acceptability					3.8 ± 0.2				2.5 ± 0.3
The meaning of color y
← Weak (for five basic tastes)				Same as reference				Strong (for five basic tastes) →
1	1.1~1.5	1.6~2.0	2.1~2.5	2.6~2.9		3	3.1~3.4	3.5~3.9		4.0~4.4	4.5~4.9	5
← Bad (for overall acceptability)								Good (for overall acceptability) →

^z December samples were evaluated using January samples as the reference, and vice versa. ^y Values were rated on a 5-point scale (1 = weak/bad; 3 = same as reference; 5 = strong/good). The five basic tastes were sweetness, bitterness, umami, saltiness and sourness. ^x Data are presented as mean ± standard error (SE) of 8 panelists (n = 8). ^w Differences between the two harvest periods were considered meaningful only when the direction of the scores (color scale) changed after switching the reference sample.

shows the reciprocal comparisons of the overall acceptability and intensities of the five basic tastes between the two harvest periods, with samples from each period alternately used as the reference. January-harvested samples showed lower overall acceptability than those harvested in December. Among the five basic tastes, both sweetness and bitterness were affected by the harvest period, and the December-harvested sample had a sweeter and less bitter taste than the January-harvested sample. Umami, saltiness, and sourness did not differ between harvest periods.

3.6. Direct Comparison of Acceptability and Sensory Characteristics of Xylem Rays by the Difference in Maturity

The mature section showed higher overall acceptability than the young and middle sections in December, whereas no differences in overall acceptability were observed among the sections in January (

Table 5. Comparison of strength of five basic tastes and acceptability among different maturity levels of the papaya xylem rays.

Harvest Season		Sensory Characteristics			Pairwise Comparisons Among Maturity Levels
					Young vs. Middle z				Young vs. Mature				Middle vs. Mature
					Young		Middle		Young		Mature		Middle		Mature
December		Sweetness			2.9 ± 0.4 x		2.9 ± 0.3		2.3 ± 0.3		3.6 ± 0.3 w		2.3 ± 0.3		3.6 ± 0.4
		Bitterness			3.4 ± 0.5		2.6 ± 0.4		3.9 ± 0.3		1.8 ± 0.3		4.3 ± 0.2		2.0 ± 0.4
		Umami			2.6 ± 0.3		3.0 ± 0.2		2.8 ± 0.3		3.3 ± 0.3		3.0 ± 0.3		3.4 ± 0.4
		Saltiness			2.9 ± 0.2		2.9 ± 0.1		3.0 ± 0.2		2.6 ± 0.2		3.0 ± 0.0		2.8 ± 0.2
		Sourness			2.6 ± 0.3		3.0 ± 0.0		3.0 ± 0.2		2.6 ± 0.2		2.9 ± 0.1		3.0 ± 0.0
		Overall acceptability			2.8 ± 0.5		2.9 ± 0.4		2.4 ± 0.3		3.8 ± 0.4		2.4 ± 0.4		3.9 ± 0.4
January		Sweetness			2.8 ± 0.3		3.1 ± 0.3		2.8 ± 0.3		3.1 ± 0.4		2.8 ± 0.3		2.9 ± 0.3
		Bitterness			3.8 ± 0.3		1.9 ± 0.3		3.9 ± 0.2		2.0 ± 0.2		2.4 ± 0.3		3.1 ± 0.2
		Umami			3.0 ±0.3		3.0 ± 0.3		3.0 ± 0.2		3.3 ± 0.3		3.0 ± 0.4		3.0 ± 0.2
		Saltiness			3.0 ± 0.2		2.4 ± 0.3		3.0 ± 0.2		2.6 ± 0.2		2.8 ± 0.3		2.9 ± 0.3
		Sourness			2.9 ± 0.1		2.5 ± 0.3		3.0 ± 0.2		2.6 ± 0.3		2.5 ± 0.3		2.6 ± 0.3
		Overall acceptability			2.5 ± 0.3		3.3 ± 0.3		2.5 ± 0.3		3.3 ± 0.3		3.9 ± 0.2		3.1 ± 0.3
The meaning of color y
← Weak (for five basic tastes x)						Same as reference					Strong (for five basic tastes) →
1	1.1~1.5		1.6~2.0	2.1~2.5		2.6~2.9		3		3.1~3.4	3.5~3.9	4.0~4.4		4.5~4.9		5
← Bad (for overall acceptability)											Good (for overall acceptability) →

^z Each pairwise comparison was conducted using reciprocal evaluation; each maturity-level section was evaluated relative to the other, and vice versa. ^y Values were rated on a 5-point scale (1 = weak/bad, 3 = same as reference, 5 = strong/good). The five basic tastes were sweetness, bitterness, umami, saltiness and sourness. ^x Data are presented as mean ± standard error (SE) of 8 panelists (n = 8). ^w Pairwise differences were considered meaningful only when the direction of the scores (color scale) changed after switching the reference sample.

). In December, both sweetness and bitterness differed between the mature and young or middle sections, and the mature section exhibited higher sweetness and lower bitterness than the other sections. In January, the young section exhibited stronger bitterness than the middle or mature sections, although there was no difference in sweetness among the sections. No differences were observed in umami, saltiness, and sourness, regardless of the section and harvest timing.

3.7. Effect of Pre-Treatment on Improving the Taste and Edibility of January-Harvested Xylem Rays

Pre-treatments had little effect on overall acceptability; however, soaking in water reduced the overall acceptability (

Table 6. Effect of pre-treatment methods on sweetness, bitterness and acceptability of papaya xylem rays.

Treatment Method		Solution			Sensory Characteristics				Comparison of Raw and Pre-Treated Samples z
									Raw				Pre-Treated
Soaking		Water			Sweetness				3.1 ± 0.3 x				2.8 ± 0.3
					Bitterness				2.0 ± 0.3				2.8 ± 0.3
					Overall Acceptability				3.6 ± 0.2				2.5 ± 0.3 w
					Edibility v				75.0%				50.0%
		1% saline solution			Sweetness				2.8 ± 0.3				2.9 ± 0.1
					Bitterness				2.9 ± 0.4				4.3 ± 0.3
					Overall Acceptability				2.9 ± 0.4				2.5 ± 0.3
					Edibility				62.5%				25.0%
Boiling		Water			Sweetness				2.4 ± 0.3				3.5 ± 0.3
					Bitterness				3.9 ± 0.2				1.8 ± 0.3
					Overall Acceptability				2.6 ± 0.3				3.5 ± 0.3
					Edibility				50.0%				100%
		5% rice bran suspension			Sweetness				2.5 ± 0.2				3.3 ± 0.3
					Bitterness				3.5 ± 0.2				2.9 ± 0.5
					Overall Acceptability				3.0 ± 0.3				3.4 ± 0.3
					Edibility				75.0%				87.5%
		0.2% sodium bicarbonate solution			Sweetness				2.6 ± 0.3				3.3 ± 0.3
					Bitterness				3.8 ± 0.2				2.0 ± 0.2
					Overall Acceptability				3.0 ± 0.3				3.3 ± 0.3
					Edibility				75.0%				87.5%
The meaning of color y
← Weak (for sweetness and bitterness)						Same as Reference					Strong (for sweetness and bitterness) →
1	1.1~1.5		1.6~2.0	2.1~2.5		2.6~2.9	3	3.1~3.4		3.5~3.9		4.0~4.4		4.5~4.9	5
← Bad (for overall acceptability)										Good (for overall acceptability) →

^z Raw samples were evaluated using pre-treated samples as references, and vice versa. ^y Values for sweetness, bitterness, and overall acceptability were rated on a 5-point scale (1 = weak/bad, 3 = same as reference, 5 = strong/good). ^x Data for sweetness, bitterness, and overall acceptability are presented as mean ± standard error (SE) of 8 panelists (n = 8). ^w Differences between raw and pre-treated samples were considered meaningful only when the direction of the scores (color scale) changed after switching the reference sample. ^v Edibility was evaluated without a reference and is expressed as the percentage of panelists who rated the samples as edible.

). For bitterness, boiling in water or 0.2% sodium bicarbonate solution reduced the perception, although the other pre-treatments had no effect. Boiling in water increased sweetness perception, whereas the other pre-treatments had no effect.

Although the edibility of the non-pre-treated raw samples fluctuated from 50% to 75%, the two soaking treatments reduced edibility, and the three boiling treatments increased edibility, with the water boiling treatment reaching 100% edibility.

3.8. Changes in Bitterness-Related Compounds in Xylem Rays Across Harvest Periods and Maturity Levels and the Comparison with the Green Papaya Fruit

Bitterness-related compounds (water-soluble nitrate nitrogen, water-soluble polyphenols, and glucosinolates) in xylem rays were analyzed and compared among the harvest periods and maturity levels, and with green papaya fruit, to explore the factors affecting overall acceptability. The effects of the harvest period were observed only in the young section, where the December samples had higher water-soluble nitrate nitrogen and glucosinolate contents (21 and 18 mg/100 g) than the January samples (4 and 16 mg/100 g) (Figure 4A–C).

As trunk maturity progressed, the water-soluble nitrate nitrogen and polyphenol contents tended to increase, whereas the glucosinolate content tended to decrease. Thus, in the mature section sampled in December, the water-soluble nitrate nitrogen and polyphenol contents (46 and 37 mg/100 g) were higher than those in the young section (21 and 19 mg/100 g) (Figure 4A,B). In January’s sample, the water-soluble nitrate nitrogen content in the mature section (34 mg/100 g) was higher than that in both the young and middle sections (4 and 8 mg/100 g), whereas no significant differences were observed in the water-soluble polyphenol content among any maturity levels. The glucosinolate content in the mature section sampled in December (13 mg/100 g) was lower than that in the young and middle sections (18 mg/100 g in both) (Figure 4C), and in January’s sample, no significant differences were observed among any maturity levels.

When we compared the content of these compounds in the xylem rays with those in the green papaya fruit, the water-soluble nitrate nitrogen and polyphenol contents were similar to or higher than those in the fruit (9 and 30 mg/100 g), regardless of harvest period and trunk maturity level. In contrast, the glucosinolate content in the xylem rays was lower than that in the fruit (23 mg/100 g), regardless of the harvest period and trunk maturity level (Figure 4A–C).

3.9. Changes in Antioxidant-Related Properties in Xylem Rays Across Harvest Periods and Maturity Levels and the Comparison with the Green Papaya Fruit

To evaluate the antioxidant-related properties of xylem rays, the total polyphenol content and antioxidant activity were measured and compared with those of green papaya fruit. The total polyphenol content in xylem rays (32–54 mg/100 g) was higher than that in green papaya fruit (26 mg/100 g), regardless of the harvest period and trunk maturity level (Figure 5A). The total polyphenol content in the mature section sampled in December (52 mg/100 g) was higher than that in the young and middle sections (32 and 41 mg/100 g), and that in the mature section sampled in January (54 mg/100 g) was higher than that in the middle section (39 mg/100 g). The antioxidant activity of xylem rays (0.12–0.36 TEAC) was similar to that of green papaya fruit (0.25 TEAC), regardless of the harvest period and trunk maturity level (Figure 5B). The mature section exhibited higher activity (0.36 TEAC in both) than the young section in both harvest seasons (0.12 and 0.14 TEAC).

4. Discussion

In temperate annual cultivation, papaya trunks represent the majority of plant biomass (62% of the total mass); however, they are typically discarded (Table 2). Although the proximate compositional analysis was exploratory because it was conducted using pooled samples without biological replication, the xylem rays exhibited moisture, carbohydrate, and protein contents broadly similar to those of green papaya fruit (Table 3). These preliminary findings indicate the edibility of xylem rays. Xylem rays are predominantly parenchyma cells known for radial transport and transient assimilate storage [23], suggesting that they function as storage tissues similar to fruits. In contrast, the outer fiber sheath and bark are less suitable for consumption, as they have a higher carbohydrate and lower moisture content, serve primarily structural functions, and are rich in fibrous components [24]. Thus, papaya trunks can be divided into two main organ types: the fiber sheath and bark, which provide structural support, and the xylem rays, which are involved in storage and transport and represent potential edible resources.

Sensory evaluation revealed that xylem rays harvested immediately after the final fruit harvest in December exhibited 100% edibility (Figure 3). In contrast, samples harvested in January following exposure to a cold wave showed a decline in edibility to 62.5%. The xylem rays from both harvest periods shared similar taste characteristics, including sweetness, bitterness, and umami, indicating an inherently complex flavor profile. However, the intensity of these tastes varied with harvest timing: January-harvested samples showed reduced sweetness and increased bitterness compared to December-harvested samples (Table 4).

One possible explanation for this sensory shift is reduced sugar availability prior to harvest. In many plants, leaf senescence is associated with a decline in photosynthetic source activity and assimilate supply [25]. By January, leaf senescence had progressed substantially in the present study, which likely limited the translocation of soluble sugars to the xylem rays. A reduction in sweetness could consequently enhance the perception of bitterness owing to diminished masking effects between taste components [26].

Another possible explanation is the accumulation of bitter compounds in response to environmental stress, as secondary metabolites involved in plant defense are known to increase under stress conditions [27,28]. In this study, we analyzed three bitter-related components: water-soluble nitrate nitrogen, polyphenols, and glucosinolates. The first two are known to contribute to bitterness in many plants [29,30], while the latter is known to be a bitter component in green papaya [31]. However, the concentrations of the measured bitter-related compounds did not increase in the January-harvested samples (Figure 4). The observed increase in bitterness was more likely associated with changes in taste balance rather than the biochemical accumulation of bitter substances.

Regardless of the harvest timing, mature trunk sections consistently exhibited higher overall sensory acceptability, which was primarily associated with greater sweetness and lower bitterness (Table 5). Enhanced sweetness and improved taste balance in mature sections are consistent with greater assimilate retention in developmentally advanced organs. In contrast, young sections located near the trunk apex, which contain the shoot apical meristem, function as strong metabolic sinks, where carbohydrates are rapidly utilized to sustain active cell division and tissue growth [32,33]. Among the bitter-related components, nitrate nitrogen and water-soluble polyphenols increased slightly with trunk maturity (Figure 4), and these trends did not correspond to the sensory patterns of bitterness. In contrast, glucosinolate levels decreased with maturity, partially aligning with the sensory results. However, their concentrations were lower than those in green papaya fruit measured in this study and also lower than values reported in previous studies [31], suggesting that they are unlikely to be the primary determinants of bitterness in the xylem-ray organs. These results suggest that maturity-related changes in sweetness and bitterness are not determined by a single compound but rather reflect complex shifts in the balance of multiple components. Importantly, mature sections yielded the largest amount of xylem-ray organs, indicating that a substantial proportion of xylem rays possessed favorable sensory qualities.

From a practical standpoint, although xylem rays harvested immediately after the final fruit harvest can be consumed raw, growers may delay harvesting to distribute labor over time. Thus, we examined pretreatment procedures to mitigate bitterness, which may intensify after exposure to low temperatures. As mentioned above, the main components responsible for bitterness were not identified; however, many water-soluble bitter-related substances, including alkaloids, phenols, and various glycosides, are known [34]. Thus, we attempted to reduce bitterness by soaking both in water and/or saline, but these procedures did not improve the overall acceptability (Table 6). Xylem rays are relatively hard organs with a radish-like texture, indicating that soaking did not effectively leach the bitter compounds from the cells.

In contrast, boiling treatments significantly improved edibility by reducing bitterness and increasing sweetness. Thermal processing is known to soften plant tissues and weaken cell adhesion, which facilitates the release or dilution of soluble compounds retained within cells [35,36]. Consistent with this mechanism, boiling likely promoted the elution of bitter-related components from the xylem-ray organs in the present study. Although boiling with sodium bicarbonate or rice bran, which are traditionally used in Japanese households to soften hard plant tissues, also improves edibility, simple boiling in water alone is sufficient to achieve this effect. These results suggest that delayed-harvested xylem rays may be suitable for use as cooked vegetables with minimal household processing.

When evaluating the potential of a novel food resource, its nutritional attributes should also be considered. Total polyphenol content and antioxidant activity (DPPH radical scavenging activity) were measured as preliminary indicators of antioxidant-related properties (Figure 5). Regardless of the harvest period or maturity level, the total polyphenol content of xylem rays was higher than that of green papaya fruit, whereas the antioxidant activity was similar. These results indicate that papaya xylem rays may represent a source of antioxidant-related compounds, although further validation is required.

Notably, previous studies on the nutritional composition of trunk tissues in the family Caricaceae have been limited to a report on the medullary parenchyma of Vasconcellea quercifolia A. St-Hil., which is rich in ash, protein, carbohydrates, fiber, and carotenoids [15]. In contrast, this study provides preliminary evidence that trunk xylem rays of Caricaceae species may also exhibit antioxidant-related properties, highlighting their potential as nutritionally valuable edible plant resources.

Both the total polyphenol content and antioxidant activity tended to be higher in the mature sections. As discussed above, the mature sections also exhibited superior sensory quality and yield, suggesting their high potential for practical utilization, including raw consumption. However, delayed-harvested xylem rays may require boiling treatment to improve palatability, and heat treatment may reduce antioxidant-related properties. Therefore, further studies evaluating the effects of processing conditions on these properties would be valuable.

Nevertheless, several limitations should be considered when interpreting the findings of this study. The study was conducted using a limited number of biological replicates (n = 3), which may restrict the generalizability of our findings. In addition, the proximate composition analysis was conducted using pooled samples without biological replication and was performed using a different cultivar and cultivation season from those used in the other experiments; therefore, these compositional comparisons should be interpreted as exploratory. The sensory evaluation involved only eight untrained panelists and was based on questionnaire-based subjective assessments using a paired comparison approach under limited experimental conditions rather than standardized sensory evaluation protocols. Therefore, the sensory findings should be interpreted as preliminary observations. Future studies using larger and better-characterized sensory panels, standardized sensory evaluation designs, and more robust sensory assessment protocols are needed to validate these preliminary observations. Furthermore, the compositional and antioxidant-related analyses, including polyphenol content and DPPH radical scavenging activity, were conducted as preliminary indicators rather than comprehensive evaluations. In addition, the compounds responsible for undesirable bitterness could not be conclusively identified. Further identification of these compounds and optimization of preparation or cooking methods based on their properties may be useful for improving the palatability and practical utilization of these resources.

5. Conclusions

From a practical perspective, papaya xylem rays harvested immediately after the final fruit harvest may be suitable for raw consumption, particularly the mature sections, which exhibit high sensory acceptability. Mature sections also exhibited relatively high antioxidant-related properties, further supporting their potential as favorable portions for food utilization.

However, when harvesting of xylem rays is delayed due to the need to distribute labor among other agricultural operations, sensory acceptability may decline because of increased bitterness. In this study, boiling for approximately 10 min effectively improved sensory acceptability, indicating that papaya xylem rays may also be utilized as cooked vegetable materials.

The ability to remain suitable for consumption under both immediate and delayed harvesting conditions indicates that papaya trunk xylem rays may be utilized under flexible harvest handling conditions, suggesting their potential utility as a biomass resource in temperate production systems. This flexibility may contribute to improved biomass utilization and waste reduction in temperate papaya production systems.