Improvement of Moso Bamboo (Phyllostachys pubescens) Properties Using a Heat Treatment Process for Landscaping Materials and Evaluation of Its Durability against Biotic/Abiotic Factors

Abstract

This study aimed to assess the effectiveness of large-scale heat treatment on Moso bamboo (Phyllostachys pubescens) grown in South Korea. The process involved multiple stages, including pretreatment, boiling, steaming, heating, and cooling. Heat treatment successfully reduced the water content to below 3% and increased the specific gravity from 0.62 to 1.12, thereby enhancing dimensional stability and strength. Following an ultraviolet-accelerated weathering test, the heated Moso bamboo exhibited improved color stability (ΔE 5.84) compared to untreated bamboo (ΔE 9.92). Furthermore, the heat-treated bamboo demonstrated high resistance against wood-rot fungi (weight loss < 10%) and termites (weight loss approximately 2%). In contrast to small lab-scale drying processes, this study employed a pilot-scale kiln for mass production, resulting in large-sized Moso bamboo with enhanced properties. This study revealed that distinct results, including extractives and lignin-degraded compounds, persisted in heated Moso bamboo cells after the heat treatment. The overall improvement in deterioration resistance, achieved through heat treatment, significantly contributes to the durability and longevity of bamboo materials in outdoor settings, such as landscape facilities.

1. Introduction

To address global environmental concerns related to climate change, there is an increasing need for plant-based materials that can replace glass and plastics while also exhibiting CO₂ storage effects. Bamboo, a widely utilized plant resource, serves as an example and finds common use in furniture, sculptures, and landscaping. According to the National Institute of Forest Science, the total distribution area of bamboo resources in South Korea was estimated to be 24,111 ha in 2016, accounting for 0.36% of the country’s total forest area [1]. Additionally, the CO₂ absorption capacity of this crop was reported to be 29.34 tons/ha per year, which is more than three times higher than those of pine, larch, and nut pine, indicating its significant CO₂ reduction effect [2]. Bamboo is characterized by its rapid growth rate, reaching up to 1 m per day.

South Korea hosts five genera and 19 species of native bamboo, including the Phyllostachys, Sasa, and Arundinaria genera. Among them, economically valuable bamboo species in Korea include Phyllostachys bambusoides (giant bamboo), Phyllostachys nigra variety henosis (henon bamboo), and Phyllostachys pubescens (Moso bamboo) [3]. Notably, Moso bamboo, characterized by abundant bamboo shoots with a diameter of approximately 20 cm, possesses excellent antibacterial properties, rapid drying capability, and deodorizing and dehumidifying properties. Extracts from bamboo have also been reported to possess various active effects, inhibiting free radicals and preventing cell damage [4,5,6,7].

However, bamboo is susceptible to rot when infected by fungi and molds and prone to insect damage due to its high sugar, starch, and protein content compared to other types of wood [8,9,10,11]. To address these issues, heat treatment technology has been evaluated as a physical and chemical modification method. Typically, heat treatment is applied to wood as a raw material, subjecting the wood specimen to high temperatures (i.e., 160–260 °C) for a specific period [12,13,14]. Effects include weight reduction, high dimensional stability, excellent weather resistance, and color stabilization due to chemical changes and degradation of cell wall components [15,16]. For instance, the degradation of hemicelluloses during heat treatment promotes dimensional stability by increasing hydrophobicity through elevated lignin content [17]. Various heat treatment methods exist, categorized as steam-, vacuum-, nitrogen-, or oil-based, depending on the heat transfer medium and treatment conditions. Commercialized treatment technologies have been developed in France (Rétification), Finland (Thermowood), Germany (Menz Holz), and the Netherlands (Plato Wood) [18,19].

Steam-based heat treatment approaches, in addition to drying and sterilizing wood, have been applied to treat food and sewage sludge [20]. Wood generated by this treatment method can be efficiently dried using gas convection, and the discharged steam can be recycled or used for electricity generation, making this process more energy-efficient and eco-friendly than other heat treatment approaches [21]. It is crucial to note that the widely used oil-based heat treatment technique poses disadvantages due to the high risk of fire, increased transportation costs resulting from oil retention in the wood, and the generation of unpleasant odors.

While previous research has explored the application of heat-treated bamboo resources, these studies did not employ bamboo in its natural state [22,23,24,25]. Instead, the outer and inner parts of bamboo or single plates were examined for use as sticks and boards. The present study reports the pilot-scale application of steam heat treatment for the mass production of bamboo, aiming to promote the utilization of modified products by evaluating their durability against biological and abiotic factors. After developing suitable steam heat treatment conditions to minimize defects and enhance the durability of Moso bamboo, the obtained product is characterized, and its durability is evaluated. The durability assessment Is divided into the effects of abiotic factors (ultraviolet (UV) rays, temperature, and dryness) and biotic factors (rot fungi and termites) to analyze the application potential of this technique and the obtained heat-treated bamboo.

2. Materials and Methods

2.1. Bamboo Preparation

Three- to four-year-old Moso bamboo specimens were gathered from Geoje-si, Gyeongsangnam-do, South Korea. Six bamboo poles, each measuring 3.7 m, were cut into two segments of 60 cm each (0–60 cm above the ground, untreated Moso bamboo, and 60–120 cm above the ground, heat-treated Moso bamboo), as illustrated in Figure 1. Each sample from the same bamboo pole received the same number and underwent heat treatment, with six replicates utilized for each analytical test.

2.2. Drying and Heat Treatment Processes

The mass production of Moso bamboo involved drying and heat treatment processes using a cylindrical commercialized kiln (1.8 m (w) × 1.8 m (h) × 8 m (l), HT Co., Ltd., South Korea). The treatment schedule comprised (i) pretreatment, (ii) boiling, (iii) steaming, (iv) maintaining, and (v) cooling stages, as depicted in Figure 2. To stabilize the internal temperature of the bamboo sample, pretreatment was carried out at 65 °C for 2 h, followed by boiling at 100 °C for 4 h. Subsequently, steaming occurred at 100–140 °C for approximately 26 h, and heating was executed at 140 °C for 2 h. Finally, the bamboo specimen was cooled down under 80 °C for 5 h and removed after 3–4 d.

2.3. Weight Loss, Moisture Content, and Specific Gravity

The weight loss of the samples after heat treatment was calculated using Equation (1).

$$\mathrm{WL} (\%)=\mathrm{Wg}-\mathrm{Wht}/\mathrm{Wg}\times 100$$

(1)

where Wg and Wht are the weight of green and heat-treated Moso bamboo, respectively.

The moisture content and specific gravity of each Moso bamboo sample, both before and after heat treatment, were measured based on KS F 2199 and KS F 2198 standards [26,27]. For moisture content determination, untreated and heat-treated Moso bamboo samples were prepared by cutting them into specimens measuring 10 mm (w) × 20 mm (h) × 10 mm (l). The volume was measured using a vernier caliper after air-drying. Each sample was weighed before and after drying in an oven at 105 °C to calculate wet- and dry-basis moisture contents using Equations (2) and (3), respectively.

$$\mathrm{Wet}-\mathrm{basis} \mathrm{moisture} \mathrm{content} (\%)={\mathrm{M}}_{\mathrm{W}}/({\mathrm{M}}_{\mathrm{W}}+{\mathrm{M}}_{\mathrm{D}})\times 100$$

(2)

Dry-basis moisture content (%) = M_W/M_D × 100

(3)

where M_D and M_W (g) are the weights of dried and wet moisture, respectively.

In addition, specific gravity was measured following the standard KS F 2198 by cutting the material into specimens measuring 20 mm (w) × 20 mm (h) × 20 mm (l). The oven-dried weight (W_o) and oven-dried volume (V_o) of each sample were measured according to the change in water height in the mass cylinder, and the oven-dried specific gravity was calculated using Equation (4).

Oven-dried specific gravity = W_o/V_o

(4)

2.4. Chemical Composition

The chemical compositions of untreated and heat-treated Moso bamboo samples were determined following the NREL laboratory analytical procedure [28]. Samples were ground to a 40-mesh size for determination.

2.4.1. Ash Content

A powdered sample from each experimental group (2 g) was weighed in a crucible and subjected to combustion at 575 °C for 15 h using an ash furnace (DMF-4.5, Lab House, South Korea). The ash content was obtained by calculating the yield.

2.4.2. Extractives Content

Another powdered sample from each experimental group (2 g) had its extractives content determined by Soxhlet extraction using ethanol/benzene (1:2, v/v) as the extraction solvent. The constant temperature water bath was set at 80 °C for the extraction, followed by concentration using a rotary evaporator. The extractives content was obtained by calculating the yield after 24 h of drying in an oven at 105 °C.

2.4.3. Lignin Content

Klason lignin and acid-soluble lignin contents were analyzed using extractives-free samples prepared as described in Section 2.4.2. For determining Klason lignin content, 72% sulfuric acid was added to a sample from each experimental group (0.3 g) and reacted in a constant temperature water bath at 30 °C for 1 h. Subsequently, the sulfuric acid was diluted to a concentration of 3%, and the mixture was reacted in an autoclave at 121 °C for 1 h, followed by filtration using a weighed glass filter with a known oven-dried weight. The filtrate was recovered for analyzing acid-soluble lignin content, and the solids were dried in an oven at 105 °C to calculate Klason lignin content using Equation (5).

$$Acid insoluble Lignin\left(\mathrm{\%}\right)=\frac{weight of residue on the glass filter}{weight of dry sample}\times 100$$

(5)

The collected filtrate was subjected to UV-visible (UV-vis) spectrometry (Optizen Pop-S spectrometer, KLab, Korea), ensuring that the absorbance at 205 nm ranged from 0.2 to 0.7. The acid-soluble lignin content was calculated using Equation (6).

$$Acid Soluble Lingin (\mathrm{\%})=\frac{{UV}_{abs}\times {Volume}_{filtrate}\times Dilution}{ϵ\times {ODW}_{sample}}\times 100$$

(6)

UV_abs = average UV-Vis absorbance for the sample at appropriate wavelength
Volume_filtrate = volume of filtrate, 100 mL

$$\mathrm{Dilution}=\frac{{Volume}_{sample}+{Volume}_{diluting solvent}}{{Volume}_{sample}}$$

ε = Absorptivity of biomass at specific wavelength
ODW_sample = weight of sample in milligrams

2.4.4. Polysaccharide Content

Polysaccharide (i.e., glucose, xylose) content was determined using the Klason lignin filtrate. An UltiMate3000 liquid chromatograph (Thermo Dionex, Berlin, Germany) equipped with an Aminex 87H column (300 × 10 mm, Bio-Rad, Hercules, CA, USA), a refractive index detector (ERC, RefractoMAX520, Tokyo, Japan), and a 210 nm UV source was employed for the analysis. The analysis was performed using a 0.01 N H₂SO₄ solution (Fluka, USA) at a flow rate of 0.5 mL/min and an injection volume of 10 µL.

2.5. Microscopic Analysis

Microscopic analysis of untreated and heat-treated Moso bamboo samples was conducted using optical microscopy and scanning electron microscopy (SEM). For optical microscopy, the samples were cut into sections of 10 × 10 × 10 mm³ and softened in a glycerin/distilled water (1:1, v/v) solution at approximately 120 °C. Cross-sectional sections were then cut into 20–25 µm-thick sections using a microtome (Nippon Optical Works Co., Ltd., Nagano, Japan) and double stained with safranin and light green. The prepared sections were observed under an optical microscope (Eclipse E600, Nikon Corp., Tokyo, Japan). For SEM analysis, the samples were cut into specimens measuring 10 × 10 × 5 mm³ and coated with platinum. The sample cross-sections were observed using a JSM-5510 SEM (JEOL, Tokyo, Japan) at an acceleration voltage of 15 kV.

2.6. Durability Evaluation

2.6.1. Effects of Abiotic Factors

Accelerated Weathering

A UV-promoted weathering tester (QUV/se, Q-LAB, Baltimore, MD, USA) was employed to observe color changes in samples induced by artificial UV irradiation. Each sample underwent UV light exposure for 10 d (240 h) at a light intensity of 0.89 W/m² and a temperature of 60 °C, following the ASTM G154 standard, utilizing a fluorescent UV lamp apparatus for the exposure of nonmetallic materials. Color difference measurements (ΔE) resulting from artificial UV irradiation were quantified at 24 h intervals using a colorimeter (CR-10 Plus, KONICA MINOTA, Tokyo, Japan). ΔE values based on L*, a*, and b* were calculated using the following equations:

ΔL* = L₀ − L_a (L₀: initial L* value, L_a: L* value after change)

(7)

Δa* = a₀ − a_a (a₀: initial a* value, a_a: a* value after change)

(8)

Δb* = b₀ − b_a (b₀: initial b* value, b_a: b* value after change)

(9)

$$\Delta E^{*}=\sqrt{{(L_0-L_a)}^2+{(a_0-a_a)}^2+{(b_0-b_a)}^2}$$

(10)

Hot/Cold Cycling

As Moso bamboo products are frequently used in outdoor settings, a hot/cold cycling test was conducted to assess the durability of each specimen under extreme temperature conditions. Untreated and heat-treated Moso bamboo specimens, approximately 30 cm in length, were employed. The hot stage of the test involved a dryer set at 80 °C (2 h per cycle), and the cold stage involved a freezer set at −20 °C (2 h per cycle), with each cycle repeated ten times before visually observing the post-test samples.

Contact Angle

To indirectly assess changes in water absorption, the contact angles of the prepared specimens were measured. A droplet of distilled water (10 μL per 1 drop) was applied to the outer surface of each sample of untreated or heat-treated Moso bamboo using a Theta Lite Optical Tensiometer (Biolin Scientific Corp., Västra Frölunda, Sweden), and the contact angle for each specimen at ambient temperature was then determined.

2.6.2. Effects of Biotic Factors

Decay Resistance

Fungal resistance was evaluated following the KS F 2213 standard, using Fomitopsis palustris (FOM) as the brown-rot fungus and Trametes versicolor (TRA) as the white-rot fungus [29]. Untreated and heat-treated Moso bamboo samples were air-dried for 24 h, followed by drying in an oven at 60 ± 2 °C for 48 h. Samples were then placed in bottles (4 samples per bottle, 2 bottles per repetition, specimen size: 20 mm (D) × 20 mm (W) × 9 ± 0.5 mm (L)), and the inoculum was cultured as per KS F 2213. Rotting was carried out by heating in an incubator at 26 ± 2 °C for 10 weeks. Fungi attached to sample surfaces were wiped off, and sample weight was measured after drying under the same conditions as before the test. Weight loss of each sample was calculated using Equation (11), and decay resistance grade (

Table 1. KS F 2213 grades for decay resistance test.

Average Weight Loss (%)	Antifungal Grades
0–10	Highly resistant
11–24	Resistant
25–44	Moderately resistant
45>	Slightly resistant or nonresistant

) was determined based on the calculated weight loss with reference to the KS F 2213 standard.

$$\mathrm{Weight} \mathrm{loss} \mathrm{after} \mathrm{rotting} (\%)=({\mathrm{W}}_1-{\mathrm{W}}_2)/{\mathrm{W}}_1\times 100$$

(11)

where W₁ and W₂ are the weights of the sample before and after rotting, respectively.

Termite Resistance

To evaluate termite resistance, an anti-termite effect test from the Korea Forestry Promotion Institute was employed. Reticulitermes speratus was used as the test insect, and each container was equipped with an acrylic cylinder (diameter: 8 cm, length: 6 cm) with plaster on the bottom and a lid on top. The container was lined with absorbent cotton, and the dried (at 60 ± 2 °C) test specimen (1 × 1 × 2 cm³) was placed inside with 200 worker ants and 20 soldier ants, up to 15 repetitions. Containers were maintained at 28 ± 2 °C for 21 d under dark conditions.

After removing foreign substances from the sample surfaces, samples were dried at 60 ± 2 °C until reaching a constant weight, and the weight loss rate was calculated using Equation (12). Additionally, dead insects were removed from the sample containers every 7 d, and the average mortality rate was calculated using Equation (13).

$$\mathrm{Weight} \mathrm{loss} \mathrm{rate} (\%)=({\mathrm{W}}_1-{\mathrm{W}}_2)/{\mathrm{W}}_1\times 100$$

(12)

where W₁ and W₂ are the sample weights before and after the test (g), respectively.

$$\mathrm{Average} \mathrm{mortality} \mathrm{rate} (\%)=\frac{Number of dead insects}{Initial number of insects}\times 100$$

(13)

Detection of Active Compounds by Thin-Layer Chromatography

Thin-layer chromatography (TLC) was performed to analyze the distribution of extract components of Moso bamboo samples before and after heat treatment. Hexane/ethyl acetate (1:2, v/v) was used as the developing solvent, and substances were visualized under 254 and 365 nm UV irradiation.

Detection of Active Compounds by Gas Chromatography/Mass Spectrometry (GC/MS)

Gas chromatography/mass spectrometry (GC/MS) was conducted to analyze components in the ethanol/benzene (1:2, v/v) extractives of untreated and heat-treated Moso bamboo. Extracted samples were subjected to GC/MS after derivatization. For GC/MS, a DB-5MS column was used, injection volume was set to splitless mode, flow rate was 1 min/1 min, and the oven was programmed with a 10 °C/min increase to a final temperature of 280 °C and held for 10 min.

3. Results and Discussion

3.1. Weight Loss, Moisture Content, and Specific Gravity

The moisture contents of the Moso bamboo specimens before and after heat treatment are presented in

Table 2. Moisture content and specific gravity of untreated and heat-treated Moso bamboo.

	Moisture Content (%)		Specific Gravity
	Dry Basis	Wet Basis
Untreated	51.56 ($\pm$15.84) a	33.33 ($\pm$6.70) a	0.62 ($\pm$0.08) a
Heat-treated	2.85 ($\pm$0.27) b	2.77 ($\pm$0.26) b	1.12 ($\pm$0.08) b

Note: Numbers in parentheses are standard deviations. The listing of the same superscript letters beside the mean values within columns denotes insignificant outcomes at the 5% significance level for comparisons between species.

, along with the results of the specific gravity measurements. Despite the initially high moisture content, the heat treatment significantly reduced the moisture content of Moso bamboo to <3%, registering 51.56% and 33.33% on a dry basis and wet basis, respectively. The weight loss of the heat-treated bamboo was approximately 33%, attributed to the decrease in moisture content. Notably, the material’s moisture content plays a crucial role in its mechanical and physical properties when exposed to external environments [30,31]. Consequently, Moso bamboo with markedly lower moisture content is anticipated to exhibit enhanced strength. Additionally, lower moisture content is beneficial in resisting degradation by fungi and borer insects [32]. Interestingly, the specific gravity experienced a significant increase from the initial value of 0.62–1.12. This result is attributed to the compaction of bamboo cells as they shrink. This compaction resulted in a rise in specific gravity, aligning with the expectation of superior strength performance in heat-treated samples [33,34].

Tang et al. (2019) observed that heat treatment of Moso bamboo with tung oil at 100, 140, 180, and 200 °C led to an increase in density at 100 and 140 °C [35]. However, at temperatures surpassing 140 °C, density decreased compared to untreated samples, attributed to changes in chemical composition under harsh heat treatment conditions.

3.2. Chemical Composition

Examining the chemical compositions of Moso bamboo before and after heat treatment (

Table 3. Chemical composition of untreated and heat-treated Moso bamboo (%).

	Untreated	Heat-Treated
Ash	0.93 (±0.46) a	0.90 (±0.43) a
Extractives	3.76 (±1.19) a	3.44 (±0.86) a
Total lignin	28.80 (±1.35) a	29.48 (±1.24) a
Klason lignin	27.36 (±1.32) a	28.13 (±1.21) a
Soluble lignin	1.45 (±0.06) a	1.35 (±0.09) a
Glucose	53.39 (±4.51) a	49.50 (±0.91) a
Xylose	15.58 (±0.42) a	16.35 (±0.69) a

Note: Numbers in parentheses are standard deviations. The listing of the same superscript letters next to the mean values within columns denotes insignificant outcomes at the 5% significance level for comparisons between species.

), no significant alterations were found in our study. Typically, wood undergoes changes in chemical composition from the temperature at which hemicellulose decomposition occurs during heat treatment. However, our study, conducted on a larger scale with an elongated bamboo shape maintained in a drying kiln, resulted in minimal chemical changes, likely due to the decomposed components remaining within the bamboo fibers.

Meng et al. (2016) analyzed the chemical composition of bamboo slivers after removing the outer and inner segments using high-temperature heat treatment at 180 and 200 °C, respectively, and reported that the contents of holocellulose and cellulose decreased after heat treatment, while the contents of lignin and extractives increased [36]. Additionally, Wu et al. (2021) analyzed the chemical compositions of the fibers and parenchyma cells of bamboo subjected to high-temperature heat treatment between 100 and 220 °C (heating at 20 °C intervals, 6 h hold at each temperature) and reported that heat treatment below 220 °C had a slight effect on the chemical composition [11]. Wang et al. (2020) reported that during the saturated steam heat treatment of Moso bamboo at 180 °C for 10, 20, 30, 40, and 50 min, the hemicellulose content decreased over time, while the cellulose and lignin contents relatively increased [37].

3.3. Microscopic Analysis

The cross-sectional images of untreated and heat-treated Moso bamboo samples were examined using optical microscopy and SEM, with the results displayed in Figure 3 and Figure 4. The optical microscopy findings, categorized into outer, middle, and inner bamboo sections, indicated that heat-treated Moso bamboo exhibited parenchyma cell shrinkage upon drying, transforming from a round to a rhombic shape. The SEM results revealed the presence of starch granules in both untreated and heat-treated Moso bamboo samples. Chen et al. (2019) highlighted the significant presence of starch grains in Moso bamboo, particularly in parenchyma cells, and similar SEM images were presented by Li et al. (2022) after hydrothermal treatment at 140 and 160 °C, where starch granules were observed in the cell lumen [14,38]. The persistence of a significant amount of starch post-heat treatment raised concerns, necessitating an analysis of potential sample degradation based on biological factors.

Fabiani et al. (2023) recently reported on flame-based heat treatment of Phyllostachys viridiglaucescens, observing microstructural changes using an environmental electron microscope (ESEM) [39]. Their results revealed small cracks along the xylem components of heat-treated samples. Wang et al. (2020) studied the microstructure of Moso bamboo subjected to heat treatment at 140, 160, and 180 °C. Using SEM, they observed minimal changes in the microstructure of Moso bamboo heated at 140 °C compared to the untreated sample [37]. However, at temperatures ≥160 °C, parenchyma cells shrank, and the intercellular volume decreased.

3.4. Accelerated Weathering Test

Photographic images of untreated and heat-treated Moso bamboo samples exposed to UV light for 240 h (10 d) are presented in Figure 5. In the untreated samples, a color change occurred during the sample preparation process (compared to the original green raw material). However, the observed changes in L*, a*, and b* values were more pronounced compared to those in the heat-treated Moso bamboo specimens (Figure 6). After 240 h of UV irradiation, the ΔE value for untreated Moso bamboo was approximately 9.9, whereas for heat-treated bamboo, it was approximately 5.8, indicating that heat treatment did not induce a significant color change. Despite potential differences in initial L*, a*, and b* values due to heat treatment, color stabilization occurred in terms of the ΔE value attributed to UV exposure.

In a broader context, Yildiz et al. (2013) investigated the degree of color change after UV irradiation (400, 800, and 1600 h) of untreated samples and those subjected to high-temperature heat treatment at 212 °C for softwood and 190 °C for hardwood [40]. Their experiments involved Scots pine (Pinus sylvestris L.) sapwood, spruce (Picea orientalis L.), Iroko (Chlorophora excelsa), and ash (Fraxinus excelsior L.). The results indicated increased L* values and decreased a* and b* values for both untreated and high-temperature heat-treated samples. Among samples irradiated for 400 h, the Scots pine and Iroko wood specimens showed ΔE values of 9.38 and 9.61 following high-temperature heat treatment, lower than the untreated samples. For all other conditions and species, the untreated samples exhibited smaller color changes than the heat-treated samples, suggesting a color-stabilizing effect of heat treatment, albeit diminishing over prolonged exposure times.

3.5. Hot/Cold Cycling Tests

Repeated hot/cold cycling tests were executed on both untreated and heat-treated Moso bamboo samples to simulate Korean weather, characterized by four seasons and a temperature difference of at least 50 °C throughout the year. As illustrated in Figure 7, the surface color of Moso bamboo, darkened by heat treatment, remained consistently stable. Notably, the untreated Moso bamboo developed new cracks on the surface that were absent before testing. In contrast, the heat-treated Moso bamboo exhibited no new defects on its surface. Further observations revealed that cross-sections of untreated Moso bamboo samples developed cracks, while those of the heat-treated specimens showed no such defects.

3.6. Contact Angle

To assess the moisture resistance of the heat-treated surface, contact angle measurements were conducted. The average contact angles for untreated and heat-treated Moso bamboo samples were 79.78 and 99.02°, respectively, indicating a superior moisture resistance effect on the heat-treated Moso bamboo surface. Despite the sample composition remaining unaffected by heat treatment, the variation in contact angle was attributed to changes in the chemical structure and surface functional groups following heat treatment. This finding aligns with the results reported by Li et al. (2022) that indicate an increased contact angle for Moso bamboo after heat treatment at 140 °C [13]. However, after 300 s, water absorption into the treated surface occurred, similar to the untreated sample.

3.7. Decay Resistance Tests

Rot fungi represent a significant biological degradation factor for plant-based materials. The resistance of untreated and heat-treated Moso bamboo samples to rot fungi degradation was evaluated, as presented in

Table 4. Weight loss of untreated and heat-treated Moso bamboo after decay resistance test.

	Untreated	Heat-Treated
FOM	10.51 (±0.62) a	6.05 (±0.51) b
TRA	12.52 (±0.51) a	5.04 (±1.02) b

Note: Numbers in parentheses are standard deviations. The listing of the same superscript letters beside the mean values within columns denotes insignificant outcomes at the 5% significance level for comparisons between species.

. The average weight loss percentages for untreated and heat-treated specimens were 10.51 and 6.05% for F. palustris, respectively, and 12.52 and 5.64% for T. versicolor, respectively, indicating lower degradation levels for the heat-treated samples. According to the decay resistance evaluation rating of wood defined in the KS F 2213 Korean standard, the antifungal grades for the heat-treated samples against both F. palustris and T. versicolor were classified as “Highly resistant”.

In a similar study, Yoon et al. valuated the decay resistance of untreated and 200 °C oven heat-treated tulip tree samples [41]. The average weight loss rates for the untreated and heat-treated samples were 48.24 and 8.13% for F. palustris, respectively, and 27.28 and 5.69% for T. versicolor, respectively. Additionally, Cheng et al. conducted surface fungus resistance tests on Moso bamboo samples subjected to oil-based heat treatment at 160, 175, and 195 °C, reporting a lower average weight loss rate for the heat-treated Moso bamboo specimens compared to the untreated samples [42].

Although no changes in chemical composition were detected after heat treatment, it is presumed that the significant reduction in the weight loss rate during degradation by rot fungi can be explained by the pilot-scale nature of the current study. Unlike previous laboratory-scale experiments with small sample sizes, it is expected that any degradable material would remain within the bamboo fibers after the pilot-scale heat treatment. Given the directional arrangement of bamboo fibers along its long axis, no material migration is anticipated in the heat-treated bamboo samples. Hence, it is assumed that decomposed or eluted wood components could be deposited within the Moso bamboo fibers, hindering the growth of rot fungi.

3.8. Termite Resistance Tests

The untreated and heat-treated Moso bamboo specimens underwent termite resistance tests, and the results are presented in

Table 5. Weight loss and mortality of untreated and heat-treated Moso bamboo after termite resistance test.

	Untreated	Heat-Treated
Weight loss	5.31 (±0.26) a	2.04 (±0.11) b
Mortality	38.70 (±23.88) a	66.70 (±16.03) a

Note: Numbers in parentheses are standard deviations. The listing of the same superscript letters beside the mean values within columns denotes insignificant outcomes at the 5% significance level for comparisons between species.

and Figure 8. Specifically, the average weight loss rates for the untreated and heat-treated samples were 5.31 and 2.04%, respectively, while the termite mortality rates were 38.70 and 66.70%. In a similar evaluation, Manalo and Garcia treated Dendrocalamus asper with oil-based heat at temperatures of 140, 160, 180, and 200 °C [43]. Samples treated for 30 min exhibited increased termite resistance at higher temperatures; however, samples treated for 120 min showed no significant changes in termite resistance, except for the sample treated at 200 °C. Moreover, all treated samples demonstrated lower weight loss rates than the untreated samples. Brito et al. subjected Dendrocalmus giganteus to heat treatment at temperatures ranging from 140 to 200 °C [44]. According to the termite test results, there was a 9.28% mass loss at 140 °C, with 100% mortality reported. The observed differences are attributed to variations in species and heat treatment conditions. In the current study, although the weight loss rates of the untreated and heat-treated samples were not significantly different, the termite mortality rate was approximately 30% higher in the heat-treated Moso bamboo sample. This was attributed to the presence of wood decomposition products in Moso bamboo and the elution of some components, such as extractives and lignin, during heat treatment, thereby increasing the termite mortality rate.

3.9. Identification of Bamboo-Derived Products

Finally, TLC was performed to identify the wood-derived products responsible for the enhanced resistance to rot fungi and termites following heat treatment. As depicted in Figure 9, the distribution of substances in untreated Moso bamboo, obtained by extraction using organic solvents, differed from that of heat-treated Moso bamboo. Although the chemical compositions of the bamboo specimens were similar, the TLC results indicate a significant quantity of Moso bamboo-derived substances due to degradation, leading to a reduction in sample molecular weight during heat treatment.

The GC/MS analysis results of untreated and heat-treated Moso bamboo extracts are presented in

Table 6. Lignin-derived substances in extractives of untreated and heat-treated Moso bamboo.

RT	Untreated	Heat-Treated
17.692		4-Trimethylsilyloxybenzaldehyde
17.779		Trimethyl-(3-trimethylsilyloxyphenoxy)silane
17.827	Trimethyl-(4-trimethylsilyloxyphenoxy)silane	Trimethyl-(4-trimethylsilyloxyphenoxy)silane
19.683		Vanillin
20.616		p-Hydroxybenzoic acid
20.731		4-Hydroxyphenylacetic acid
21.548	Syringaldehyde	Syringaldehyde
22.096	Vanillic acid
22.106		p-Hydroxybenzoic acid
22.442		p-Coumaryl alcohol
22.615		Benzoic acid, 3,4-bis(trimethylsiloxy)-, trimethyl silyl ester
23.356		Trimethylsilyl 3,5-dimethoxy-4-(trimethylsilyloxy)benzoate
23.442	Syringic acid
23.452		Trimethylsilyl 3,5-dimethoxy-4-(trimethylsilyloxy)benzoate
23.923	p-Coumaric acid	p-Coumaric acid
24.644	Sinapaldehyde	Sinapaldehyde
25.337	Ferulic acid	Ferulic acid
26.644	Sinapinic acid	Sinapinic acid
29.115	Arbutin

. Newly produced lignin-derived substances, absent in untreated Moso bamboo extracts, were observed in heat-treated Moso bamboo extracts. Retention times of 20.616, 20.731, and 22.106 indicated the additional detection of the H (p-hydroxyphenyl) type lignin monomer in the heat-treated Moso bamboo extract. This is a result of the decomposition of bamboo cell wall components due to heat treatment, with the decomposed products being retained within the bamboo specimen. Despite no changes in chemical composition in heat-treated bamboo, this can explain the resistance to fungi and termites. Yu et al. analyzed the acetone/methanol extract of Moso bamboo by GC/MS, finding compounds such as vanillin and syringaldehyde in untreated Moso bamboo [45]. H (p-hydroxyphenyl), G (guaiacyl), and S (syringyl) types of lignin-derived substances were detected. The presence of such phenolic compounds can also account for the increased resistance of the heat-treated Moso bamboo specimens to rot fungi and termites [46].

In conclusion, while the change in chemical composition due to heat treatment was not explicitly analyzed, the durability of Moso bamboo increased due to the structural change in chemical composition caused by the heat treatment.

4. Conclusions

In this study, we developed a pilot-scale heat treatment technology and conducted basic characterization of heat-treated Moso bamboo. To achieve this, we considered the effects of abiotic factors (ultraviolet rays, temperature, and dryness) and biotic factors (rot fungi and termites) to analyze the application potential of this technique and the resulting heat-treated bamboo. Specifically, chemical composition analyses of the untreated and heat-treated samples revealed relatively few differences between the specimens, with no significant weight loss observed following heat treatment. This is a result of the decomposition of bamboo cell wall components due to heat treatment, with the decomposed products being retained within the bamboo specimen. Additionally, moisture content and specific gravity measurements indicated a low moisture content of <3% after treatment, coupled with a significant increase in specific gravity (approximately two times), signifying that this treatment was advantageous in terms of dimensional change and sample strength. Lastly, the heat-treated Moso bamboo exhibited superior decay resistance and enhanced termite resistance effects compared to the untreated Moso bamboo, partly attributed to the presence of phenolic-based compounds originating from degraded lignin in the treated sample. The effects of various abiotic and biotic factors demonstrated that the treated bamboo specimens exhibited greater resistance compared to the untreated sample, suggesting that treated Moso bamboo would demonstrate improved durability for exterior applications. Overall, the results obtained suggest that the described pilot-scale steam heat treatment technology can be employed for mass-producing Moso bamboo with enhanced quality, indicating its potential applicability in various landscaping facilities in the near future.