The ash, crude fiber, and carbohydrate content of HPP powder were determined during the experiment. Adapted from Hazrati et al. [2], the methodologies used to explore acid detergent fiber (ADF), neutral detergent fiber (NDF), ash, crude fiber, lignin, hemicellulose, and cellulose in HPP powder were used. The chemical composition of HPP powder was determined using the ADF and NDF. Eqs. (4) and (5) were used to calculate the amounts of hemicellulose and cellulose.

(4) Cellulose=ADF−lignin

(5) Hemicellulose=NDF−ADF

The samples’ surface morphology was determined using the Hitachi S-3400N scanning electron microscope (Hitachi Science Systems Ltd., Japan). To establish an electron beam, the samples were coated with a layer of gold, and a 20 kV voltage was delivered through them in a high vacuum environment. The electrons were coupled to the sample atoms and generated signals that provided a report on the surface topography by generating high-resolution images.

The chemical composition and structure of the sample were determined using Fourier transform infrared (FTIR) Spectroscopy. Spectra of the sample were obtained using JASCO FTIR-6100 Spectrometer (JASCO Corporation, Japan). Spectrum was plotted between 4000–400 cm⁻¹ wavenumbers.

A 2500 X-ray diffractometer (Instrument-Rigaku, Tokyo, Japan) with an angular range of 5 to 60° (2θ) and a scattering speed of 0.02θ s⁻¹ was used for the XRD analysis. 35 mA and 40 kV were chosen as the operating current and voltage, respectively. Using the approach of Segal et al. [23], the crystallinity index (CI) of the sample was quantitatively determined from the diffraction intensity data. Eq. (6) was used to get the crystallinity index (%).

(6) CrystallinityIndex(CI)=I002−IamI002×100%

where I₀₀₂ is the diffraction intensity close to 2θ = 22° and represents a crystalline material; I_am is the diffraction intensity close to 2θ = 18° and refers to amorphous material in cellulosic fibers.

Thermogravimetric analysis was employed to characterize the material’s thermal degradation behavior in terms of weight loss owing to temperature increase. TGA/DSC 3+ (Mettler-Toledo AG, Analytical, Switzerland) was used in the testing. The weight samples range was 10 ± 2 mg. In an aluminum pan, the test was conducted at a temperature range of 25°C to 600°C at a heating rate of 10 °C min⁻¹ in a dynamic nitrogen environment.

The presence of cellulose, lignin, and hemicellulose in HPP powder is one of the most relevant findings in the study of fiber composition. According to Table 3, the chemical composition analysis results were compared to those of prior studies. It was discovered that the cellulose content of HPP powder (34.45%) was significantly higher than the cellulose content of passion fruit peel fiber (28.58%) and prickly pear fruit peel (27.00%). Furthermore, the cellulose percentage of HPP powder was higher than that of cellulose derived from other bio-peel waste, such as cucumber peels, banana peels, orange peels, and mango peels, which were 18.22%, 16.9%, 11.93%, and 9.19%, respectively [35,36]. Additionally, the hemicellulose content of HPP fiber was not detected. These findings were consistent with the results of Bakar et al. [19] and Chia et al. [29], who conducted the same study of fiber. Furthermore, the lignin content of HPP powder (6.73%) was significantly lower than the other peel of fiber, such as passion fruit peel (36.18%). This could be since the percentage of amorphic (lignin, hemicellulose) and crystalline (cellulose) components in natural fibers vary depending on the location and condition of the plant [9]. Chen et al. [37] explained that cellulose is a critical component of natural fiber structure and plant structural strength. The amount of lignin present in the fibrous residue, on the other hand, was evaluated to assess the relative number of resistant components present in the fibrous residue, which plays an important role in the strength and stiffness of the fiber-based walls [2].

On the other hand, natural fibers mainly comprise holocellulose (hemicellulose, cellulose), lignin, crude fiber, and ash. Increased composite tensile strengths were achieved by introducing high cellulose amounts, which allowed for better matrix interactions [38]. When natural fibers were mixed with thermoplastic starch and its derivatives, the mechanical properties of the composite were significantly improved [39]. The chemical similarity between matrix and fibers was discovered to contribute to this property, resulting in optimal composite compatibility [40]. Numerous researches have established the optimal performance of polymer-based biodegradable composites [41]. According to Uitterhaegen et al. [42], inconsistency in fibers could attribute to differences in lignin content or origin, as well as differences in bioplastic manufacturing techniques.

The microstructures of the HPP powder are shown in Figs. 3a and 3b, respectively. The peeled surface appeared shriveled and had irregular geometry. The finding was in agreement with studies conducted by Chia et al. [29] and Bakar et al. [19] for the drum drying and spray-drying of dragon fruit peel powder. The shriveled look might be related to the sluggish drying rate during the manufacture of the peel powder, as Tonon et al. [49] found that when the inlet air temperature was low, most powder particles stay shrunk with a shriveled surface. Meanwhile, Alamilla-Beltrán et al. [50] stated that the physical features of the crust, which might be flexible and collapsed (when low and intermediate temperatures were used) or stiff and porous (when high temperatures were employed), could explain morphological variations between powders formed at various temperatures. Similarly, Gan et al. [51] found that due to the physical grinding process performed on a millstone, the powders particle were subjected to friction and shearing force randomly, resulting in the powder surfaces being rough and irregular.

The FTIR technique was used to investigate the functional groups available in Hylocereus Polyrhizus peel powder. In the region between 4000–400 cm⁻¹ of FTIR spectra, major functional groups in HPP powder were identified, as presented in Fig. 4. The hydrophilicity of the HPP powder was indicated by the broad absorption band in the 3650–3000 cm⁻¹ regions, attributable to the O–H groups presented in the components. The peak at 3288 cm⁻¹ was attributed to intramolecular hydrogen bonding in cellulose II [52]. Furthermore, the peak at 2937 cm⁻¹ was caused by cellulose aliphatic saturated C–H stretching vibration [53].

The presence of a peak located at ~1743 cm⁻¹ indicated the presence of C=O stretching of the acetyl and uronic ester groups of polysaccharides, such as pectin, lignin, and hemicellulose [54,55]. The finding was almost similar to that reported by Ribeiro et al. [56] in untreated mandarin peels, where the peak was located around ~1750 cm⁻¹. The p-coumeric acids of lignin and/or hemicellulosic acids were also linked to this peak [57]. Peak 1602 cm⁻¹ appeared to represent the stretching vibration of carboxyl ions (COO–) which indicated the presence of pectin in HPP powder [10]. This is according to the spectral region between 1500 and 1800 related to carboxylic acids and carboxylic esters, which are the key functional group responsible for the characteristics of pectin [58]. Furthermore, the carbohydrate fingerprint area found at wavenumber 800–1300 cm⁻¹ can be utilized to identify the major chemical groups of polysaccharides [59].

Hemicellulose and lignin are amorphous, whereas cellulose has both crystalline and amorphous domains in its structure [60]. The crystalline structure of cellulose influences its mechanical and thermal properties. The reinforcing ability and mechanical strength of cellulose, in particular, are critical characteristics for its use in environmental remediation technologies. Fig. 5 displays the X-ray diffractogram of the Hylocereus Polyrhizus peel powder. From the observation, there were three obvious peaks shown in the XRD pattern of the sample at diffraction angles of 14.55°, 22.17°, and 32.36°. The crystallinity pattern of cellulose for HPP powder was seen by the peak at 2θ = 22.17° [61], which corresponded to the 200 lattice plane of cellulose I structure. Meanwhile, the characteristic peak of HPP powder identified at 2θ = 14.55° and 32.36° were 110 and 004 lattice planes of cellulose I, indicating the presence of an amorphous region. Similar results were reported by Abiaziem et al. [44] for cellulose nanocrystal obtained from sugarcane peel and Huang et al. [62] for corncob pretreatment on the delignification and enzymatic hydrolysis.

Moreover, the crystallinity index for HPP powder was calculated using Eq. (1). which is described in the experimental section and found to be 32.76%. This result showed a high degree of crystallinity than cellulose from banana peels (15%) [53] and cellulose from Pomelo albedo (25.1%) [63], but lower than Cucumis sativus peels (56.7%) [35] and cassava root peel (56.3%) [64]. As a result, the crystallinity value varied based on the plant type and the method of fiber processing. The value of the crystallinity degree region and the material properties were related, with an increase in the crystallinity degree region enhancing the material’s strength [46].

Thermal stability and weight loss of HPP powder were determined using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG). The TGA and DTG curves in Fig. 6 depict three distinct stages of weight loss (Table 4), as shown by significant peaks on the DTG curve. Overall, the weight loss of HPP powder occurred in the temperature range of 30°C–600°C. The initial weight loss was noticed in the TGA curve between 42°C and 108°C. The elimination of moisture and volatile organic compounds such as oils, terpenes, and pigments was responsible for this weight reduction [35,65,66]. The second weight loss was recorded between 204°C and 306°C, corresponding to the decomposition of cellulose [67]. The decomposition in the region of 318°C and 480°C referred to cellulose degradation; meanwhile, the lignin degradation took a wide temperature range between 100°C and 900°C [53,68].

In the DTG curves for HPP powder, three main peaks were observed located at 78°C (peak 1), 294°C (peak 2), and 479°C (peak 3), with mass losses of 7.16%, 34.92%, and 14.05%, respectively. The HPP powder demonstrated thermal decompositions of cellulose contents, as indicated by the strong U-shaped peak detected at 294°C. The presence of a final large thermal degradation peak near 479°C verified the degradation of the fiber’s cellulose content as well as the removal of other non-cellulosic chemical elements. Overall, the thermal stability of the HPP powder was approximated to be 204°C, comparable to the thermal stability of Dioscorea hispida tubers [2].

The residual char in the TGA analysis indicates the leftover or remaining materials after all volatile components were eliminated during the pyrolysis process [34]. Residual char of HPP powder was found to be 31.46%. This finding was parallel with the reported study conducted by Sahari et al. [69] that revealed the residues might be associated with the presence of inorganic material and silicon dioxide in the natural fiber, which can only be decomposed at a very high temperature. On the other hand, the composition of lignin and ash in Hylocereus Polyrhizus peel powder also contributed to char residue content.