Evaluation of the Resource Utilization Potential of Capsicum Residue for Sustainable Industrial Capsaicin Extraction

Abstract

Capsicum residue generated from industrial capsaicin extraction is rich in nutrients and represents a significant fraction of solid waste in the food processing industry. Despite its potential value, limited efforts have been devoted to its resource recovery, leading to considerable resource loss and environmental burdens. This study systematically evaluates the applicability of existing food waste recycling technologies for capsicum residue and assesses its valorization potential through comprehensive characterization. The results indicate that capsicum residue holds promise as a feedstock for pectin extraction and as a component in animal feed. Regarding anaerobic fermentation for acid production, the maximum volatile fatty acids (VFAs) yield and VFAs/SCOD ratio reached 462.09 mg·L⁻¹ and 3.16%, respectively, suggesting moderate potential for acidogenic conversion but limited suitability for methanogenesis. Fluorescence spectroscopy of dissolved organic matter revealed that microbial humic-like substances (C1) were the dominant fluorophore, accounting for 42.64% of the total fluorescence, followed by terrestrial humic-like (C2, 19.28%), fulvic-like (C3, 19.12%), and tryptophan-like (C4, 18.95%) components. The favorable C/N ratio of amino acids and humic substances supports the feasibility of composting. Additionally, trace levels of residual capsaicin may confer antibacterial benefits and enhance soil fertility, further supporting the potential of capsicum residue as a value-added resource.

1. Introduction

The rapid growth of global population and shifting consumption patterns have led to a significant increase in food waste (FW) generation. Food waste, which includes vegetable residues, fruit peels, and by-products from meat processing, represents a complex and heterogeneous biomass stream with high organic content and moisture levels [1]. Improper management of FW not only occupies vast land resources but also poses serious environmental risks, including leachate infiltration and greenhouse gas emissions. It is projected that FW generation in Asia will reach 4.16 billion tons by 2025 (China National Bureau of Statistics, 2020). The high moisture content, variable composition, and organic richness of FW complicate its treatment and resource recovery [2]. Currently, landfilling and incineration remain the predominant disposal methods [3]; however, these approaches fail to recover valuable organic matter and may lead to soil and groundwater contamination [4]. Therefore, developing tailored and sustainable treatment strategies based on the specific characteristics of FW is urgently needed.

Capsicum frutescens L., commonly known as chili pepper, is a widely cultivated and consumed crop, particularly in China [5]. In addition to its culinary uses, chili pepper is extensively utilized in industrial applications, especially for the extraction of capsanthin, a natural red pigment. Capsanthin exhibits excellent color stability, non-toxicity, and strong coloring capacity, making it highly desirable in food, pharmaceutical, and cosmetic industries. For instance, it is commonly used in artificial food coloring, sugar coatings, and enteric capsules, and has been shown to disperse uniformly in medicinal syrups, making it suitable for pediatric formulations [6]. Thus, capsanthin holds significant commercial potential across multiple sectors.

However, the extraction of capsanthin generates a considerable amount of solid residue, the quantity of which varies depending on the extraction method. For example, solvent-based extraction yields 30–50% residue by dry weight [7], while supercritical CO₂ extraction produces 15–25% residue [8]. The accumulation of this residue places increasing pressure on waste management systems. Traditional disposal methods such as landfilling and composting are limited by land requirements, leachate generation, and potential groundwater pollution. Therefore, developing innovative and sustainable strategies for the valorization of capsicum residue is essential.

Previous studies have explored limited applications of capsicum residue. For instance, it has been evaluated as a low-value animal feed [9], a source of pectin [10], and a substrate for protein extraction [11]. Akinci et al. [12] incorporated chili pulp protein and oil into corn starch films, enhancing their mechanical strength and antibacterial properties. Anaerobic digestion of chili residues for biogas production has also been investigated, with untreated chili pulp yielding 464 ± 25 NL CH₄/kg VS [13]. However, challenges such as VFA accumulation, process instability, and foaming remain unresolved [1]. Other studies have explored the conversion of chili residues into biofuels [14], but high processing costs and technical complexity hinder large-scale application [15]. While composting is a mature and environmentally friendly option, issues such as gas leakage and inefficient carbon sequestration persist [16,17,18].

In China, chili pigment extraction is concentrated in five major regions, Xinjiang, Inner Mongolia, Gansu, Hebei, and Yunnan, with Xinjiang being the most prominent [19]. In 2023, the chili cultivation area in Xinjiang reached approximately 500,000–600,000 mu (33,333–40,000 hectares), with a total fresh chili output of 1.5–2 million metric tons [20,21]. Consequently, large volumes of capsicum residue are generated, containing valuable components such as pectin, vitamins, and bioactive compounds. This underscores the industrial significance and environmental urgency of developing high-value utilization pathways for capsicum residue.

Despite its potential, the high-value resource utilization of capsicum residue remains underexplored. This study aims to address this gap by investigating the physicochemical properties of capsicum residue and evaluating its potential for valorization through various bioconversion and extraction technologies. The findings are expected to contribute to (i) the development of sustainable waste management strategies for the chili processing industry and (ii) the promotion of circular economy practices in agricultural waste utilization.

2. Materials and Methods

2.1. Source and Physical Properties of Capsicum Residue

Figure 1 illustrates the industrial extraction process used to obtain capsaicin from chili peppers via solvent extraction and leaching. The production process begins with harvested capsicum, which is industrially ground into particles to facilitate extraction. These particles are then loaded into extraction equipment where a solvent is used to dissolve the target compounds, primarily capsaicinoids. Following this extraction, the process splits into two streams: the solid leftover, now depleted of most solubles, is discharged as the capsicum residue, while the liquid extract is concentrated through evaporation to remove the solvent. This concentrate is subsequently purified by passing it through decolorizing particles to remove impurities and pigments, resulting in the semi-finished capsaicin product. After capsaicin is separated and packaged, the by-product—capsicum residue—is generated. The residue used in this study was supplied by a biotechnology company. It contains trace amounts of capsaicin, carotenoids, vitamin C, and other organic compounds. Physically, the residue is a deep-red, semi-solid material with high viscosity and fluidity at room temperature. It has a moisture content of 65.25%, a pH of 4.53 (indicating weak acidity), an oxidation–reduction potential (ORP) of 218.8 mV, and an ash content of 11.86%. It also emits a pungent odor.

2.2. Chemical Properties of Capsicum Residue

The pectin content in capsicum residue was analyzed using the carbazole-sulfuric acid spectrophotometric method. Briefly, the dried residue was subjected to acidic hydrolysis to solubilize pectic polysaccharides. An aliquot of the hydrolysate was then reacted with a carbazole solution in concentrated sulfuric acid. The resulting pink chromogen was measured at a wavelength of 530 nm using a UV-Vis spectrophotometer (TU-1800, Beijing Purkinje General Instrument Co., Ltd., Beijing, China). A standard calibration curve was prepared using galacturionic acid, and the pectin content was expressed as galacturonic acid equivalents. The pectin yield was subsequently calculated using Formula (1).

$$\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}={\displaystyle \frac{\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d} \mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{n}}{\mathrm{d}\mathrm{r}\mathrm{i}\mathrm{e}\mathrm{d} \mathrm{c}\mathrm{a}\mathrm{p}\mathrm{s}\mathrm{i}\mathrm{c}\mathrm{u}\mathrm{m} \mathrm{p}\mathrm{o}\mathrm{w}\mathrm{d}\mathrm{e}\mathrm{r}}}\times 100\mathrm{\%}$$

(1)

The Vitamin C content in capsicum residue was quantified using high-performance liquid chromatograph (HPLC). The analysis was performed on an HPLC system (Agilent 1260 Infinity II, Agilent Technologies, Santa Clara, CA, USA) equipped with a UV detector and a reversed-phase C18 column (ZORBAX Eclipse Plus, Agilent Technologies, Santa Clara, CA, USA, 4.6 × 150 mm, 5 μm). The capsicum residue was extracted with a meta-phosphoric acid solution to stabilize vitamin C prior to analysis. The mobile phase consisted of a potassium dihydrogen phosphate buffer (pH 2.5) and was run under isocratic conditions at a flow rate of 1.0 mL/min. Detection was carried out at a wavelength of 245 nm. Quantification was achieved by comparing the peak areas of the samples to a Vitamin C standard calibration curve.

Fluorescent organic components in the residue were identified via three-dimensional excitation–emission matrix (EEM) fluorescence spectroscopy using a Shimadzu RF-6000 spectrofluorometer (Shimadzu Corporation, Kyoto, Japan).

2.3. Anaerobic Sludge Co-Fermentation

The anaerobic sludge inoculum, characterized by 16S rRNA gene sequencing, was dominated by bacterial phyla essential for hydrolysis and acidogenesis, including Bacteroidetes (~28%) and Firmicutes (~22%). The archaeal community, responsible for methanogenesis, was primarily composed of the acetolactic genus Methanosaeta (~61%) and the hydrogenotrophic genus Methanobacterium (~17%), indicating a robust methanogenic potential. The anaerobic sludge collected from the mesophilic digester of Beijing Jingcheng Huitong Environmental Protection Co., Ltd. (Beijing, China). (pH 7.3, ORP −120 mV, vs. 24.8%) was blended with the residue to give a VS-based ratio of 6:1, and the mixture was adjusted to the desired starting pH. The initial total solids (TS) concentration of the fermentation system was 6.4%. Two series of 250 mL serum bottles were prepared, each sealed with butyl rubber stoppers, flushed with N₂ and connected to gas-sampling bags to ensure strict anaerobiosis. One series was incubated at 25 °C and the other at 35 °C, both shaken at 120 rpm. Over 20 days, aliquots were withdrawn every 48 h, centrifuged and filtered through 0.22 µm membranes for subsequent analyses. Volatile fatty acids (VFAs) were quantified using a Gas Chromatograph (Agilent 7890B, Agilent Technologies, Santa Clara, CA, USA) equipped with a flame ionization detector (FID) and a DB-FFAP capillary column (30 m × 0.32 mm × 0.25 μm). High-purity nitrogen was used as the carrier gas at a constant flow rate of 2.0 mL/min. The injector and detector temperatures were set at 250 °C and 300 °C, respectively. The oven temperature program was as follows: initial temperature held at 70 °C for 1 min, then increased to 140 °C at a rate of 10 °C/min, and finally ramped to 240 °C at 20 °C/min and held for 2 min. VFAs converted to COD equivalents using standard stoichiometric factors of 0.35, 1.07, 1.51 and 1.81 g COD g⁻¹ for formic, acetic, propionic and n-butyric acids, respectively.

3. Results

3.1. Nutrient and Water-Soluble Organic Components in Capsicum Residue

Table 1. Comparative analysis of Pectin and Vitamin C contents in capsicum residues and other commonly occurring fruit and vegetable waste materials in research studies (Source: Authors’ own data).

Sample	Pectin (%)	Vitamin C (mg/100 g)	Common Treatments
Capsicum residues in this research	8.09	27.65	-
Capsicum residue	9.64–15.62 [10]	-	Animal feed, composting
Apple pomace	23.26 [23]	2.545 [24]	Antioxidant substance extraction [25]
Banana peel	11.31 [26]	24.11 [27]	Minerals and antioxidant extraction
Mango peel	18.5 [26]	4.57–5.20 [28]	Phenolic composting
Grape pomace	32 [29]	10–20 [30]	Composting
Orange peel	11.6–17.5 [31]	26.0 (Dried orange peel from sunlight drying) 50.2 (Fresh orange peel) [32]	Essential oils, biofuels, pectin extraction
Passion fruit peel	6.5 [33]	-	Dietary fiber extraction
Coffee pulp	6.7 [34]	20–60 (Fresh coffee pulp) 5–20 (Dried coffee pulp) [35]	Pectin extraction and composting
Beet pulp	15–30% [36]	4–8 [37]	Cellulose extraction
Berry pulp	5–8 [38]	5–12 [39]	Pigment extraction, composting
Potato pulp	4–15 [40]	5–15 [41]	Starch recovery and composting

presents the pectin and vitamin C contents in capsicum residue compared with other common fruit and vegetable wastes reported in previous studies. The pectin was extracted from the residue using a hydrochloric acid solution (pH 2.0) at a solid-to-liquid ratio of 1:25 (w/v), 85 °C, for 90 min, yielding 8.09%. The extracted pectin was characterized as a high-methoxyl pectin with a degree of methylation of 65% and a galacturonic acid content of 75%, meeting the key quality criteria outlined in the Chinese industry standard QB 2484-2000 [22]. Meanwhile, the vitamin C content in fresh chili peppers is approximately 110 mg/100 g, whereas the residue contained significantly less vitamin C (27.65 mg/100 g).

The water-soluble organic components in capsicum residue were analyzed using excitation–emission matrix (EEM) fluorescence spectroscopy. After preprocessing to eliminate Rayleigh and Raman scattering, the fluorescence contour plot (Figure 2) revealed the presence of aromatic compounds, particularly fulvic-like and humic-like acids, as well as protein- and peptide-like substances [42,43].

Figure 3 shows the 4 fluorescent components (C1, C2, C3, and C4) in capsicum residue obtained by Parallel factor analysis (PARAFAC) analysis, where colors represent the fluorescence intensity in Raman units (R.U.). The model generated by PARAFAC was uploaded to OpenFluor for querying to match the carrier spectra with the data from other studies. The minimum similarity of the models was above 0.95.

Component C1 exhibits its maximum excitation when the excitation wavelength Ex < 250 nm, with a lower excitation peak at 300 nm and an emission peak at 390 nm. Its spectral characteristic is peak M, which mainly represents low molecular weight microbial humus-like fluorescent groups. In anaerobic systems, these microbial-derived humic substances are not merely metabolic byproducts; they can act as electron shuttles, potentially facilitating direct interspecies electron transfer (DIET) to enhance the efficiency of acidogenesis and methanogenesis [44,45]. The dynamics of C1 may thus reflect the activity of the key microbial consortia and the overall stability of the fermentation process.

C2 shows a major excitation maximum at 260 nm, followed by a lower intensity excitation peak at 350 nm and a maximum emission peak at 490 nm. The C2 spectral characteristics are similar to the traditional humus peaks A and C [46], and are considered to be high molecular weight terrestrial source humus material [47]. The C3 excitation peak is located at Ex < 300 nm, with a slightly lower excitation peak at 350 nm and an emission peak at 425 nm. The C3 component is mainly composed of terrestrial yellow rot compounds [48]. The C4 excitation peak is located at Ex < 250/275 nm, with an emission peak at 335 nm. The component is considered to be protein-like substances [49].

Furthermore, the presence of residual capscaicin in the substrate may have a shaping influence on this fluorescent organic matter profile. Its antimicrobial properties could selectively inhibit certain microbial populations, thereby indirectly affecting the synthesis and degradation pathways of components like C1 (microbial humics) and C4 (protein-like substances). This potential interaction offers a plausible explanation for the unique evolution of the fluorescent components observed in capsicum residue compared to other organic wastes.

Figure 4 illustrates the relative abundance of these components. Microbial humic-like C1 was the dominant fraction, accounting for 42.64% of total fluorescence. Terrestrial humic-like C2, fulvic-like C3, and protein-like C4 contributed 19.28%, 19.12%, and 18.95%, respectively.

3.2. VFAs Production in the Anaerobic Fermentation

Poly-hydroxy-alkanoates (PHAs) are bioploymers derived from bacteria, which can serve as carbon sources for biological culture media and essential nutrients for growth limiting concentrations [50] The microbial synthesis of PHAs mostly uses high-value pure substances such as glucose and pure acid as carbon sources, but the production cost is relatively high [51,52]. VFAs and lactic acid can serve as low-cost carbon source for PHAs synthesis. Many researchers have studied VFAs, anaerobic fermentation products from bagasse, straw, food processing waste, and municipal sludge as the low-cost resources for PHAs synthesis [53,54]. This study aims to determine the potential of capsicum residue as a raw material for synthesizing PHAs by analyzing the yield and composition of VFAs and lactic acid produced by anaerobic fermentation of capsicum residue under different temperature and pH conditions.

The VFAs composition and concentration of capsicum residue after 20 days of anaerobic fermentation under initial pH values of 4, 5, 6, and 7 at 25 °C and 35 °C are shown in Figure 5. In general, the VFAs concentration reached a maximum of 462.09 mg/L after 14 days of fermentation at a temperature of 35 °C and initial pH of 6, with the concentrations of Formic Acid, Lactic Acid, Acetic Acid, Propionic Acid, N-butyric Acid of 108.78 mg/L, 45.42 mg/L, 172.33 mg/L, 68.06 mg/L, 67.50 mg/L, respectively.

The composition of VFAs formed by anaerobic fermentation of capsicum residue for 20 days was analyzed, and it was found that isobutyric acid, n-valeric acid, and isovaleric acid were not detected in several samples. Butyric acid was only detected at a temperature of 35 °C and an initial pH of 5. When the pH is 4 and 5, formic acid is the main component of VFAs, and when the pH is 6 and 7, acetic acid is the main component of VFAs. Under the conditions of producing the highest concentration of VFAs (temperature of 35 °C pH of 6), acetic acid is the main component, accounting for 37.39%.

3.3. Lactic Acid Production in the Anaerobic Fermentation

Jiang et al. [55] found that lactic acid as a carbon source has more advantages in synthesizing PHAs, and the higher the content of lactic acid, the higher the content of synthesized PHAs. In addition, lactic acid has high stability and its properties are not easily changed during storage and transportation [56]. Therefore, the lactic acid content in anaerobic fermentation broth is also an important factor to consider when using capsicum residue as a raw material for PHA synthesis.

The proportion of lactic acid after 20 days of anaerobic fermentation under initial pH of 4, 5, 6, and 7 at temperatures of 25 °C and 35 °C is shown in Figure 6. The proportion of lactic acid in pH of 4 and 5 is higher than that at pH of 6 and 7, indicating that acidic conditions are more conducive to lactic acid production. When the temperature is 35 °C and the pH is 5, the proportion of lactic acid is highest at 44.35% after 2 days of fermentation.

3.4. VFAs/SCOD Ratio as an Indicator of Acidification Efficiency

Soluble COD (SCOD) characterizes the hydrolysis efficiency of anaerobic fermentation, including the content of VFAs, soluble carbohydrates, and proteins [57,58]. The ratio of VFAs/SCOD is the ratio of the sum of COD concentrations corresponding to VFAs to SCOD, indicating how much dissolved organic matter in the fermentation broth is converted into VFAs, reflecting the anaerobic acid production effect. The larger the ratio, the better the acid production effect [55,57].

The results of the VFAs concentration and the ratio of VFAs/SCOD under different temperature and pH were shown in Figure 7. Under the temperature of 35 °C and pH of 6, the VFAs/COD ratio of anaerobic fermentation of capsicum residue was higher than that of other conditions, indicating that the acid production effect was the best under this condition, which was similar to the trend of VFAs production. Under the temperature of 25 °C and pH of 7, VFAs concentration and VFAs/SCOD ratio increased slowly with anaerobic fermentation time, and the increase value of VFAs was higher than other conditions at 25 °C. However, under other conditions, the VFAs concentration and VFAs/SCOD ratio fluctuated with time, which may be due to the inhibition of the fermentation process due to the limitation of the activity of acidogenic bacteria. Under the condition of temperature of 35 °C and pH of 7, VFAs/SCOD ratio showed an upward trend in the first 4 days, and gradually decreased in the last 16 days. Under the condition of temperature of 35 °C and pH of 6, there was a minimum total VFAs/SCOD ratio at 12 days of fermentation. With the increase in fermentation time, the limit was also lifted, and reached the highest VFAs/SCOD ratio of 3.16% after 14 days of fermentation. In addition, it was found that there was no obvious gas production in the anaerobic fermentation process except under the condition of temperature of 35 °C and pH of 7, indicating that the substrate was not converted into methane and other gases in the whole anaerobic fermentation process.

4. Discussion

4.1. Feasibility of Recycling Valuable Substances from Capsicum Residue

Pectin extracted from various agro-industrial residues is widely used as a thickening, gelling, and stabilizing agent in the food, pharmaceutical, and cosmetic industries [59]. For optimal performance in food matrices such as confectionery, yogurt, and fruit juices, pectin must meet specific requirements regarding degree of methoxylation and molecular weight [10]. Wang et al. [60] optimized ultrasound-assisted acid extraction of pectin from chili peppers, achieving a yield of 16.85%, which is comparable to that of citrus peel—a conventional pectin source. The extracted pectin also met the quality standards specified in the Chinese National Standard QB 2484-2000.

In this study, the pectin extraction yield from capsicum residue was 8.09%, slightly lower than the 8.72% reported by Xu et al. [10]. This discrepancy may be attributed to differences in chili varieties, capsaicin extraction methods, or pectin degradation during storage and transportation. It is well documented that pectin-rich biomass is sensitive to moisture, temperature, and processing conditions, all of which can affect extractability and functionality [61]. Therefore, while capsicum residue shows potential as an alternative pectin source, further optimization of storage protocols and extraction techniques is necessary for its industrial application.

Vitamin C content in capsicum residue is significantly influenced by processing, storage, and environmental factors, as ascorbic acid is highly susceptible to degradation under light and heat [62]. Although the residue retains a relatively high vitamin C content compared to many other fruit and vegetable wastes, its recovery is less economically attractive due to lower yields and higher sensitivity to handling conditions. Thus, from a valorization perspective, pectin extraction appears to be the more viable route.

As shown in Table 1, apple pomace, mango peel, and grape pomace exhibit the highest pectin contents, while capsicum residue, banana peel, and citrus peel fall within the intermediate range. In contrast, passion fruit peel and coffee pulp show relatively low pectin levels. Regarding vitamin C, capsicum residue, banana peel, and citrus peel contain notably higher concentrations than apple pomace and mango peel. These results highlight the dual-value potential of capsicum residue as a source of both pectin and vitamin C, supporting its candidacy for integrated biorefinery approaches.

The fluorescence characteristics of capsicum residue in this study are consistent with those reported by Pérez-Murcia et al. [43] prior to composting. However, their study showed that protein- and peptide-like substances were largely degraded during aerobic composting, while fulvic- and humic-like compounds accumulated. This suggests that aerobic composting of capsicum residue not only stabilizes organic matter but also enhances the formation of humic substances, offering both economic and environmental benefits. Therefore, depending on the target product—pectin, vitamin C, or humified organic matter—different processing strategies (e.g., extraction vs. composting) can be applied to maximize resource recovery from capsicum residue.

4.2. Evaluation of Anaerobic Fermentation Potential

The production of volatile fatty acids (VFAs) during anaerobic fermentation is strongly influenced by pH and temperature. An initial pH of 6 favors the enrichment of hydrolytic and acidogenic microbial communities, promoting the activity of extracellular hydrolytic enzymes. These enzymes catalyze the breakdown of complex organic macromolecules into soluble monomers, which are subsequently metabolized by acidogenic bacteria to produce VFAs [63].

The optimal temperature range for acidogenic bacteria is 35–55 °C. Elevated temperatures accelerate hydrolysis rates and enhance microbial metabolic activity, leading to increased conversion of soluble carbohydrates and proteins into VFAs [64].

VFAs such as acetic, propionic, n-butyric, and isobutyric acids are primarily derived from the fermentation of soluble proteins, carbohydrates, and lipids, whereas n-valeric and isovaleric acids are typically produced via proteolysis [57]. Notably, branched-chain VFAs (e.g., propionic, butyric, and valeric acids) are readily degraded into acetic acid prior to methanogenesis.

At pH 6, methanogenic activity is inhibited due to suboptimal conditions (optimal pH for methanogens: 6.5–8.5) [65], resulting in acetic acid accumulation and minimal degradation of intermediate VFAs [66,67]. This explains the absence of isobutyric, n-valeric, and isovaleric acids, and the dominance of acetic acid at pH 6–7.

Lactic acid production was favored under acidic conditions (pH 4–5), consistent with Tang et al. [68], who reported that neutral to alkaline pH suppresses lactic acid bacteria (LAB) activity. At pH 6, LAB activity declines prematurely, limiting lactic acid accumulation [69]. Additionally, lactic acid can be further metabolized into propionic acid, which is subsequently converted to acetic acid [70]. The limited dissolution of organic matter in the fermentation broth resulted in the reduction in the content of soluble organic matter, thus affecting the ratio [71].

The higher the pH, the more likely it will produce more soluble chemical oxygen demand, such as soluble proteins and carbohydrates, which will make the substrate easier to acidify [72]. Higher pH values generally enhance the solubilization of proteins and carbohydrates, increasing substrate bioavailability for acidogenesis. However, under these conditions, methanogens may become active, reducing VFA accumulation through methanogenic consumption [71].

4.3. Environmental Implications

As a by-product remaining after capsaicin extraction, capsaicin residue retains various nutrients such as vitamins, pectin, and amino acids. However, due to the influence of the extraction process, the actual measured contents of pectin and vitamins, and other bioactive compounds are considerably lower than those in the original capsicum material. A preliminary techno-economic analysis reveals that the dedicated re-extraction of these components from the residue is currently challenged by high operational costs and limited yield, resulting in a unit cost that is not yet competitive with established pectin sources. Thus, while capsicum residue holds potential as a raw material for future extraction applications, its utilization must be evaluated in terms of technical feasibility, economic viability, and environmental impact to identify more sustainable approaches.

Capsicum residue is rich in organic matter, humus, and protein, making it a suitable feedstock for composting and bioenergy production. However, for large-scale implementation, practical barriers must be addressed. The high moisture content (65.25%) significantly increases transportation and handling costs, suggesting that pre-treatment (e.g., mechanical dewatering) or decentralized processing near source locations is critical for economic feasibility.

Studies on the anaerobic co-fermentation of capsicum residue with sewage sludge have shown that, under conditions of 35 °C and pH at 6, the maximum VFAs/SCOD ratio reached 3.16% after 14 days within a 20-day fermentation period. However, throughout the entire process, the VFA/SCOD ratio remined below 4%, indicating a limited conversion efficiency. This suggests that while capsicum residue has the potential for PHA synthesis, the low VFA yield may be attributed to insufficient organic content, low biodegradability, or suboptimal fermentation conditions. Furthermore, at scale, operational challenges such as foam control would need to be managed. To enhance acidogenic performance, an integrated biorefinery approach is recommended. Further investigation should focus on adjusting parameters such as C/N ratio, N/P ratio, and hydraulic retention time (HRT) within a cascading valorization framework. For instance, the solid fraction after fermentation retains value and can be directed to composting, thereby improving the overall benefit–cost ratio of the waste management chain.

In a related study, Du et al. [73] investigated the effect of capsaicin as an additive on short-chain fatty acid (SCFA) production during food waste recycling. They found that low concentrations of capsaicin (4 mg/g VS) promoted solubilization, hydrolysis and acidification processes, accompanied by a mild accumulation of reactive oxygen species (ROS) that stimulated microbial activity, resulting in a 12.5% increase in SCFA yield. In contrast, high concentrations exerted an inhibitory effect. These findings suggest that determining the residual capsaicin content in the residue and utilizing it as an additive to enhance fermentation of other organic substrates may represent a viable pathway for valorizing capsicum residue. This strategy of using the residue as a low-cost bio-stimulant could offer a higher economic return and simpler implementation pathway than dedicated extraction or standalone fermentation, presenting a promising avenue for industrial application.