Agricultural Pectin Extraction in Iranian Experimental Settings

Abstract

Pectins are belonged to one important group of polysaccharides extracted from the plant cell walls. Commercial pectins are widely used in the cosmetic, pharmaceutical, and food industries, mainly as texturizing, emulsifying, stabilizing, and gelling agents. Due to rich contents of partially esterified galacturonic acid (GalA) found in agri-food waste, the valorization of recovery process needs to be further developed with economic and environmental benefits. Accordingly, in order to maximize the utilization of these residues, some researchers attempted to extract multiple valuable products from plant waste like pectin from mango peel or simultaneously extracted pectin and polyphenols from pomegranate peels, because the simultaneous extraction seems more efficient due to decreased process time and cost. The characteristics and applications of pectins are strongly influenced by their structures depending on plant species, tissues, and extraction methods. This review aims to review the optimal extraction conditions using new promising methods in order to obtain pectin from Iran’s Agro waste and assess physicochemical parameters in recent Iranian experimental study designs, including microwave heating processes and ultrasonic treatment.

1. Introduction

Plant pectins are complex polysaccharides with an acceptable content of galacturonic acid (GalA) determined as 65% for commercial purposes like pectin from apple pomace, citrus peels, or sugar beet pulp [1, 2] with different applications based on low (<50%) and high (>50%) methoxyl pectin [3]. High- and low-methylated pectins are often applied according to their different DE, physicochemical properties, and applications; for example, the latter are used in low-calorie products like dietetic jams and jellies [2].

There are various potential sources in pectin production like agricultural wastes [1], for instance, the extracted pectin from mango peel, pomegranate peels, or sour cherry pomace through different extraction procedures and their physicochemical, structural, and functional properties of the extracted compounds have been studied in the literature [3].

Conventionally, pectin can be easily extracted through a cheap and time-consuming acidic hot water extraction (HWE) procedure based on mineral acids like hydrochloric acid, nitric acid, and sulfuric acid [1, 2, 4] based on some factors like raw material, the type of purposed pectin, and manufacturer’s instruction [1]. Then, the pectin is recovered by precipitation using ethanol with higher extraction yield. But unfortunately, mineral acids cause serious toxicity and hazardous effect toward the environment, and organic acid like citric acid can be an alternative to this problem [2].

Hence, the extracted pectin is affected by degradation in both quantity and quality aspects with the consequent huge pollutant effluent [1, 4]. It is required to investigate highly efficient and eco-friendly alternative procedures with different plant sources and experimental settings through various optimization protocols for extraction processes [1]. An eco-friendly extraction process can maximize the use of agricultural waste, minimize the volume of the remained waste, and produce the valuable compounds from the global increased agricultural waste in order to have the sustainable waste valorization and the highest utilization of food waste. For example, a large quantity of the industrially processed fruit converts into a large volume of the inevitably produced pomace. The fruit waste is usually discarded, while this valuable source of phenolics and pectin can be extracted in order to increase the financial benefits of production units and reduce the volume of fruit waste and the subsequent environmental problems [5].

These methods include pulsating hydrodynamic action, microwave-assisted extraction (MAE), ultrasound-assisted extraction (UAE), and ultrasound microwave-assisted extraction (UMAE) which lead to increased extraction yield and rate, reduced process time and costs, as well as low environmental hazards aimed to obtain the desirable pectin stability and minimized pectin degradation [1, 4]. Other MAE advantages include reduced wastewater with lower use of organic solvents, increased pectin purity, improved heating rates, decreased equipment sizes, and greater control on the extraction parameters during the process [4]. Based on the cavitation effect of ultrasound waves, eco-friendly UAE methodology is based on the increased destruction of plant cell wall, better penetration, and increased rate of mass transferring with lower consumed solvents and energy [6]. The integrated ultrasonic-microwave synergistic extraction (UMSE) utilizes high energy produced in both devices and removes the defects of HWE, MAE, and UAE at an atmospheric environment with low temperature [4].

In the food industry, serious challenges for the environment are developed with massive quantities of agri-food residues and wastes, including peel, husk, seed, pomace, etc., with the loss of extractable and re-usable valuable compounds at disposal. The designed procedures can prevent the depletion of natural resources by enhancing the economic agricultural opportunities for the rural livelihoods [5].

The average annual agricultural waste is estimated as 35% in Iran with different proposed management procedures. The highest pectin content is found in premature fruits. During ripening, pectin esterase and pectinase enzymes make the pectin percentages decreased, and hence fruits gradually soften. Chemical compounds, structure, and percentage of pectin vary in various plants. Pectin plays a role in food industries as a colloidal additive, thickening/gelling agent, stabilizer, immobilizer, condenser, and emulsifier for traditional use in the production of marmalades, jams, and fruit jellies [7, 8]. According to the experimental literature, industrial pectin can be potentially extracted from the noncommercial sources of agricultural wastes from different fruits and vegetables including peach pomace, watermelon rind, pomegranate peel, papaya peel, sisal waste, pumpkin, soy hull, sunflower oilseed, lemon sour, banana peel, husk of blackberry tree branch, grapefruit peel, Akebia trifoliata husk, peanut, cocoa bean husk, grape pulp, golden kiwi, tomato, carrot, pistachio green husk, and eggplant peel/cap [2, 7].

For example, orange can play an important role in the industry and economy of Iran with producing 2.3 Mt./y in 2019. Citrus peels contain about 20–25% pectin as one of the rich sources of commercial pectin. One of the chemical pretreatments for orange wastes is dilute acid treatment with high pectin recovery in order to hydrolyze it to GalA and other sugars and dissolve the main part of hemicellulose. The valorization of orange waste is oriented on (i) direct utilization (as fertilizer and animal feed), and (ii) extraction of pectin, enzymes, and bioactive compounds with designed biorefinery systems. The relevant studies were oriented toward (a) ultrasounds and microwave treatments to obtain pectin and limonene extraction from citrus waste and (b) to cut the high price of enzyme and its long reaction time in order to eliminate the non-eco-friendly enzymatic hydrolysis [8].

2. Structural and functional properties

Physicochemical structure and functional properties are studied according to (i) gelling properties, (ii) water/oil-holding capacity (WHC/OHC), and (iii) emulsion. The structures are represented by some parameters (molecular weight, MW; degree of esterification, DE; GalA content, and monosaccharide composition) which depend on plant sources and extraction methods and determine the final application. Regarding gelling properties, high-molecular-weight pectins (≥ 300 kDa) can produce a kind of gel which shows network structures with high mechanical strength, rupture strength, and viscosity. In low-molecular-weight pectins, increased medium acidity and higher extraction time are observed. In high-methylated pectin, the gel is formed faster even at acidic pH due to reduced electrostatic repulsion. Low-methylated pectin is able to form a gel by strongly binding divalent ions. Moreover, distribution of non-methoxylated GalA in regular blocks results in gels with a better gelation ability and a stronger mechanical strength. The degree of methoxylation strongly affects Ca-pectin gel properties, and subsequently, acetylation of GalA results in reduced Ca-binding sites and unfavorable gel formation. High active Ca-binding sites induce the percolating network structures and improved rheological properties such as faster formation kinetics, enhanced viscosities, and higher elastic modulus. Regarding WHC/OHC, the hydrophobic/hydrophilic pectin constituents, total charge density and their functions affect texture through the interaction between food product components. High-OHC pectin plays a role in stabilizing or emulsifying in producing high-fat meat foods. WHC shows the hydration ability based on the OH group. The high absorption of water in pectin reduces the syneresis rate in yogurts and dairy desserts [9].

Regarding emulsion, pectin can increase the viscosity of the aqueous phase partly due to homogalacturonan domains with the contribution to emulsion stabilization and higher solution viscosity. The hydrophilic and hydrophobic groups with different amounts and distribution patterns characterize the solubility and rheological properties of pectin-treated liquid food products. In different pectin extraction procedures, different viscosities are created in aqueous solution, affecting emulsion characteristics. Those protein moieties bonded to pectin arabinogalactan and also pectin methyl, acetyl, and ferulic acid ester contents result in high hydrophobicity with an ability to be adsorbed at the oil-water interface. After emulsion formation, the emulsion instability remains to be limited or prevented based on the carbohydrate domain in pectin structure. Moreover, neutral side chains potentially interact with ferulic acid and/or proteins resulted in emulsion stabilization. The commercial pectins produced from citrus peel and apple pomace did not show strong emulsifying properties compared to sugar beet pectin which has the higher protein and ferulic acid contents [9].

3. Iranian literature review

The characteristics and applications of pectins are strongly influenced by their structures depending on plant species and tissues, as well as extraction methods. The aim of this review is therefore to highlight the structures of pectins and the various methods used to extract them, including conventional ones but also microwave heating, ultrasonic treatment, and dielectric barrier discharge techniques, assessing physicochemical parameters which have significant effects on pectin characteristics and applications as techno-functional and bioactive agents [9].

The amount and composition of secondary metabolites produced in plants are controlled significantly by environmental factors, including temperature, carbon dioxide, lighting, ozone, soil water, soil salinity and soil fertility, and climate change which make them to accumulate lower or higher in plants [10, 11]. The adaptation of plant morphology, anatomy, and physiological functions to the changes in biotic and abiotic may influence the accumulation of secondary metabolites. The pathways of secondary metabolites and their regulation are highly susceptible to environmental stresses due to the alteration observed in the involved gene expression [12]. The secondary metabolites play a variety of functions in plant growth and developmental processes, immunity and defense, and finally interaction with environmental stresses [13]. It demonstrates that the multifunctionality of plant secondary metabolites drives interactions between abiotic and biotic factors, with potential consequences for plant resistance in variable environments [14]. The plant has to produce a specified quantity and quality of secondary metabolites to encounter the environmental stress that determines the adaptability and availability of plant in a particular region [11]. In other words, external factors can adversely affect some process associated with biosynthesis of secondary metabolites that ultimately leads to variation in their overall phytochemical profiles, which play important roles in the production of bioactive substances [15].

Therefore, it is essential to perform repeated study designs on plants based on different geographical regions over the world. Here, we have reviewed briefly recent Iran-affiliated studies on pectin extraction procedures performed with agri-food wastes grown in different climatic and geographical settings in order to obtain pectin profiles.

In Kashani et al. [16], three variables were studied in related to their effects on pectin yield, GalA percentage, and DE of pectin including temperature (35, 65, and 95°C), time (40, 120, and 200 min), and pH (1, 2, and 3) with the extracted samples obtained from the potato peels using the acidic or citric acid extraction method in order to optimize the extraction condition profile based on the response surface method. Results of potato peel showed that pectin yield, GalA percentage, and DE ranged 7.15–14.87%, 14.45–36.37%, and 15.35–41.82% in 15 extraction treatments. The physicochemical properties were compared among the potato peel pectin, commercial citrus pectin, and commercial apple pectin according to pectin flow behavior tests at different concentrations, Fourier transform infrared (FT-IR) spectrum, and Mw. In potato peel pectin, the optimized single-independent variables showed that the highest extraction yield was 14.87% with the highest percentage of GalA as 36.37% at 95°C, 120 min, and pH 1.0. Also, the highest DE was 41.820% at 65°C, 40 min, and pH 3.0. Simultaneous optimization for both pectin yield and GalA showed that the highest pectin yield was 15.23% with favorite GalA as 38.0712% at 95°C, 200 min, and pH 1.0. The highest stability of extracted pectin emulsion obtained from potato peel was at 4°C on the first day compared to the stability at 23°C on the 30th day. According to FT-IR results, the strong absorption seen between 3200 and 3500 cm⁻¹ was related to the intracellular/extracellular vibration of the hydrogen bonds in the GalA polymer. At increased pectin concentration (0.1–2%), the viscosity was increased and Newtonian behavior was observed in all samples with flow index of 1. In potato peel, Mw of the extracted pectin was 53.46 kDa after 30 days of storage under optimal conditions at 4 and 23°C with emulsion stability (ES) of 85.1 and 63.1, respectively. Therefore, the produced pectin obtained optimally from agricultural wastes using citric acid procedure can be introduced to the market with Newtonian behavior and optimal gel grade.

In Kazemi et al. [17], the cantaloupe rind was effectively valorized into food-grade pectin by an environmentally friendly MAE process without the application of mineral acid. Then, the extraction factors were optimized by Box-Behnken design (BBD), and the extracted pectin was characterized according to various physicochemical, structural, functional, and bioactivity properties. Four variables of the extraction process were successfully optimized (microwave power: 700 W, irradiation time: 112 s, pH 1.50, and liquid-solid ratio (LSR): 30 mL/g) with a yield of 181.4 g kg⁻¹. After analysis, it was found that the isolated pectin was a high-methylated GalA-rich sample (703.4 g kg⁻¹) with an average Mw of 390.475 kDa. Also, the isolated pectin was a high-potential sample with favorite functionality and antioxidant ability in comparison with commercial citrus pectin according to FT-IR, Hydrogen-1 nuclear magnetic resonance (1H-NMR), and X-ray diffraction (XRD) spectroscopies. The main functional groups, structural characteristics, and crystallinity showed that the assayed samples had a significantly higher value of OHC, emulsifying capacity (EC), ES, α-diphenyl-β-picrylhydrazyl (DPPH), and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) scavenging activity, and reducing power assay with very promising quantity (yield) and quality values. The potential of MAE process included the remarkable reduction of both production time (instead of hours into minutes) and energy consumption, economically and environmentally promoted productivity. In industrial scale, it is necessary to evaluate the MAE process to demonstrate much better pectin yield, operating costs, and environmental burdens. Moreover, the repeatability and the functionality of isolated pectin are essential in future studies.

In Peighambardoust et al. [18], the comparative study objective was to identify the properties of beet pectin based on a novel and ecofriendly technology of subcritical water extraction (SWE) instead of traditional procedures in order to maximize pectin extraction efficiency. The advanced modeling procedures were applied including response surface methodology and optimization of operational parameters. The results indicated a promising scalable approach for converting the beet waste to pectin based on SWE with improved pectin properties. The optimal conditions for obtaining the highest pectin yield were determined using the central composite design for both comparative methods. In traditional procedure, the temperature, time, pH, and pectin recovery yield were 90°C, 240 min, 1, and 20.8%, respectively. In the subcritical water extraction, LSR was 30% (v/w) at temperature of 130°C for 20 min with a comparable yield of 20.7%. The effect of obtained pectin samples on viscoamylograph pasting and differential scanning calorimetry (DSC) thermal parameters of corn starch was assessed. The GalA content, degree of methylation, acetylation, and ferulic acid content were higher in the pectin obtained using SWE, while their Mw was lower. Both pectin samples have similar chemical groups according to FT-IR with similar colors. At lower concentrations (0.5–1%), pectin solution obtained in both techniques nearly indicated a Newtonian behavior according to the rheological measurements. The addition of both pectin samples to corn starch decreased both pasting (except hot paste viscosity) and DSC gelatinization (peak temperature or Tp, conclusion temperature or Tc, and Tc − To (onset temperature) of starch parameters, while increased ∆H to higher values. The To was minimally affected after the addition of pectin. Brabender viscoamylograph results were in good agreement with the DSC results. SWE was more efficient with the same extraction yield in a much shorter time due to nearly being 12 times faster; therefore, it can reduce pectin extraction time on an industrial scale and also facilitates the achievement of the pectin with improved specifications.

In Vaez et al. [8], a multipurpose platform was developed to dilute acid pretreatment as a multipurpose process to recover pectin, hydrolyze hemicellulose, and open up the cellulose structure. Some studies used separated fractions of orange wastes divided into pulp and peel, but it is not possible to practically separate them at the industrial scale and according to a conventional laboratory setting it is required to study their potential separately. Vaez et al. [8] designed a dilute acid treatment method on orange waste to extract pectin and fermentable sugars as well as breaking down the recalcitrant structure of the remained lignocellulose. The fermentable sugars dissolved in the supernatant were used for ethanol production without any further procedures for enzymatic hydrolysis. After acid treatment, the pretreated remained solid fraction was used for biogas production. One advantage of their designed biorefinery platform is related to the removal of enzymatic hydrolysis, that is a necessary step in a conventional ethanol production process. Therefore, biogas-ethanol-pectin integrated production is practiced in their study based on a dilute acid treatment procedure on orange waste with sulfuric acid (1% w/v) (94, 100, 140, and 180°C; 60, 30, and 0 min). Finally, the pectin was extracted from the hydrolysate, the liquor was used to produce ethanol, and the pretreated solid was anaerobically digested to produce biogas. The highest pectin extraction yield was 24.7% (w/w) and 23.7% (w/w) from orange peel and pulp fractions, respectively, from the supernatants of liquor treatment at 94°C for 60 min. FT-IR results confirmed the similar characteristics of the extracted pectin to the commercial sample. The GalA content (as pectin purity) was 70.2 and 69.9% from orange peel and pulp, respectively, at the optimal conditions. The acid treatment at 94°C for 60 min achieved a pectin product with approximately 69% of DE compared to approximately 45% in the treatment procedure at 140°C for 30 min. The highest ethanol yields of 81.5 and 82.9% were achieved from orange peel and pulp, respectively, after the acid treatment at 140°C for 30 min. The highest methane yields were 176.8 and 191.8 mL/g as volatile solids (VS) from the untreated orange peel and pulp, respectively. The highest total product value was 2472.9 USD/t orange wastes with dilute acid treatment at 94°C for 60 min. At the optimal conditions related to high pectin production with no enzyme, 244 kg of pectin, 26.5 L of ethanol, and 36 $m^3$ of methane were obtained from 1 t of orange wastes. If biogas is intended, the treatment procedure of citrus waste is not required. The proposed biorefinery platform can increase the total products value up to 75 times compared to the traditional anaerobic digestion of citrus waste [8].

In Ezzati et al. [19], the extracted pectin of sunflower by-product was obtained using UAE technique. The UAE variables were successfully optimized using BBD optimization process (irradiation time: 30 min, temperature: 33°C, ultrasound power: 400 W) with 11.15% of pectin yield. It was found that the extracted pectin was proved to be a high-purity sample and rich in low-esterified GalA content (72.94%) with long-side galactan branches, arabinogalactan, and arabinan, with an average Mw of 175.353 kDa. The functional groups and structural characteristics were determined by FT-IR, 1H-NMR, and XRD spectroscopies. According to the results obtained from DSC and thermogravimetric analysis (TGA) procedures, the thermal analysis suggested a suitable thermal stability for the extracted pectin. Other functional parameters were measured including the solubility, WHC, OHC, EC, ES (in different conditions), foam capacity, foam stability, DPPH, and ABTS inhibitions for assaying antioxidant properties and reducing power assay in order to prove the higher value of the extracted pectin for the potential of replacing to commercial food ingredients. Therefore, the obtained pectin can be used as a high-quality pectin sample with good functional and technological properties in pharmaceutical or food industries.

Hosseini et al. [3] studied the optimized MAE conditions for the simultaneous recovery of pectin and phenolic compounds from sour cherry pomace. An annual production of >109,000 tons in Iran makes this country as the seventh producer of sour cherry in the world, and the residual pomace that is discarded as a side product in food industry and is rich in different polysaccharides such as pectin can lead to environmental problems; but the reuse of fruit wastes for their production has high benefits for nutritional and environmentally friendly. They found that the highest yield of pectin (14.65 ± 0.39%, Mw: 472.977 kDa) was obtained with microwave power of 800 W, irradiation time of 300 s, pH 1.0, and LSR of 20 v/w. The structural analysis indicated that the obtained supernatant was rich in high-methoxyl pectin with amorphous structure. The moisture, ash, and protein contents as well as total carbohydrates were nearly 8.32, 3.73, 1.41, and 26.43%, respectively. The high purity was proved from approximate GalA content of 72.86%, suitable thermal stability was obtained due to degradation temperature of 252.15°C, and also high-methoxyl pectin was proved from DE of 68.37 ± 2.78%. According to the FT-IR, 1H-NMR, and XRD analysis, the obtained sample was rich in esterified polygalacturonic acid with an amorphous structure.

In Kazemi et al. [20], BBD was applied to optimize the conditions of the UAE and heating extraction (HE) procedures according to pectin yield from pistachio green hull as a response. At optimal condition, the pectin extraction yield of the UAE (ultrasound power: 150 W, pH 1.5, time: 24 min) and HE (temperature: 70°C, time: 90 min, LSR: 40 v/w) methods were 12.0 ± 0.53% and 10.3 ± 0.75%, respectively. Also, the GalA content of pectin was about 59.33 and 75.11% in UAE and HE methods, respectively, which was higher in the latter. Both methods were able to achieve good emulsifying activity, WHC and OHC. Moreover, total phenol content (TPC) and antiradical activity (DPPH radical scavenging) of pectin samples were higher in HE method compared to UAE procedure. But the surface tension value was lower in the former which was in agreement with the results obtained from foaming properties. The decreased surface tension with the increased TPC can be probably attributed to the accumulation of phenolic compounds in the air-water interface with a resulted increase in the surface pressure and finally a resulted decrease in surface tension. In the HE method, DE was higher compared to the pectin produced in the UAE method according to FT-IR and 1H-NMR analysis. The results of XRD patterns revealed an amorphous structure with some crystalline portions in the fruit pectin. The surface morphology showed more surface roughness for pectin obtained from the HE method compared to the UAE method. According to the rheological properties of pectin solutions, G′ and G″ of pectin extracted from the HE method in 2% w/v were much higher than the UAE method. Both samples showed similar elastic behavior in high frequencies, while pectin sample obtained from HE method had a viscous behavior in low frequencies. The UAE method had significantly increased pectin yield with a lower processing time and consumed solvent volume; however, pectin sample from HE method presented a better quality. Therefore, there are some limitations and disadvantages for running UAE procedure in industrial scale.

Khodaiyan and Parastouei [5] investigated pectin extraction from black mulberry pomace based on an eco-friendly extraction process of MAE procedure. The variables were successfully co-optimized using BBD, and then the optimized condition yielded about 10.95% pectin as response. The compounds produced under optimum conditions (microwave power: 700 W, irradiation time: 300 s, pH 1.42, and LSR: 20 mL/g) were characterized based on physicochemical, structural, and functional properties. The increased production of global agricultural waste urges critical attention to the concept of sustainable waste valorization and maximum utilization of food waste; therefore, the production of several products from waste has attracted high interests. According to the physicochemical analysis, there was a highly esterified amorphous pectin (DE: 62.21%) with an average MW of 620.489 kDa based on XRD analysis and a highly esterified GalA content of 70.15% with further confirmation based on FT-IR and 1H-NMR spectroscopies. Also, the DSC showed higher thermal stability for the assayed pectin than commercial pectin (degradation temperature: 251.82°C). The designed procedure provides a promising management of black mulberry waste generated in food industry with high quality for being applied as natural ingredients in various food and pharmaceutical products. The final aim is to maximize the waste use for the production of the valuable compounds and minimize the waste volume.

Nouri and Mokhtarian [7] studied on the pectin processed from walnut green husks and found them as good sources for pectin extraction. According to the response surface statistical methodology, they assessed extraction efficiency rate, DE, and GalA of the obtained pectin in different pH (1.0, 1.5, 2.0), temperature (60, 70, 80°C), and process time (60, 90, 120 min) values. The optimal samples were selected, and total ash, MW, emulsifier, rheological, and FT-IR spectroscopy assessments were performed. The highest efficiency rate (25.84%) was obtained at optimal conditions (pH 1.62, 80°C, and 120 min). The highest DE (63.19%) occurred at optimal conditions (pH 2.0, 72.92°C, and 87.27 min, range: 52.30–60.20). The GalA proportion indicating purity of pectin was normal. The highest GalA (68.53%) was recorded at pH 1.44 and 72.92°C in 93.33 min. Some viscous and pseudoplastic behaviors were assayed with the extracted pectin. According to FT-IR spectral diagrams, the optimal pectin samples have shown the presence of GalA as a rich source of pectin.

Gharibzahedi et al. [4] studied on the comparative pectin extraction procedures from common fig skin, including HWE, UAE, MAE, and UMAE. The results showed that UMAE (11.71%) significantly obtained a more extraction yield than MAE (9.26%), UAE (8.74%), and HWE (6.05%). The UMAE-pectin with the highest GalA content (76.85%) and MW (6.91 × 10 3 kDa) had the highest emulsifying activity (61.2–61.3%) and ES (94.3–95.2%) with a monomodal droplet size distribution at both cold and ambient storage temperatures. A non-Newtonian shear-thinning behavior was recorded at 1.5–3.0% pectic solutions. XRD analysis showed noncrystalline pectin extracted by UMAE. FT-IR spectroscopy and high-performance liquid chromatography (HPLC) photodiode array detector proved that both conventional and novel extraction technologies do not change the chemical structure and monosaccharide composition of pectin significantly. The UMAE at operating conditions (pH 1.4, 1:20 g/mL CFS¹/water, sonication time: 25 min, irradiation time: 3.5 min and microwave power: 600 W) was proved to be a successful strategy to extract high-MW pectin from fig skins with the highest extraction yield, total GalA, viscosity of pectic solution, emulsifying activity, and ES at different assay conditions. The pectin functionality for food-grade emulsions was also proved because oil-in-water emulsions stabilized with fig skin pectin extracted by UMAE had the lowest droplet size with a monomodal size distribution. All extracted pectin samples had a DE being lower than 50 and can be applied in stable formulations of many low-sugar dietary foods. A pseudoplastic flow behavior was observed at the high concentration of pectin. The main functional groups and monosaccharides determined based on FT-IR spectroscopy and pulsed amperometric detection (HPLC-PAD), respectively, are clues for the extracted polygalacturonic acid-rich pectin. However, it is essential to conduct an optimization study in order to find the functional conditions for finding the best UMAE extraction method and obtaining the highest extraction yield of fig skin pectin.

Hosseini et al. [6] performed a study for optimization and characterization of pectin extracted from sour orange peel by UAE procedure. Their aims were as follows:

1. new UAE-assisted optimization of fruit pectin in order to find the effects of various extraction factors on pectin yield and properties,

2. structure, monosaccharide, and chemical compositions of the extracted pectin in related to extraction methodology,

3. physicochemical and functional properties of fruit pectin.

In this work, BBD was applied with three variables (ultrasound power, irradiation time, and pH) for pectin extraction optimization from sour orange peel in three levels by ultrasound waves. The physicochemical, structural, and functional properties of fruit pectin were evaluated in optimal extraction point. According to the obtained results, the highest extraction yield was 28.07 ± 0.67% in optimal conditions (ultrasound power: 150 W, irradiation time: 10 min, pH 1.5). Also, ash, moisture, and protein contents of fruit pectin were 1.89 ± 0.51%, 8.81 ± 0.68%, and 1.45 ± 0.23%, respectively. Moreover, 65.3% of the extracted pectin was GalA with approximately 72% of total neutral sugars as galactose according to HPLC findings which showed the fruit pectin has a suitable purity. In the optimized pectin, there are TPC of 39.95 ± 3.13 mg gallic acid equivalents/g pectin, the surface tension of 46.56 ± 0.23 and 42.14 ± 0.61 mN/m in concentrations of 0.1 and 0.5%w/v, and WHC and OHC of 3.10 ± 0.12 and 1.32 ± 0.21 g water or oil/g pectin, respectively. In addition, the emulsifying activity of fruit pectin extracted by ultrasound waves was higher than those samples from other sources, and emulsions were more stable in low temperature. Moreover, DE of 6.77 ± 0.43% was proved the fruit pectin to be a low-methoxyl pectin according to FT-IR and 1H-NMR analysis. Therefore, the procedure with ultrasound waves showed a high efficiency based on quantity/quality of the extracted pectin. These waves improve the destruction of plant cell wall by their cavitation effect and increase the rate of mass transferring resulted in the higher extraction yield of pectin in a shorter extraction time [6].

In Kazemi et al. [21], eggplant peel was used for pectin extraction through UAE technique. The optimization process was carried out using BBD in order to optimize the extraction process factors, and the results showed that the highest experimental extraction yield (33.64 ± 1.12 g/ 100 g, the predicted yield: 35.36 g/100 g) was achieved with optimal conditions (ultrasound power: 50 W, irradiation time: 30 min, pH 1.5). The assay of chemical, physicochemical, functional, and structural pectin features indicated that it was rich in GalA (66.08 g/100 g) and has both high DE (61.22%) and TPC (96.81 mg GAE/g pectin) and both low ash and protein contents. Also, the extracted pectin showed favorite measurements of functional properties including WHC, OHC, emulsifying and foaming properties, and antioxidant activity. In addition, FT-IR and 1H-NMR spectroscopy proved a high-methylated pectin structure in the obtained samples. Moreover, assaying DE suggested that the extracted pectin is in the group of high-methoxy pectin with further confirmation based on those measurements from FT-IR and 1H-NMR spectroscopy. XRD pattern proved a high crystallinity for eggplant pectin. Given the high extraction yield and favorite properties, the eggplant peel pectin can be a good replacement for commercial pectin.

In Jafari et al. [2], the central composite design was used with four variables in five levels to determine the effects of pH (0.5–2.5), temperature (50–90°C), heating time (30–150 min), and LSR (10–50 v/w) on both yield and DE of the extracted carrot pectin. The highest extraction yield of pectin was 15.6 ± 0.5% at optimal conditions (90°C, 79.8 min, LSR of 23.3 v/w, pH 1.3) which was close to the predicted values (16.0%). According to the obtained findings, the extracted pectin was proved to be a low-methoxylated pectin (DE: 22.1–51.8%) with a favorite emulsifying activity (60.3%), viscosity at a wide range of frequencies (0.1–50 Hz, 1% w/v), and pseudoplastic flow behavior at the same concentration. With the optimal extraction conditions, the GalA content and emulsifying activity were 75.5 and 60.3%, respectively; moreover, the emulsions had a high stability (80.4–80.3%, 74.7–74.4%) at two different storage temperatures (4 and 23°C) after 1 and 30 days, respectively.

Bagherian et al. [1] performed a comparative study on the conventional and microwave- and ultrasound-assisted methods for the extraction of pectin from grapefruit. In this study, the effect of microwave power and heating time was assayed on both pectin yield and quality in grapefruit. It was found that the highest pectin yield was 27.81% (w/w) at 6 min and 900 W. It was observed that pectin yield, the GalA content, and DE increased with the increased microwave power and heating time. But Mw decreased with an increase in heating time; however, the effects of power on Mw were dramatically more than heating time. In addition, laboratory studies on the extraction of pectin treated with high-intensity ultrasound were carried out. The effects of temperature and time on quality and quantity of extracted pectin were investigated. The highest yield was for sonication time of 25 min (17.92%) in a constant bath temperature of 70°C. Furthermore, before applying MAE the grapefruit solution was treated by a preliminary ultrasonic heating and a higher yield was proved. Intermittent sonication was so efficient than continuous procedure. The studied parameter in microwave extraction included microwave field power and heating time for improving both qualitative and quantitative characteristics of extracted pectin. Finally, 2 min of microwave heating was able to induce the same amount of pectin obtained as with 90 min of conventional extraction procedures. On the other hand, sonication was performed with water bath, and the effect of sonication time and bath temperature were measured on the pectin extraction in order to find the optimal factors. The conventional procedure was not able to compete with sonication method being 3 times faster. Also, when the ultrasound pretreatment was performed before microwave heating, better results were obtained than MAE.

4. Methodology

4.1 Case study No. 1

An optimal biorefinery development for pectin and biofuels production from orange wastes without enzyme consumption—A case study by Vaez et al. [8].

4.1.1 General procedure

The orange wastes were treated with sulfuric acid (1% w/v) at 94, 100, 140, and 180°C for 60, 30, and 0 min in order to extract pectin from the hydrolysate. The highest yield of pectin extraction was 24.7% (w/w) and 23.7% (w/w) in orange peel and pulp, respectively, in the supernatants at 94°C for 60 min. The results of FT-IR confirmed that the characteristics of the extracted pectin were similar to the commercial product. Regarding the pectin purity, the GalA content was 70.2 and 69.9% in orange peel and pulp, respectively. The acid treatment (94°C, 60 min; 140°C, 30 min) showed a DE being higher than 69% and less than 45%, respectively. At the optimal conditions, high production of pectin was 244 kg/t of orange wastes without enzyme procedure.

4.1.2 Preparation of material

Fresh Thomson navel orange (Citrus sinensis) from northern Iran was used in this study. After the fruits were washed, they were juiced, and the separated pulp and peel were dried at ambient temperature for 2 weeks and then they were grinded. Those particles being sized as 60.85–0.180 mm (20–80 mesh) were collected and kept in sealed plastic bags for further analysis at room temperature. The prepared orange peel and pulp contained 92.6 and 93.6% total solids (TS) and 87.3 and 89.9% volatile solids (VS), respectively.

4.1.3 Dilute acid treatment

Sulfuric acid solution (1% w/v) was applied with different temperatures (100, 140, and 180°C) and times (0, 30, and 60 min) in 101SSHPR² reactor warmed up in an oil bath. In order to extract pectin, the optimal acid treatment condition was encountered at 94°C for 60 min with 140 mL of sulfuric acid (1% w/v) and 10 g of loaded orange peel/pulp in the reactor. Then, the temperature was quickly decreased using an ice bath. The suspension was filtered for the solid residues that rinsed with distilled water to remove chemicals. Then, they were freeze-dried and kept at room temperature, but the liquid fraction was kept at −20°C for pectin extraction and purification.

4.1.4 Pectin extraction and purification

For the highest pectin yield, pH was adjusted to 3.5 with sodium hydroxide. The solution was dispersed in ethanol, and the precipitates were obtained after it remained for a night at 4°C with centrifugation (4000 rpm, 20 min). Ethanol at a 70 and 96% (v/v) alcohol content was used for rinsing the separated solids. The mixture was centrifuged (4000 rpm, 30 min) to separate the pectin. The separated pectin was dissolved in deionized water, freeze-dried, and maintained at room temperature. The yield of pectin extraction was calculated using an equation (Bi and P as initial quantity in gram of substrate and pectin, respectively):

$$Y_{pec}\left(\%\right)=\frac{P}{B_i}\times 100$$E1

4.1.5 Analytical methods

4.1.5.1 Substrate characterization

The TS, VS, and extractive contents of the orange wastes were measured according to the National Renewable Energy Laboratory (NREL) protocol. The morphology of the freeze-dried substrates was assayed using gold-coated procedure and scanning electron microscopy (15 kV) in order to observe the acid treatment effect.

4.1.5.2 Pectin characterization

Pectin characteristics (chemical structure, GalA content, and DE) were determined. The chemical structure was assessed by a Fourier transfer infrared spectrometer with a deuterated-triglycine sulfate detector in comparison with commercial pectin (resolution of 1 cm⁻¹, 32 scans in 4400–400 cm⁻¹).

The GalA content was quantified based on Ramos-Aguilar et al. [22]. Pectin (5 mg) was added to 2 mL of concentrated sulfuric acid (98%) and 1 mL of deionized water and adjusted to 10 mL. After an ice bath procedure (10 min), the centrifugation was performed at room temperature (2000 × g, 10 min). The liquid fraction (400 μL) was mixed with 2.4 mL of sodium tetraborate (75 mM in concentrated sulfuric acid) and 40 μL of 4 M potassium sulfamate solution (pH 1.6). The temperature of tubes decreased using indirect contact in an ice bath after insertion in boiling water for 20 min. The M-hydroxy di-phenyl solution in NaOH was added. The absorbance of samples was assayed at 525 nm using a UV-vis spectrophotometry.

The DE was obtained according to Santos et al. [23] as follows: the dry mass of pectin (0.1 g) was dissolved with 3 mL of ethanol 96% in 20 mL of distilled water at 40°C and 100 mL Erlenmeyer flasks were magnetically stirred up. The titration was performed with sodium hydroxide solution (V1 mL for the first neutralization step, 0.1 M) and phenolphthalein indicator for appearing a pale pink color. The sodium hydroxide solution (10 mL, 0.1 M) was added to this neutralized solution and stirred. The hydrochloric acid solution (0.1 M, 10 mL) was added and stirred in order to disappear the pink color completely. The sodium hydroxide solution (V2 mL for the second neutralization step, 0.1 M) was added to neutralize the excess of acid until the solution color changed to pink (V2). The DE of the pectin was determined using V1 and V2 as follows:

$$DE\left(\%\right)=\frac{V_2}{V_1+V_2}\times 100$$E2

4.1.5.3 Statistical analysis

Analysis of variance (ANOVA) was applied using the least significant difference (LSD) and Tukey’s methods in SAS 9.1.3 software (p < 0.05).

4.2 Case study No. 2

High-quality pectin from cantaloupe waste: Eco-friendly extraction process, optimization, characterization, and bioactivity measurements—A case study by Kazemi et al. [17].

4.2.1 Extraction and optimization of pectin

Fresh cantaloupe fruits were washed, peeled, cut, and placed in an oven (60°C, 48 h). The dried pieces were powdered and stored at 25°C. The MAE process was used to produce pectin from cantaloupe powder in a microwave oven according to Kazemi et al. [17]. pH levels were adjusted using organic citric acid. The response was expressed as the extraction yield (g kg⁻¹). For optimization of the MAE process, MAE variables and their levels were selected for pectin extraction from cantaloupe rinds using BBD according to the literature. The effects of MAE-independent variables were optimized and investigated including microwave power (300–700 W), irradiation time (60–180 s), pH (1.5–3.0), and LSR (20–30 mL/g). In order to reduce systematic errors, all experiments were randomly performed, and then the results were put in a polynomial equation in order to predict the optimized condition for the MAE process.

4.2.2 Pectin characterization

Pectin characteristics (physicochemical, structure, functional, and antioxidant assay) were determined. Three experiments were performed for each measurement, and the results were calculated and reported as mean value ± SD³.

4.2.3 Physicochemical analysis

For moisture content, 1 g of fruit pectin was dried at 105°C for 24 h. Ash content was measured after dry ignition of 1 g of pectin samples (550°C, 6 h) in a muffle furnace. Protein content was determined using the Kjeldahl method (crude protein = N × 6.25). Total carbohydrate content was measured according to the phenol sulfuric acid method. The GalA content was determined using the meta-hydroxydiphenyl method. The DE was measured according to Kazemi et al. [17]. TPC was found on an SP-UV500DB model UV-vis spectrophotometer according to the Folin-Ciocalteu method using a standard curve of gallic acid. TPC was expressed as mg gallic acid equivalent per g of pectin (mg GAE/g pectin). All physicochemical measurements were compared based on the commercial citrus pectin.

Average MW was determined by injection of pectin solution (20–50 μL, 1 mg/mL) high-performance gel permeation chromatography (HPGPC) using an Ultrahydrogel™ column and a RID⁴-10A refractive index detector. For elution, NaNO3 (0.1 M) was used as mobile phase at a flow rate of 1 mL/min and temperature of 35°C. The calculation was based on the calibration curves of dextran standards.

4.2.4 Structural analysis

The isolated pectin was structurally characterized using FT-IR, 1H-NMR, and XRD spectroscopies in comparison with commercial citrus pectin. FT-IR spectrum of the KBr-dispersed samples was recorded on a Bruker Tensor 27 spectrometer by 10 scans at a resolution of 4 cm⁻¹ over 4000–600 cm⁻¹. 1 H-NMR spectrum was collected on a 500 MHz Varian Unity Inova spectrometer by eight scans (24°C, 4.0 s). XRD pattern was recorded on a Philips diffractometer (10–80° as 2θ).

4.2.5 Functional characteristics

The functional properties were measured using four methods including WHC, OHC, EC, and ES in comparison with commercial citrus pectin. WHC and OHC were measured according to Kazemi et al. [17] as the mass (g) of water or oil retained by 1.0 g of pectin sample (g/g). EC (24°C) and ES (1 and 30 days, 4°C and 24°C) of emulsions were measured according to Yapo et al. [24].

4.2.6 Antioxidant assay

The antioxidant capacity of pectin solutions was determined in concentrations of 0.1–5 mg/mL using three methods, including DPPH scavenging activity, ABTS scavenging activity, and reducing power assay in comparison with ascorbic acid, beta hydroxy acid (BHA), and commercial citrus pectin at similar concentrations. Also, the half maximal inhibitory concentration values of DPPH and ABTS scavenging activity for each sample were calculated and compared (the required concentration of antioxidant for scavenging the 50% of initial concentration of free radicals).

4.2.7 Statistical analysis

Response surface BBD was used to assess the effects of MAE process variables individually and interactively in terms of pectin yield (56–150 g kg⁻¹) and the highest pectin yield. The experimental data was applied in a second-order polynomial model developed using multiple regression analysis. Also, in order to show a good agreement between the experimental and predicted data, Pareto ANOVA was used. According to the obtained results, a fitted developed model could explain the suitable relationship between process variables and response.

5. Conclusions

In food industry, the functional properties of pectin are influenced by the source, methods, and conditions of extraction (time, pH, LSR, temperature, wave power and frequency, enzymes, and the integrated conditions). Novel extraction methods are focused in studies in order to reduce extraction time/solvent consumption and increase process efficiency/pectin yield. Agri-food wastes can be processed to produce valuable by-products like marketable pectin according to commercial standards. Their significant WHC/OHC and promising emulsifying properties make them as textural ingredients and emulsifiers in food products and pharmaceutical supplements [9]. The green chemistry extraction process for the valorization of industrial food processing waste should be designed based on circular economy concepts in order to increase its benefits [5]. However, further research is needed to understand the synergistic effect of multiple extraction factors using new techniques for the improvement of the productivity of industrial pectin processes from Iran’s Agro waste (Tables 1 and 2).

Studies published in Persian: Akbari-Adergani B, Zivari Shayesteh P, Pourahmad R. Evaluation of some functional properties of extracted pectin from pomegranate peel by microwave method. Journal of Food Technology and Nutrition. 2021;18:5–16 [In Persian].; Amanpour M, Asefi N. Effect of ultrasound time and acid type on the qualitative properties of extracted high methoxyl pectin from quince fruit. Iranian Journal of Nutrition Sciences and Food Technology. 2020;15:71–82 [In Persian].; Azadbakht M, Tabaey MH, Sabet Ahd Jahromi A. The comparison of various methods for extraction and isolation of pectin from Citrus Decumana Murry. Journal of Tabriz University of Medical Sciences (Pharmaceutical Sciences). 2003;1:21–28 [In Persian].; Bahramipour M, Akbari-adergani B. Optimization of microwave-assisted extraction of pectin from peaspod by response surface method. Journal of Food Science and Technology. 2018;80:349–360 [In Persian].; Darvishi M, Yazdanpanah S. Evaluation and comparison of emulsion, rheological and spectroscopic properties of FT-IR extracted pectin from peel and cap of pumpkin. Journal of Food Science and Technology. 2020;17:149–162 [In Persian].; Ebrahimzadeh MA, Azadbakht M. (2006). Extraction of pectin and comparison of yield and degree of esterification and percentage of galacturonic acid in the skin of some citrus. Journal of Mazandaran University of Medical Sciences. 2006;16:52–59 [In Persian].; Fathi B, Maghsoudlou Y, Ghorbani M, Khomeiri M. Effect of pH, temperature and time of acidic extraction on the yield and characterization of pectin obtained from pumpkin waste. Journal of Food Research (AGRICULTURAL SIENCE). 2012;22:465–475 [In Persian].; Haji Esmaelli F. Extraction of pectin from sugar beet pulp [master’s thesis]. Tehran (Iran): Science and Research Branch. Islamic Azad University; 2002 [In Persian].; Hosseini SS, Khodaiyan F, Barazande S. Extraction and comparison of the physicochemical properties of pectin extracted from pineapple, samsuri and galia melon peels assisted by microwave. Iranian Journal of Food Science and Nutrition. 2017;11:71–80 [In Persian].; Jannat B, Oveisi MR, Sadeghi N, Behzad M, Behfar AAA, Hajimahmoodi M, et al. Determination of pectin in sunflower and its application in food industry. Journal of Food Technology and Nutrition. 2016;13:25–34 [In Persian].; Karimifard SM. Separation of valuable chemicals from citrus waste and conversion of the produced wastes to biogas [master’s thesis]. Isfahan (Iran): Isfahan University of Technology; 2013 [In Persian].; Keramat J, Kabir GM, Ghenaati B. Qualitative and quantitative study of pectins extracted from the orange juice concentrate production process waste. Agricultural Science and Technology and Natural Resources. 2002;6:141–148 [In Persian].; Mesbahi GR, Jamalian J. Pectin extraction from sugar beet pulp and its application in food products. Journal of Agricultural Science and Technology of Agriculture and Natural Resources, Water and Soil Science. 2002;6:125–138. [In Persian].; Mirmajidi A, Babaei B. Extraction of pectin by pre-hydrolysis method from some commercial type of sugar beet. Research in Agricultural Engineering. 2007;9:103–114. [In Persian].; Mosayebi V, Emam-Djomeh Z, Tabatabaei Yazdi F. Optimization of extraction conditions of pectin by conventional method from black mulberry pomace. Quarterly Journal of Food Science and Technology. 2017;62:341–356 [In Persian].; Nateghi L. Optimization of extraction conditions of pectin from Aloe Vera leaf. Food Technology and Nutrition. 2021;18:95–115. [In Persian].; Nateghi L, Ansari S, Shahab Lavasani AR. Investigation of yield and physicochemical properties of pectin extracted from eggplant peel. Food Science and Technology. 2017;73:13–30 [In Persian].; Nateghi L, Ansari S. Investigation of yield and physicochemical properties of pectin extracted from eggplant cap. Journal of Modern Food Technologies. 2017;5:219–239 [In Persian].; Saberian H. Optimization of pectin extraction from orange waste by ohmic and enzymatic novel methods [Ph.D Thesis]. Tehrn (Iran): Tarbiat Modares University; 2017 [In Persian].; Zarei M. Effect of extraction temperature on qualitative characteristics extracted pectin from orange peel. 18th National Congress on Food Technology, Mashad, Iran; 2008 [In Persian].

Studies published in English: Akbarpour V, Hemmati K, Sharifani M. Physical and chemical properties of pomegranate (Punica granatum L.) fruit in maturation stage. American-Eurasian Journal of Agriculture and Environmental Sciences. 2009;6:411–416.; Asgari K, Labbafi M, Khodaiyan F, Kazemi M, Hosseini SS. High-methylated pectin from walnut processing wastes as a potential resource: ultrasound assisted extraction and physicochemical, structural and functional analysis. International Journal of Biological Macromolecules. 2019;152:1274–1282.; Chaharbaghi E, Khodaiyan F, Hosseini SS. Optimization of pectin extraction from pistachio green hull as a new source. Carbohydrate Polymers. 2017;173:107–113.; Faravash RS, Ashtiani FZ. The influence of acid volume, ethanol to extract ratio and acid washing time on the yield of pectic substances extraction from peach pomace. Food Hydrocolloids. 2008;22:196–202.; Gavlighi HA, Tabarsa M, You S, Surayot U, Ghaderi-Ghahfarokhi M. Extraction, characterization and immunomodulatory property of pectic polysaccharide from pomegranate peels: Enzymatic vs. conventional approach. International Journal of Biological Macromolecules. 2018;116:698–706.; Gharibzahedi SMT, Smith B, Guo Y. Ultrasound-microwave assisted extraction of pectin from fig (Ficus carica L.) skin: Optimization, characterization and bioactivity. Carbohydrate Polymers. 2019;222:114992.; Hashemi A, Razzaghzadeh S. Investigation on the possibility of ensiling cucurbit (cucurbita pepo) residues and determination of best silage formula. Journal of Animal and Veterinary Advances. 2007;6:1450–1452.; Hosseini AS, Akramian M, Khadivi A, Salehi-Arjmand H. Phenotypic and chemical variation of black mulberry (Morus nigra) genotypes. Industrial Crops and Products. 2018;117:260–271.; Hosseini SS, Khodaiyan F, Yarmand MS. Aqueous extraction of pectin from sour orange peel and its preliminary physicochemical properties. 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An integrated valorization of industrial waste of eggplant: simultaneous recovery of pectin, phenolics and sequential production of pullulan. Waste Management. 2019b;100:101–111.; Kazemi M, Khodaiyan F, Labbafi M, Hosseini SS, Hojjati M. Pistachio green hull pectin: Optimization of microwave-assisted extraction and evaluation of its physicochemical, structural and functional properties. Food Chemistry. 2019c;271:663–672.; Mesbahi GR, Jamalian J, Farahnaky A. A comparative study on functional properties of beet and citrus pectins in food systems. Food Hydrocolloids. 2005;19:731–738.; Mohammadzadeh J, Sadeghi–Mahoonak AR, Yoghbani M, Aalami M. Extraction of pectin from sunflower head residues of selected Iranian cultivars. World Applied Sciences Journal. 2010;8:21–24.; Pasandide B, Khodaiyan F, Mousavi Z, Hosseini SS. Pectin extraction from citron peel: optimization by Box–Behnken response surface design. 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Table 1.

A collection of underlined Iran-affiliated studies for the reported results about pectin extraction procedures from Agri-food wastes.

Pectin sources	Extraction conditions								Yield (%)	DM (%)	GalA (%)	MW (kg/mol)	Reference*
	Treatment	Solvent	Temperature; (°C)	pH	S/L	Time	Power	Enzyme
Beet pulp	CHE	HCl	80	1	1:50	3 h	—	—	20.0	58.92	66.18	116	Mesbahi et al. (2005)
Citron peels	CHE	citric acid	95	1.5	1:30	95 min	—	—	28.31	51.33	—	—	Pasandide et al. (2018)
Eggplant peel	CHE	citric acid	90	2.5	1:40	90 min	—	—	26.1	60.2	69.7	—	Kazemi et al. (2019b)
	UAE	citric acid	—	1.5	1:20	30 min	50 W		33.64	61.2	66.08		Kazemi et al. (2019d)
Grapefruit	UAE	HCl	70	1.5	1:50	25 min	—		17.92	75.1	68.21	68.3	Bagherian et al. (2011)
	MAE	HCl	—	—	1:50	6 min	900 W		27.81	80	75	50	Rahmani et al. (2020)
Pistachio green hull	MAE	—		1.5	1:15	165 s	700 W	—	18.13	12.1	66.0	1659	Kazemi et al. (2019c)
Sweet lemon peel	MAE	citric acid	—	1.5	—	3 min	700 W		25.31	5.80	87.2	615.8	Rahmani et al. (2020)
Sour orange peel	MAE	citric acid	—	1.50	1:15	3 min	700 W		28.8	1.5	71.0	—	Hosseini et al. (2016b)

Table 2.

The experimental profiles of pectin extraction for some Iran’s agricultural wastes [9].

^*

see Table 1.

S/L: solid-liquid ratio, DM: degree of methyl esterification, GalA: galacturonic acid content, MW: molecular weight, UAE: U-assisted extraction, MAE: microwave assisted extraction, and CHE: conventional heating extraction.