Extraction of Dietary Fibers from Plant-Based Industry Waste: A Comprehensive Review

Featured Application

This review provides a collection of information about the application of plant-based waste for the extraction of dietary fiber using different extraction methods. Favorable extraction techniques and optimal extraction conditions can, depending on the plant resources, facilitate the investigation for future researchers and the industry.

Abstract

The amount of waste generated by the production of food products has increased over the years, presenting economic and environmental problems. To minimize these problems, it is necessary to valorize food waste in order to explore its further utilization in the food industry and also in other industries. Such waste usually represents a valuable raw material in terms of dietary fibers or bioactive components. Dietary fibers, especially pectin, are usually derived from apple pomace or citrus peel. Currently, sources of dietary fibers include novel food waste streams and by-products. Also, the utilization of novel extraction techniques is in demand to limit conventional processes. This review provides information about the conventional and innovative extraction approaches for dietary fibers from different food wastes. The extraction of these fibers depends on the materials used and the extraction conditions, such as temperature, solvents, time, pH, and liquid/solid ratio. Novel green techniques may ensure an increase in fiber yield and better quality, as well as a reduction in operating time and toxic solvents.

1. Introduction

Food losses and the formation of large amounts of waste present very significant economic, as well as environmental, issues in all sectors of the food industry due to the significant loss of resources [1,2]. To minimize these losses, it is necessary to valorize food waste in order to explore its further utilization in the food industry and across other industries. Even though a large number of investigations have focused on the application of plant-based by-products, since they are known to possess significant amounts of dietary fiber and various bioactive compounds [3], it is necessary to identify and/or develop adequate extraction techniques so that these valuable compounds can be successfully utilized. The food industry in the EU annually produces over 100 Mt of waste, which is composed of inedible plant tissues (seed, peel, husk, etc.). Most of this waste has the potential to be utilized to obtain functional food ingredients [4,5]. Dietary fibers, as well as other compounds, like essential oils, proteins, pigments, and flavor compounds, can be isolated using different extraction techniques [4]. The definition of dietary fibers outlined by the American Association of Cereal Chemists [6] is that these compounds are “the remnants of the edible part of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine, with complete or partial fermentation in the large intestine”. Non-starch polysaccharides are the main dietary fiber components, which can generally be characterized according to their solubility in insoluble and soluble ones [7]. Insoluble fibers include cellulose, hemicellulose, or chitin (and its derivatives, like chitosan), whereas soluble fibers include pectin, gums, β-glucans, mucilage, oligosaccharides, or inulin [8]. Their function is to ensure structural rigidity/hardness of the plant cell wall [8]. Dietary fibers are present in cereal, fruit, and vegetable waste in significant amounts; thus, these materials present significant potential for the extraction and utilization of these valuable components. The application of dietary fibers in the food industry has been widely represented. Recently, a review paper was published on dietary fibers present in various food waste materials and the utilization of dietary fibers in the different segments of the food industry [9]. The health benefits of dietary fibers with respect to their impact on microbiota health, cancer treatment and prevention, cardiovascular health, obesity, and diabetes, have been thoroughly investigated [10,11,12].

Polysaccharides, together with other compounds, are cross-linked to the vegetable cell wall and form a structural network. To remove polysaccharides from this structure, water or other complex media is needed [13]. Key factors that govern the properties of fibers obtained via extraction are, on the one hand, the selected extraction method and solvent used, and, on the other hand, the source from which the fibers are extracted. The methods used for fiber extraction include dry/wet extraction and chemical, enzymatic, gravimetric, and microbial methods. Recently, the application of green extraction techniques was put forward by scientists and technologists. These techniques include less complicated extraction using water, ethanol, or steam. Advances in extraction techniques were also pointed out, which included pulsed electric field-assisted, ultrasonic-assisted, microwave-assisted, and high hydrostatic pressure-assisted methods [8]. Each extraction method was characterized by its advantages and some disadvantages with respect to the expenses for materials and time, food safety, and impact on the environment. In the case of enzymatic methods, a long reaction time (above 24 h), high cost of enzymes, and low efficiency present several disadvantages [14]. The advantages of ultrasound-assisted and microwave-assisted extraction are low consumption of energy, short operating time, decreased solvent amounts, and increased extraction yield [4]. Dietary fibers, especially pectin, are usually derived from apple pomace or citrus peel. Currently, the source of dietary fibers includes novel food waste and by-products, such as mango peel, guava pulp, fig seed, passion fruit peel, faba bean and cocoa hulls, watermelon rind, banana, pomegranate peels, peanut shell, rice husk, hull-less barley bran, etc. [4,15,16,17,18].

This review provides insight into the application of conventional methods and green extraction techniques, which do not use solvents that are harmful to the environment and save energy, and the application of plant-based industrial waste for the extraction of dietary fibers (Figure 1). The optimal conditions for each extraction method and the main observations of various investigations on the extraction techniques of dietary fibers from different by-products are presented. The articles were collected using scientific databases, such as Science Direct, Google Scholar, Scopus, PubMed, and ResearchGate. The search criteria were the keywords “dietary fibers”, “extraction”, “pectin”, “cellulose”, “β-glucan”, and “food waste”. The exclusion criterion was non-plant material as a dietary fiber source. The time period range of articles was not selected since the focus was on the significance of the studies selected for the areas covered in this review.

2. Dietary Fibers

2.1. Soluble Dietary Fibers

2.1.1. Pectin

Pectin is composed mainly of moieties of D-galacturonic acid which are interconnected through α-1,4-glycosidic bonds. It is located in the cell walls in the central lamella and provides firmness and resistance to plant tissues [19]. There are three basic structural domains of pectin that include homogalacturonan, rhamnogalacturonan I, and the substituted galacturonans, xylogalacturonan and rhamnogalacturonan II [20]. The ratios between these polysaccharides can vary, but the largest portion of polysaccharide is homogalacturonan (around 65%) followed by rhamnogalacturonan I (20–35%), while xylogalacturonan and rhamnogalacturonan II constitute less than 10% [16]. The carboxyl groups or hydroxyls of pectin can be esterified (methyl-esterified and/or O-acetyl-esterified). Depending on the percentage of esterified groups, the degree of esterification (DE) can be determined, which is the key factor underlying the determination of the gelling ability of pectin. Therefore, a distinction is made between the high-methoxylated pectin (DE > 50%) (for the formation of gel, cosolutes (e.g., sucrose) have to be present) and the low-methoxylated pectin (DE < 50%) (for the formation of gel, divalent cations (e.g., Ca²⁺) have to be present) [19]. The range of different pectin applications is broad, so high-methoxylated pectin is applied in the fruit industry for the preparation of jams and jellies as a gelling agent, thickener, stabilizer, and emulsifier, but also for other jell-like products, while low-methoxylated pectin is used in ice cream, bakery glazing, low-calorie products, fat substitutes, and emulsified meat products [4,21]. Pectin has the potential as a prebiotic due to its ability for modulating microbiota and its positive impact on the distal part of the colon. It is a widely distributed ingredient in the food, cosmetic, and pharmaceutical industries, in the production of animal feed, and in the formulations of edible packaging films [4].

2.1.2. β-Glucan

An important and significant source of soluble dietary fibers, especially β-glucan, is oat (Avena sativa L.) [22]. Other sources of β-glucan include barley, yeast cell walls, algae, fungi, and bacteria [23]. It has proven health benefits, such as reducing cholesterol levels, preventing diabetes, and diminishing the risk of coronary heart disorders. As an unbranched polysaccharide, β-glucan is composed of glucose moieties linked to polymers by β-(1 → 3) and β-(1 → 4) bonds with around 70% and 30% representation, respectively. The solubility of β-glucan depends on its molecular and structural properties. Thus, the existence of β-(1 → 3) links violates the regularity of β-(1 → 4) link intervals, which enables water molecules to penetrate into molecular chains causing fiber solubilization. On the other hand, nearby situated β-(1 → 4) bonds can cause inter-chain aggregation due to the formation of hydrogen bonds, resulting in the reduction of β-glucan solubility [22]. β-glucan retrieved from yeast contains a linear backbone of β-(1 → 3)-linked D-glucose with β-(1 → 6) side chains [24]. Hu et al. [25] investigated three types of β-glucans (mushrooms β-(1 → 3)/β-(1 → 6)-glucan, curdlan β-(1 → 3)-glucan, and oats bran β-(1 → 3)/β-(1 → 4)-glucan) on cognition and the gut–brain axis. All three β-glucans supplementations increased the number of positive cells (CD206) and enhanced the temporal order recognition memory. Only oat β-glucan altered gut microbiota and enhanced intestinal mucus. Furthermore, water-insoluble β-glucan had poor beneficial effects on the digestive system, while water-soluble β-glucan exhibited many potential health benefits, like immunity enhancer, lowering of total serum and low-density lipoprotein, and role in weight and diabetes control [26]. β-glucan has antimutagenic, anticytotoxic, and antitumorigenic properties, and can generally improve the resistance of the body to cancer, parasitic and infectious diseases [26].

2.1.3. Gums and Mucilage

Gums and mucilage are not components of cell walls; they are formed in specialized secretory plant cells. They are highly branched plant fibers which form gels and bind water and other organic materials. The most common gums are guar gum and gum arabic [27].

2.2. Insoluble Dietary Fibers

2.2.1. Cellulose

Cellulose is one of the most represented polysaccharides and can be obtained from the cell walls of plants, wood, algae, some species of bacteria, and tunicates, the only known animals which contain cellulose [28]. It is an unbranched polymer composed of repeating moieties of D-anhydroglucopyranose-connected β-(1 → 4) glycosidic bonds. Cellulose possesses a number of worthwhile characteristics, like biodegradability, biocompatibility, non-toxicity, renewability, and low cost. It is made of crystalline structures (the highly ordered regions) and amorphous structures (the disordered regions) [29]. Cellulose nanocrystals present nanoscale cellulose obtained from natural fibers having excellent chemical and physical characteristics. Cellulose is considered as a novel biodegradable packaging material and a food ingredient in the food industry. It has a high thermal stability, excellent mechanical characteristics, as well as rheological characteristics of shear thinning due to its large surface area, small size, and high crystallinity. Nanocrystals of cellulose are used as low-calorie replacements, carriers of flavor, or as other active ingredients and suspension stabilizers [30].

2.2.2. Hemicellulose

Hemicelluloses are polysaccharides which have backbones consisting of glucose units connected with β-(1 → 4) glycosidic linkages. They differ from cellulose in that hemicelluloses are smaller in size, are usually branched, and contain a variety of sugars [27,31]. The hemicellulose groups include xylans, xyloglucans, mannans, and glucomannans. The main biological role of hemicellulose is strengthening the cell walls via interactions with cellulose [31].

2.2.3. Lignin

Lignin is a three-dimensional long-chain and high molecular weight aromatic polymer of phenylpropane units, including sinapyl, coniferyl, and p-coumaryl alcohol, that underwent a complex dehydrogenative polymerization. Lignin comprises from 10% to 25% of the biomass in plant primary cell walls and contributes to the formation of rigid, impermeable, and resistant structures [27,32].

3. Extraction Methods

3.1. Subcritical Water Extraction

Hot water extraction is a commonly used technique for obtaining fibers from vegetable sources. Higher temperatures of water result in higher yields, but the extent of extractability obtained with enzymatic and chemical methods is higher [13]. Subcritical water, i.e., pressurized hot water, represents liquid water that is heated under pressure beyond the normal boiling temperature but not above the critical temperature of 374 °C [14]. Increasing the temperature in subcritical water leads to the weakening of the hydrogen bonds between water molecules causing an alteration of the dielectric constant. Temperatures around 270 °C cause raising of the ion product of water (Kw). As a result of the above mentioned properties, reactive selectivity and solubility of polar components in subcritical water can be regulated with temperature and pressure [33]. Important advantages of using this type of extraction are high extraction efficiency, short processing time, and positive environmental impact, without the residual organic solvents [14].

3.2. Ultrasound Assisted-Extraction

Sound waves consist of mechanical vibrations, and they can be utilized by various treatments for solids, gases, or liquids [4]. Ultrasound frequencies are higher than 20 kHz, which is above the hearing frequency range of humans (16 kHz) [4]. Ultrasound-assisted extraction is a green technique characterized by the utilization of intensive mechanical shear forces for the disruption of plant material structure, which allows the leaching of the components of interest [34]. Due to disruption, solvent entry into cells is stronger and enhanced. Ultrasound waves can facilitate hydration and swelling and result in the enlargement of pores in plant materials, which promotes diffusion and mass transfer [4]. Advantages over conventional heating methods include lower energy consumption, shorter than needed operating time, reduced solvent volumes needed, and increased yields [4,35].

3.3. Microwave-Assisted Extraction

Microwave frequencies range from 300 MHz to 300 GHz and belong to non-ionizing radiation. In the food matrix, microwaves produce heat due to their interactions with the cellular compounds of polar nature. The produced heat causes the ionic conduction and rotation of dipoles [4,36]. Over the last three decades, microwaves have been increasingly used for the preservation of food, extraction, inactivation and inhibition of enzymes, and inactivation of microorganisms. As an innovative method, microwave-assisted extraction combines the application of microwaves with the conventional extraction using solvents. The advantages of microwave-assisted extraction over conventional methods are the reduced use of applied solvents, increased yields, shorter process time, and a reduction in energy consumption [4,35].

3.4. Enzyme-Assisted Extraction

Enzyme-assisted extraction methods utilize lower extraction temperatures and higher pH values and are therefore considered a greener alternative to conventional methods. There are two explanations for the enzyme-assisted extraction of polysaccharides: (i) the enzymes disrupt the cell walls and membranes, and desirable compounds are thus liberated, and (ii) enzymes partially degrade polysaccharides. Enzymes which are commonly used for the extraction of soluble dietary fibers are cellulase, hemicellulase, protease, xylanase, pectinase, and glucanase. Since combinations of enzymes may achieve higher yields, blends of different enzymes, customized for particular cell wall type disruptions, can be bought [37]. The high cost of enzymes and their low extraction efficiency have been pointed out as some of the disadvantages of this technique [14].

3.5. Ohmic Heating Extraction

This technique is utilized for sterilizing foods and recovering valuable compounds from plants. This technique herein permits the passing of electricity through material and produces heat. Ohmic heating rapidly heats the material, resulting in the proper mass and heat transfer during extraction. This technique reduces the processing time and causes no variations in the properties of the extracted compounds [38].

3.6. Pulsed Electric Field Assisted-Extraction

The pulsed electric field technique uses short electrical pulses with high voltage into the processed material. This is a non-thermal technique that is used for the extraction of valuable compounds from plants and for the sterilization of foods [39,40]. Electrostatic charges cause an electrical field potential when the living cell reaches an electric field. A critical value of electrical potential causes irreversible or reversible electroporation in the cell membrane, resulting in the release of cytoplasmic fluid and cellular materials [40].

4. Dietary Fibers Extraction

4.1. Fiber Mixtures Extraction

Grapefruit peel, a food industry by-product, was utilized for the extraction of soluble dietary fibers. For this purpose, microwave-assisted extraction was applied with three modifications: microwave-enzymatic, microwave-sodium hydroxide, and microwave-ultrasonic treatment. Comparing the soluble dietary fibers from untreated grapefruit peel and microwave treatment alone, these three extraction modifications resulted in more complex and looser structure fibers, as well as a higher molecular weight, thermal stability, and crystallinity of fibers. Microwave-ultrasonic treatment resulted in the production of fibers with the highest water-holding and oil-holding capacities, as well as the adsorption of glucose, nitrite ion adsorption, and cholesterol adsorption capacities, thus indicating a great potential for utilization in different segments of the food industry [41]. Furthermore, citrus junos peel comprises about 50% of the whole fruit and is a valuable source of dietary fibers (pectin, cellulose, and hemicellulose) and some specific oils. Through the application of hydrothermal treatment with subcritical water in a temperature range from 160 °C to 320 °C, pectin, cellulose, and hemicellulose were successfully isolated from the residue of citrus junos peel [33].

Conventional milling of the wheat grain for flour production causes the formation of wheat bran as a by-product, which presents a rich source of soluble dietary fibers. The subcritical water extraction of wheat bran, with the objective of obtaining soluble dietary fibers, was conducted as well as subcritical water extraction with modifications in order to improve the extraction process. One modification included subcritical water extraction in aqueous citric acid (pH 5.0) while another modification comprised ultrasound-assisted subcritical water extraction in aqueous citric acid. Carboxyl groups from citric acid can cause the ionization of hydrogen ions, leading to the disruption of cell walls accompanied by acceleration of the dissolution of soluble dietary fibers. On the other hand, application of ultrasonic treatment can convert some insoluble dietary fibers into soluble ones. The pressure and temperature during this process were fixed at 5 MPa and 140 °C. Their results revealed that the utilization of ultrasound-assisted subcritical water extraction in aqueous citric acid achieved a higher yield of soluble dietary fibers, smaller particle size, lower molecular weight, higher antioxidant potential, and α-amylase inhibitory potential compared to the other applied methods. Through this method, improved homogeneity and thermal stability have been achieved; on the other side, reduction in apparent viscosity and dynamic viscoelasticity were also observed. The authors emphasised that this method is a competitive green extraction technique that can be used for the separation of soluble dietary fibers from wheat bran [14]. Green techniques (including water extraction, ethanol extraction, and steam extraction in combination with lemon juice) were also applied onto cactus rackets (Opuntia ficus indica) in order to extract dietary fibers. Also, extraction times (at 30 min, and 1, 3, and 5 h), as well as maceration or steam explosion, were investigated. The results of Fourier transform infrared (FT-IR) spectroscopy showed that the applied extractions did not violate the structure of cellulose but were able to reduce the contents of both lignin and hemicellulose. The steam extraction in combination with lemon juice resulted in higher quality fibers with the decomposed amorphous phase and preserved crystalline phase [42]. Mucilage is a complex, high molecular weight polysaccharide that is considered a dietary fiber. Its health benefits include the reduction of cholesterol and beneficial effects on intestinal function, and in the food industry, it is also used for thickening, gel formation, and as a chelator [43,44]. Chia seeds were used for the extraction of mucilage and throughout optimization of their extraction it was concluded that the optimal conditions of their extraction were as follows: temperature of 80 °C, time of extraction 4 h, and water: seed ratio 30:1. Under these conditions, an extraction yield of 4.95 g/100 g was achieved [44]. Fan et al. [40] extracted soluble dietary fibers from orange peel using pulsed electric field assisted-extraction. Their optimal conditions included a temperature of 45 °C, a pulse number of 30, an electric field intensity of 0.6 kV/cm, and a processing time of 20 min. The obtained soluble dietary fibers exhibited good properties (high water solubility, water- and oil-holding capacity, swelling capacity, emulsifying capacity, foam, and emulsion stability). The authors proposed this technique for the improvement of the physicochemical properties of orange peel soluble dietary fibers.

4.2. Pectin Extraction

Citrus peel (orange and lemon) is the most common material for pectin extraction; additionally, apple pomace or sugar beet pulp are often used. However, with the increase in pectin applications, novel sources have gained increasing importance, such as mango peel, guava pulp, fig seed, passion fruit peel, faba bean and cocoa hulls, watermelon rind, banana, and pomegranate peels [4,16].

Jackfruit (Artocarpus heterophyllus) peel presents a valuable by-product of jackfruit processing. Through the process of extraction with organic (mostly tartaric, lactic, and citric acids) and mineral acids (mostly hydrochloric, nitric, and sulfuric acids), pectin was obtained from this by-product. A comparison of pectin obtained with ultrasonic-microwave-assisted extraction in combination with citric acid, and conventional heating extraction was performed. Results indicated that through the application of the first extraction technique, a higher pectin yield was achieved. The optimal conditions for this process were as follows: extraction temperature of 86 °C; extraction time of 29 min; and solid–liquid ratio 1:48 (w/v). Pectin obtained with ultrasonic-microwave-assisted extraction exhibited a stronger antioxidant activity under certain assays. Therefore, this method can be a suitable alternative to the conventional extraction method as citric acid is a “green” and safe acidic solvent [45]. Eggplant waste currently has no commercial value but presents a great source of polyphenols and polysaccharides [5]. Pectin was extracted from eggplant peel and calyx through the employment of microwave-assisted extraction under the following conditions: microwave power of 700 W; irradiation time of 2 min; pH 1.5; liquid-to-solid ratio 20 (v/w). Following the application of these conditions, the extracts were centrifuged, treated with ethanol, and dried. A higher pectin extraction yield was obtained for eggplant peel (29.17%) than for eggplant calyx (18.36%). Results showed the presence of a high content of homogalacturonan (58.6%) in eggplant peel pectin and rhamnogalacturonan-I (44.9%) in eggplant calyx pectin. The first one demonstrated higher values for the content of polyphenols, antioxidant activity, and emulsifying and foaming properties, as well as water-holding and oil-holding capacities. This type of extracted pectin has the potential for its utilization as an ingredient for different food systems [5]. However, these authors only applied microwave-assisted extraction for eggplant waste, indicating that this extraction technique may not be reliable for other applications. It will be necessary to also perform other extraction techniques, using combinations of microwave and ultrasound to achieve improved extraction yields, and demonstrate better properties of obtained pectin. Another source of pectin can be fig (Ficus carica L.) skin. Four methods for pectin extraction (hot water, microwave-assisted, ultrasound-assisted, and ultrasound-microwave-assisted extraction techniques) were applied to select the optimal one. The highest yield was observed for ultrasound-microwave assisted extraction (11.71%) at operating conditions of 25 min of sonication time, 3.5 min of irradiation time, 600 W of microwave power exposure, pH 1.4, with a fig skin/water ratio of 20 g/mL. Pectin obtained through this method, with 76.85% galacturonic acid content and 6.91 × 10³ kDa molecular weight, had the highest emulsifying properties and emulsion stability [46]. Furthermore, pectin from tomato peel was extracted through five different extraction methods (ultrasound-assisted, microwave-assisted, ohmic heating-assisted, ultrasound-microwave-assisted, and ultrasound-assisted ohmic heating extraction techniques). All obtained pectin via these techniques had acceptable purity and typical functional groups were recorded through FT-IR screening. The extraction yield was highest in microwave-assisted extraction (25.42%) with a small difference for ultrasound-microwave-assisted extraction, but with a significant variation in esterification degree (59.76% versus 73.33%, respectively). Considering the quality and yield, ultrasound-microwave-assisted extraction can be recommended for utilization for pectin extraction from tomato peel [47]. Extraction of pectin from banana peels with citric acid was optimized using response surface methodology. The optimal recommended conditions which ensured maximum galacturonic acid yield and a degree of methoxylation above 51% include the temperature of 87 °C, lasting for 160 min at pH 2.0 [48]. Optimal conditions for the extraction of pectin from Citrus medica peel through applying water have also been defined and include a temperature of 90 °C, 180 min duration, with a liquid/solid ratio of 40 v/w. The pectin extraction yield was 21.85% and this pectin was characterized by the degree of esterification of 77.2% and emulsifying activity of 46.5% [49]. Also, artichoke by-products were utilized as the material for the extraction of pectin with different extraction methods (ultrasound-assisted, enzyme-assisted, ultrasound-enzyme-assisted, and acid extraction techniques). Ultrasound-enzyme-assisted extraction had the highest yield with a pectin molecular weight ranging from 160 to 267 kDa. Under ultrasound-assisted extraction, pectin of the lowest molecular weight (146–155 kDa) was obtained, while the pectin obtained via acid extraction had the highest molecular weight (329–352 kDa) and galacturonic acid content (82.2–90.2%). Considering all these results, the authors concluded that the ultrasound-enzyme-assisted extraction of pectin is a quick method with high yield, without molecular modifications and low co-extractions of other polysaccharides, with a disadvantage of lower galacturonic acid content [50]. Citric acid was used for the extraction of pectin from the pistachio green hull under optimized conditions. These conditions included a pH of 0.5, temperature of 90 °C, processing time of 30 min, and liquid-to-solid ratio of 50 v/w. Under optimal conditions, the achieved pectin yield was 22.1% and the galacturonic acid content was around 65% [51]. Mango peel turned out to be a good source of pectin. The optimized extraction with hydrochloric acid suggested ideal conditions of 35–48 min of operating time, 85.4–97 °C temperature range, and a 1.66–2.4 pH value range. Under these conditions, both energy savings and hydrochloric acid volume reduction have been achieved [52]. Grassino et al. [53] compared the conventional method of pectin extraction (with ammonium oxalate/oxalic acid as their solvent) and ultrasound-assisted extraction (at 37 kHz) under two different temperatures (60 °C and 80 °C, respectively). As a source of pectin for extraction, tomato waste was used. Although conventional extraction at 60 °C resulted in the highest pectin yield, a better quality of pectin was obtained with ultrasound-assisted extraction. Also, shorter extraction times and the eco-friendly approach give priority to ultrasound-assisted extraction. In this case, only the ultrasound technique was applied as a novel extraction technique. Previously, the extraction of pectin from tomato peels by five different methods was considered [47], and in view of the obtained yield and pectin quality, the ultrasound-microwave-assisted extraction was recommended. In order for this particular study to be relevant for potential industrial application, comparison with other extraction techniques would also need to be performed. Microwave and conventional methods (Soxhlet extraction) were applied for pectin extraction from orange peels. It was observed that for microwave-assisted extraction, the highest pectin yield was 5.27% using 15 min of extraction time. Regarding the implemented pH values (1.5, 2, and 10) the highest amount of pectin was obtained at pH 1.5, and in systems with ethanol and EDTA [54]. The conventional acid technique was performed for pectin extraction from fresh watermelon rinds and lyophilized watermelon rinds. Pectin was extracted from alcohol-insoluble residues with boiling nitric acid (0.1 M) under reflux for a duration of one hour where the solid: liquid ratio was 1:25 (w/v). Precipitated pectin was washed with ethanol and dried under vacuum. The obtained results showed a higher yield for the fresh watermelon rinds than the lyophilized material. The obtained pectin showed a relatively high viscosity, good foaming and emulsifying properties, and could have potential applications in the food industry with great economic prospects [55].

Table 1. Studies of pectin extraction from various material sources.

Plant Resource		Reference
Musa balbisiana waste		[56]
Method	Ultrasound-assisted with citric acid-mediated extraction.
Conditions	Optimized conditions were as follows: 323 W—ultrasound power; 3.2—pH; 28 min—extraction time; and 1:15 g/mL—solid: liquid ratio.
Observations	The experimental pectin yield was 8.99% while the predicted yield was 9.02%.
Passion fruit peel		[57]
Method	Ultrasound-assisted extraction.
Conditions	The conditions were as follows: 644 W/cm2—power intensity; 85 °C temp; 1:30—dried peel: solvent ratio; and 10 min—sonication.
Observations	The achieved pectin yield under these conditions was 12.67% and the esterification degree and galacturonic acid content were 60.36% and 66.65%, respectively. The authors made a comparison of this extraction method with the conventional method under similar conditions and the obtained pectin yield was higher with the ultrasound-assisted extraction.
Lime peel		[58]
Method	Microwave-assisted and conventional extraction.
Conditions	Hydrochloric and citric acids—solvents; 1:20 or 1:40—peel: solvent ratio; 95 °C—temp for conventional method; 700 W—microwave oven power.
Observations	Pectin yield was higher with the conventional method with hydrochloric acid. Microwave-assisted extraction with citric acid resulted in pectin without loss in quality (higher equivalent weight and degree of esterification) and this method also resulted in a reduced energy consumption due to the shortened extraction time.
Sisal waste		[59]
Method	Ultrasound-assisted extraction.
Conditions	Optimized conditions were: 61 W—ultrasonic power; 50 °C temp; 26 min—time of sonification; and 1:28 g/mL—solid: liquid ratio.
Observations	The experimental pectin yield was 29.32%, and the authors highlighted the advantages of this method due to reduced energy consumption, lower temperatures, and a shorter extraction time.
Malus domestica ‘Fălticeni’ apple pomace		[60]
Method	Microwave-assisted extraction. Ultrasound-assisted extraction. Enzyme-assisted extraction. Ultrasound-assisted extraction—heating treatment. Enzyme-assisted extraction—ultrasound treatment. Conventional citric acid extraction.
Conditions	pH 2.2; 560 W—microwave power; and 120 s—duration. pH 1.8; 30 min—sonification. With cellulase: pH 4.5 (adjusted with citric acid); 47 °C temp; and 20 h duration. With Celluclast 1.5 L: pH 4.5; 48 °C temp; and 18 h and 14 min duration. After ultrasound-assisted extraction, the sample was heated for 2 h at 86 °C. After enzyme-assisted extraction with cellulase, the sample was sonicated for 21 min. pH 1.9; 90 °C temp; and 148 min duration.
Observations	The highest extraction yield was obtained with microwave-assisted extraction while the lowest was with enzyme-assisted extraction (with Celluclast 1.5 L). Pectin obtained via microwave-assisted extraction had a high galacturonic acid content, acceptable color parameters, increased equivalent weight, high molecular weight, and degree of esterification. Pectin obtained through ultrasound-assisted extraction also had a high galacturonic acid content, degree of esterification, and molecular weight. Pectin obtained with enzyme-assisted extraction had a lower degree of esterification and was classified as low-methoxylated pectin.
Gold kiwifruit (Actinidia chinensis)		[61]
Method	Conventional citric acid extraction, water extraction, and enzyme-assisted extraction.
Conditions	Acid extraction: pH 2.2; 1:3 w/v—solid: liquid ratio; 50 °C temp; and 60 min duration. Water extraction: pH 3.7; 1:3 w/v—solid: liquid ratio; 25 °C temp; and 30 min duration. Enzymatic extraction: Celluclast 1.5 L; pH 3.7; 1:3 w/v—solid: liquid ratio; 25 °C temp; and 30 min duration.
Observations	Pectin extracted with water exhibited the properties closest to its native form. The highest extraction yield was obtained with enzyme-assisted extraction.
Sour orange peel		[62]
Method	Microwave-assisted extraction.
Conditions	Optimized conditions: pH 1.5; 700 W—microwave power; and 3 min—irradiation time.
Observations	Pectin yield was 29.1% and degree of esterification ranged from 1.7% to 37.5%, which indicated low-methoxylated pectin. Emulsifying activity and galacturonic acid content was 40.7% and 71%, respectively.
Lemon, mandarin and kiwi peels		[63]
Method	Microwave-assisted extraction and ultrasound-assisted extraction.
Conditions	Hydrochloric acid and nitric acid—solvents. For microwave-assisted extraction: 360–600 W—microwave power; and 1, 2 and 3 min—irradiation time. For ultrasound-assisted extraction: 60 °C and 75 °C temp; and 15, 30 and 45 min—sonification.
Observations	The highest yield was obtained for kiwi peel using hydrochloric acid (17.97% for microwave-assisted extraction at 360 W for 3 min and 17.30% for ultrasound-assisted extraction at 75 °C for 45 min, respectively). Generally, for all materials, microwave-assisted extraction resulted in higher extraction yields.
Apple peel		[64]
Method	Organic acid extraction.
Conditions	Citric, malic, and tartaric acids—solvents; 85 °C temp.
Observations	Citric acid extraction resulted in pectin with the highest molecular weight and apparent viscosity. Pectin obtained with all tested organic acids was highly methoxylated.
Melon peel		[65]
Method	Conventional citric acid extraction.
Conditions	Optimized conditions: pH 1; 95 °C temp; 10 v/w ratio; and 200 min—duration.
Observations	The extraction yield was 29.48% and the galacturonic acid content was 48%, respectively. The emulsifying activity was 35% and at concentrations of 1% w/v, the obtained pectin behaved as a weak gel.
Banana peel		[66]
Method	Microwave-assisted extraction (continuous and intermittent).
Conditions	Optimized conditions for continuous extraction: pH 3; 900 W—microwave power; and 100 s duration. Optimized conditions for intermittent extraction: pH 3; 900 W—microwave power; and 0.5—pulse ratio.
Observations	The highest extraction yield for continuous extraction was 2.18%, while for the intermittent extraction it was 2.58%, respectively.
Lemon wet and dried peels		[67]
Method	Water-based extraction. Microwave-assisted extraction.
Conditions	Acidified water (with sulfuric acid)—solvent; pH: 1, 2, and 3; temperatures: 80, 90, and 95 °C; and time: 60 and 90 min. Distilled water, 0.05 M ethylenediamine tetra acetic acid (EDTA), and 1 M sodium hydroxide—solvents; time: 5, 10, and 15 min.
Observations	The maximum pectin yield with water-based extraction was obtained at 95 °C, pH of 1, for 90 min for both wet and dried lemon peels, at 46% and 16%, respectively. For microwave extraction, the maximum yield was obtained using EDTA at an extraction time of 15 min.
Banana peel		[68]
Method	Ultrasound-assisted extraction.
Conditions	The conditions were as follows: 0.1 M of citric acid with pH 1.5, solid: liquid ratio 1:33.3 g/mL, temperature of 75 °C, and sonicated for 23 min.
Observations	Using the optimal extraction conditions, the achieved pectin yield was 6.08%. The obtained pectin was high-methoxyl pectin with improved gelling time.

summarizes the studies conducted on the extraction of pectin from different plant materials using different extraction techniques.

4.3. β-Glucan Extraction

Across the food industry, there has been an increased interest focused on the investigation of β-glucan extraction and purification techniques, as well as ways of incorporating β-glucan into food products. Techniques which achieve the highest yields and do not affect the molecular weight of β-glucan are favored [69]. Various extraction methods were developed for acquiring β-glucan from oats and barley. These methods include dry and wet processes. Milling and sieving, i.e., the dry extraction methods, serve to obtain flour fractions rich in β-glucan [22]. Dry fractionations include various operations, such as hammer milling, roller milling, abrasion milling, shifting, and pearling [69,70]. The main disadvantage of these methods is the poor yield of β-glucan (of around 30%) [71]. Concentrations of β-glucan obtained through dry methods can be increased with the application of ultrafine grinding and electrostatic separation, as reported by Sibakov et al. [72], with a 56.2% β-glucan concentration in the final product. Wet techniques include a solvent for a wet extract, and they comprise hot water extraction, solvent extraction, alkali extraction, and enzymatic extraction [22]. Using these methods, β-glucan may be concentrated up to 95% [69]. β-glucanase is an endogenous β-glucan enzyme which degrades and ferments β-glucan. During water extraction, this enzyme may be activated, and the result produced via this technique is the lowering of β-glucan molecular weight and viscosity. This problem can be overcome through the inactivation of β-glucanase with ethanol (80% v/v) before extraction [69,73]. Contaminants, such as starch and proteins, are usually removed using hydrolytic enzymes or through selective adsorption [74]. Problems with wet extraction can be caused by the high viscosity of β-glucan aqueous extracts, even at low concentrations. Also, low pH values and the presence of endogenous enzymes can cause a reduction in the molar mass of β-glucan [74].

Yoo et al. [22] performed β-glucan extraction from oat flour using subcritical water extraction under optimized conditions (200 °C—extraction temperature, 10 min—extraction time, 4.0—solvent pH, and 425–850 µm—particle size). Extraction yield was 6.98 g/100 g oat flour, which was twice the yield value achieved using hot water extraction at 60 °C for 3 h [22]. Hot water extraction was performed on the oat bran to extract β-glucan. Milled oat bran was first mixed with ethanol to deactivate β-glucanase, followed by the addition of water (55 °C) to the mixture and mixing for 2 h. After centrifugation, the supernatant was adjusted to pH 8.5 using NaHCO₃ and then to pH 4 with citric acid. The slightly alkaline solution and acid solution were necessary for the separation of proteins and fibers. Finally, β-glucan was precipitated with ethanol and the yield obtained was 5.3% [75]. Barley (Hordeum vulgare L.) β-glucan was extracted with subcritical water at temperatures of 130–170 °C and under pressures of 10–40 MPa in a semi-batch system. UV–Vis spectra and FT-IR spectra confirmed the extraction of β-glucan. These authors improved their extraction process with CO₂ addition and their results showed that the extraction yield was significantly increased [76]. The same group of authors found that the entire amount of water-soluble β-glucan was extracted at 170 °C [77].

Table 2. Studies of β-glucan extraction from various material sources.

Plant Resource		Reference
Highland barley		[78]
Method	Alkaline-acid-alcohol extraction method.
Conditions	Barley bran was mixed with water at pH 8 adjusted with NaOH and incubated for 2.5 h at 80 °C. The mixture was centrifuged two times (after the first one, the pH of the supernatant was adjusted to 4.5 with HCl, and after the second, the supernatant was reduced at 100 mL and mixed with ethanol) and after 10 h of incubation at 4 °C the precipitate was freeze dried.
Observations	β-glucan extract showed great thermal and pH stability and its solubility was influenced by temperature.
Hull-less barley bran		[18]
Method	Ultrasound-assisted extraction. Microwave-assisted extraction. Microwave-ultrasound-assisted extraction. Hot water extraction.
Conditions	A 1:25 w/v—solid: liquid ratio; 50%—amplitude level; 20 min duration; and 50 °C temp; A 1:25 w/v—solid: liquid ratio; 3 min duration; and 800 W—microwave power; A 1:25 w/v—solid: liquid ratio; microwave heating for 1.5 min at 800 W; and sonicating for 10 min at 50 °C; A 1:15 w/v—solid: liquid ratio; 80 °C temp; 2 h duration; enzymatic treatment for removing starch and protein—α-amylase and trypsin; precipitation in ethanol for 12 h; and freeze drying.
Observations	The microwave-ultrasound-assisted extraction resulted in the highest extraction yield (2.16%) in the shortest amount of time. β-glucan obtained with this method had the highest apparent viscosity, stronger foam stability and emulsifying properties than β-glucan obtained with ultrasound-assisted extraction, which had the stronger foaming capability.
Barley bran		[79]
Method	Enzyme-assisted extraction.
Conditions	α-amylase—pH 6.5, 96 °C temp; glucoamylase—pH 4.5, 50 °C temp; protease—pH 7.5, 60 °C temp; pullulanase—4.5, 50 °C; two types of xylanase—pH 4.75, 50 °C temp.
Observations	Results showed that the three-step purification using α-amylase, protease, and xylanase for 4 h increased the β-glucan content, and removed starch, protein, and pentosans. Also, the highest β-glucan purity (around 89%), as well as the lowest molecular weight, (2 × 104 g/mol) were achieved with this method.
Oat bran		[80]
Method	Alkaline extraction.
Conditions	Endogenous enzyme inactivation and fat removal: water/ethanol (50:50 w/w)—solvent; and 80 °C temp. Extraction: pH 8.5 (NaOH/water, 1:40 w/w). Deproteinization: at pH 4.5 and centrifugation. Enzymatic treatment: pancreatin, thermostable α-amylase, and amyloglucosidase.
Observations	The obtained β-glucan was successfully purified, and it can be concluded that a complex process, which includes different enzymes, is required for the removal of residuals from β-glucan.

lists the investigations conducted on β-glucan extraction from various material sources.

4.4. Cellulose Extraction

Cellulose is the most abundant natural macromolecule in the world. As tree resources are decreasing, more attention is paid to the search for new natural cellulose resources and green extraction processes [81]. Usually, cellulose extraction is performed using alkali treatment, and it is an effective method for the removal of non-cellulosic components, but results in the production of pollutants [81,82,83,84]. Water is an ecological solvent and can be used for the extraction of soluble dietary fibers. Subcritical water has a higher concentration of hydroxide and H₃O⁺ ions and can act as an acid or base catalyst precursor during hydrolysis. Also, organic compounds are more soluble due to their low dielectric constant and density of subcritical water [81]. Subcritical water can be applied to obtain pure cellulose because during subcritical water treatment hemicellulose gradually hydrolyzes at 180 °C, and with the increase in temperature levels, the depolymerization and condensation of lignin also occurs [81,85].

During sesame hulling, high amounts of sesame hulls are produced which are typically used as domestic animal feed or discarded. These hulls contain significant amounts of cellulose and are valuable to the food industry. Subcritical water and alkali pretreatment were performed for the extraction of purified cellulose from the sesame hulls. The results indicated that the extraction yields were similar for both techniques while the crystallinity of cellulose obtained with the subcritical water pretreatment was significantly higher [81]. Rice straw is a by-product of rice production that includes leaf blades and sheaths, stems, and the remains of the panicle, and it is an abundant lignocellulosic material [34]. For the extraction of cellulose from rice straw, ultrasound, combined with the reflux heating method, was applied. In comparison with the alkaline process (29%), a higher yield was obtained with the combined method (37%), also resulting in a more hydrophilic cellulose with lower aggregation tendency. Both cellulose fibers had similar crystallinity, morphogeometric characteristics, thermal behavior, as well as similar performance after incorporation into a polymer matrix. This new combined method was four times faster than the alkaline process and is also an eco-friendlier process that does not require a toxic solvent [34]. Alkaline treatment for cellulose extraction from apple pomace was optimized using response surface methodology. The optimal conditions were 161.54 min for extraction time, 10.23% for the NaOH concentration, and 69.82 °C for the temperature. Under these conditions, the cellulose yield was 27.96%, the α-cellulose content was 85.31% and the whiteness index was 47.79%. To produce cellulose nanocrystals, acid hydrolysis and ultrasonication treatment were used and resulted in a higher crystallinity index (78%) than for α-cellulose (69%) [86]. Preparation of cellulose nanofibrils from sugarcane bagasse was performed by Feng et al. [87]. High-speed dispersion combined with ultrasonication, and preceded by pretreatments (including continuous steam explosion, dilute alkali-catalyzed hydrothermal treatment, and bleaching with hydrogen peroxide), was conducted. Continuous steam explosion pretreatment caused the violation of the middle lamella and the primary wall and also resulted in the defibration of the cellulose fibers. In the alkali treatment, dilute NaOH (0.4 wt%) solution heated at 200 °C removed hemicellulose and lignin. Bleaching with H₂O₂ was the final step for cellulose production. The obtained cellulose nanofibrils possessed gel-like behavior in suspension and good thermal stability, with an environmentally friendly approach to preparation.

Table 3. Studies of cellulose extraction from various material sources.

Plant Resource		Reference
Tomato pomace		[88]
Method	Alkaline and bleaching treatment for cellulose production and extraction of cellulose nanocrystals with acid hydrolysis.
Conditions	Acid hydrolysis-optimized conditions: 45 °C temp; 30 min duration.
Observations	The obtained crystallinity was 97% and the particle average diameter was 104 nm. The results showed that tomato pomace, as food waste, could be used for the extraction of cellulose nanocrystals, which is an environmentally friendly material.
Peanut shell		[17]
Method	Alkaline and bleaching treatment for cellulose production.
Conditions	For alkaline treatment: 0.5 M sodium hydroxide; 90 °C temp. Washing with nitric acid in ethanol and finally bleaching with sodium hypochlorite (10%).
Observations	A total of 0.39 g/g of cellulose (dry wt%) was extracted from the tested material.
Mengkuang leaves (Pandanus tectorius)		[89]
Method	Alkaline and bleaching treatment for cellulose production and extraction of nanocrystals with acid hydrolysis.
Conditions	For alkaline treatment: 4% sodium hydroxide; 125 °C temp; and 2 h duration. Bleaching treatment: 1.7 w/v% NaClO2; pH 4.5; 125 °C; and 4 h duration. Acid hydrolysis: 60 wt% H2SO4; 45 °C temp; and 45 min duration.
Observations	The raw material contained 37.3% cellulose. After alkaline treatment and bleaching treatment, cellulose content was 57.5% and 81.6%, respectively.
Rice husk		[15]
Method	Alkaline and bleaching treatment for cellulose production and extraction of nanocrystals with acid hydrolysis.
Conditions	For alkaline treatment: 4 wt% sodium hydroxide; 2 h duration. Bleaching treatment: buffer solution of aqueous chlorite (1.7 wt%), acetic acid and distilled water; 4 h duration; and 100–130 °C temp. Acid hydrolysis: 10 M sulfuric acid 50 °C temp; 40 min duration.
Observations	The raw material contained 35% cellulose. After alkaline treatment and bleaching treatment, cellulose content was 57% and 96%, respectively.
Orange peel		[90]
Method	Extraction with sodium sulfite and sodium metabisulfite.
Conditions	The applied conditions were as follows: 98 °C temp; 7.5:1—liquid: solid ratio. Bleaching treatment was performed under an alkaline medium, slightly acid medium, and hydrogen peroxide at 25 °C temp for 240 min, 120 min, and 60 min, respectively.
Observations	The optimum yields were 45.2% and 40.4% for sodium metabisulfite and sodium sulfite, respectively. The results showed good purity, whiteness, water retention, molecular weights, and low crystallinities.
Lemon (Citrus limon) seeds		[29]
Method	Water extraction of cellulose and preparation of cellulose nanocrystals through sulfuric acid hydrolysis, ammonium persulfate oxidation, and TEMPO oxidation.
Conditions	Extraction of cellulose from lemon seed powder with water at 80 °C for 2 h. Lignin was removed with 5% w/v NaClO2, pH 3.8–4, at 75 °C for 5 h. Hemicellulose and residual lignin were removed with 10% w/v NaOH at 30 °C for 12 h. Sulfuric acid (64% w/w) hydrolysis was conducted for 1.5 h at 45 °C. Ammonium persulfate (1 M) oxidation was performed at 70 °C for 14 h. TEMPO oxidation was conducted with the TEMPO/NaBr/NaClO system in water (pH 10) for 4 h.
Observations	The lemon seed cellulose yield was 14.6% (w/w). The yield of cellulose nanocrystals obtained with sulfuric acid hydrolysis, ammonium persulfate oxidation, and TEMPO oxidation was 27.61%, 13.02%, and 52.01%, respectively.

presents the studies conducted on cellulose extraction from various material sources.

5. Conclusions and Future Perspectives

Generally, large amounts of waste, after processing of raw materials into different products, are left over, creating economic and environmental issues and concerns across all types of food industries. Very often, these by-products, especially plant-based ones, still contain high amounts of valuable compounds, especially bioactives. In the modern world, it is necessary to minimize these losses and wastes, and significant efforts have been made in order to maximize sustainability in the overall production process of food products, from field to table. In this review, we concentrated on dietary fibers as valuable compounds known for their health benefits. Nutritional guidelines have emphasized the importance of dietary fibers in our daily diets and recommend their constant intake or increased intake, depending on the nutritional practices and behavior of the individuals. Dietary fibers extracted from plant-based by-products can be used for the enrichment of foods with the objective of the formulation and production of functional foods. On the other hand, they can be used in the formulation of foods with specific properties, due to their interesting and valuable technological characteristics. Reuse of plant-based by-products is a step in the right direction in ensuring sustainability of the food industry and agro-industrial complex and can provide a major contribution to the general sustainability movement all over the world. Application of the scientific results of extraction of different resources from plant-based by-products and scaling-up of laboratory practices to industrial throughputs has been highly appreciated. Conventional extraction methods are still represented, to a certain high degree, in industrial processes, in spite of the developments of new methods capable of delivering higher yields and shorter times of extraction. Plant-based industry wastes have proven to be highly valuable materials for the extraction of dietary fibers, resulting in fibers of high quality and desirable physical and chemical properties. Microwave-assisted and ultrasound-assisted extraction methods have shown to be the most effective extraction techniques, both individually, and in combination with each other and with other methods. By studying scientific publications on this topic, it was observed that in most studies, only two methods of extraction were carried out experimentally and compared, and from the obtained results, one of the investigated methods is proposed as promising for the investigated by-products. For the actual application of these methods, several possibilities of extraction methods should be examined. Subsequently, the ones with the highest capability of exhausting by-products and achieving maximum product quality should be chosen. Application of green extraction techniques, which do not use solvents that are harmful to the environment and minimize energy use, in combination with the use of plant-based industrial waste to extract components of interest, would provide a foundation for sustainable food production with an ecologically friendly approach. Investigation of new industrially relevant extraction techniques is expensive; however, reducing the time of extraction, energy consumption, and reagent expenses, while increasing extraction yields and achieving improved quality of the final products, can result in the economic and commercial validation of these novel methods since they can achieve profitability in the long run. Although numerous studies have been performed on dietary fiber extraction by implementing new extraction techniques using various plant-based by-products, which resulted in promising results both in terms of yield and product quality, there are still many challenges and obstacles to their industrial applications. Some of these challenges are the lack of providers of such equipment, the previously mentioned high costs, and also the lack of professional operators, as well as programs for their training and education. Also, not enough is known about the applicability of these techniques after their scale-up to industrial production capacities, but successful laboratory studies on various plant-based by-products are an invitation to industries to cooperate with the scientific sector.