# Extraction of Dietary Fibers from Plant-Based Industry Waste: A Comprehensive Review
## 4 Dietary Fibers Extraction
### 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]. Jackkruit (_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 \({}^{\circ}\)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 rhamnostoman-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 ex traction 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 \(\times\) 10\({}^{3}\) 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.