# Extraction of Dietary Fibers from Plant-Based Industry Waste: A Comprehensive Review
## 4 Dietary Fibers Extraction  
### 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 \({}^{\circ}\)C to 320 \({}^{\circ}\)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 \({}^{\circ}\)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 \(\alpha\)-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].