# Extraction of Dietary Fibers from Plant-Based Industry Waste: A Comprehensive Review
## 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 largeintestine\". 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, \(\beta\)-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].