# Extraction of Dietary Fibers from Plant-Based Industry Waste: A Comprehensive Review
## 4 Dietary Fibers Extraction
### Pectin Extraction
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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)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]. [PERSON] 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 \({}^{\circ}\)C and 80 \({}^{\circ}\)C, respectively). As a source of pectin for extraction, tomato waste was used. Although conventional extraction at 60 \({}^{\circ}\)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.