# Harnessing Agri-Food Waste as a Source of Biopolymers for Agriculture
## 2 Sources of Biopolymers
### Pectin: Importance and Applicability as Biopolymer  
Pectin is a heteropolysaccharide mainly composed of methylated D-galacturonic acid units (Figure 2); this biopolymer can have a different composition, length, degree of esterification, and ramifications. The different characteristics that pectin can undertake are directly linked to the natural variability of the polymer. Pectin is present in all plant tissues, but it is particularly abundant in fruit peels and pomace; for this reason, peels are the part used more often for pectin extraction, with being citrus peel the most common AFW source for commercial manufacturing of pectin [42]. Pectin can be extracted using different methods: the most used one is the extraction under conventional heating (70-100 \({}^{\circ}\)C) and acidic conditions (pH 1.5-2.5) [43]. After the extraction step, pectin is usually isolated through ethanol precipitation. However, in conventional processes, pectin yield is lower compared to novel techniques [44]. In Table 2, some recent works on pectin extraction from biomass are reported. The yields and quality of pectin, measured by the degree of esterification, vary depending on the plant source, cultivar, growth conditions, and part of the plant being extracted. Figure 2: Pectin chemical structure. For instance, subcritical water extraction (SWE) reveals different optimal extraction temperatures for apple pomace and citrus peel. The yield for apple pomace is 16.68% and for citrus peel is 21.95%, achieved through extractions at 150 \({}^{\circ}\)C and 120 \({}^{\circ}\)C, respectively. (Entry 1) [45]. In Entries 2 [46] and 3 [47] pectin was extracted from grape pomace and pomeramate peels using similar protocols, but the yield on pomeramate peel is 1.25 times higher in comparison to grape pomace. Furthermore, it can be noticed in Entry 4 [48] that the yield obtained from _Citrus maxima_ albedo is significantly higher compared to the one of flavedo. The extraction method also influenced the amount of pectin recovered: for example, in Entry 5 [49] the yield of pectin on ananas peel was higher by 21.50% using microwave-assisted extraction (MAE) compared to conventional conditions (Table 2, Entry 1). The usage of different methods and technologies to valorise AFW as a source of pectin represents a promising path, since pectin has different agricultural applications, such as an antimicrobial agent [52] that will help in the biotic control of the soil and will promote growth of crops; used as a hydrogel to retain water, thus helping to maintain soil moisture; and it is also used as a biopolymer to cover fruits, increasing its shelf-life. Therefore, AFW represents a low-priced source of this polysaccharide. For instance, pectin can be applied directly in the agricultural field as a hydrogel agent for drought stress management. A common virulence factor of plant pathogenic bacteria and fungi is the ability to produce pectinases, so plant pathogenic microbes can degrade petiens [53]. However, it has been found that the addition of agricultural waste polysaccharides improved plant growth and increased plant resistance against soil-borne pathogens [54]; additionally, pectin was found to be beneficial to _Flavobacterium_, a common plant holbiont [55]. [PERSON] et al. [56], for example, fabricated a guar gum-pectin/polyacrylamideamide/ZnO super absorbent hydrogel with satisfactory water retention and adequate biodegradation in soil, and [PERSON] et al. [57] created pectin nanoparticles which showed good performance in the retention of soil moisture. Applying a cascade treatment to orange peel, [PERSON] et al. (Table 2, Entry 6) [50] were able to obtain pectin with high anhydrouronic acid content and hesperidin, a high added-value compound. [PERSON] et al. [51] (Table 2, Entry 7) observed that the combination of pulsed ultrasound assisted extraction (PUAE) and hot acid extraction (HAE) brought the highest yield on onion waste biomass compared to the two methods used separately.