**3. Integration of the process for pectin extraction under the biorefinery concept**

It is useful to study how different processes can be integrated with the existent pectin production process under the biorefinery concept to improve the integral sustainability of the valorization of citrus residues. This means that the sustainable use of citrus residues implies the maximization of possible products and energy obtained from this feedstock. For that, it is crucial to consider a logical order in which the different compounds are extracted or produced, as the presence of some of them can impact the quality of other compounds later in the process, which relates to the concept of biomass cascading applied to the biorefinery design process [29, 30]. Additionally, other reagents used along the steps should be carefully selected and studied as they may impact the desired product itself, cause environmental issues, or affect the economic viability of the whole process. Finally, the technical aspects of each step should always be considered to guarantee the quality and yield of the different products.

In this context, citrus residues constitute the primary raw material derived from biomass, and the different processes discussed earlier help to separate it and transform said reagents into chemical substances that can be used as final bioproducts. Nonetheless, there are opportunities to produce more value-added products by integrating the pectin production process with several configurations of other technologies, which are summarized in **Figure 8**. For example, Hilali et al. proposed an orange peel biorefinery that obtains essential oils and pectin but extracts additional value from the solar hydrodistillation process by retrieving partially solubilized polyphenols (flavanones) such as Narirutin and Hesperidin [12]. In another work, Budarin et al. proposed the use of microwave-assisted steam distillation (using only the water present in the peel) and microwave-assisted hydrothermal treatment to obtain essential oils, pectin but also hydroxymethylfurfural and 5-chloromethyl furfural (CMF) which can be used as platform chemicals to produce herbicides, insecticides, pharmaceuticals, monomers, solvents and fuels [31]. Ortiz-Sanchez et al. proposed the anaerobic digestion of the solid residue obtained after acid hydrolysis to produce biogas with a high methane content [9], and also the use of hydrolyzed pectin in a fermentation process with fungi (*T. reesei*) to produce mucic acid [23]. Hydrolysates from orange peel have also been evaluated for their potential to produce other organic acids, such as succinic acid, with the help of fermenting bacteria [15, 32]. Kyriakou et al. extracted more value from orange residues by including an enzymatic hydrolysis step to the solid residue left after pectin extraction to obtain sugars that can be fermented into ethanol and produce biogas from the solid residue from the enzymatic hydrolysis [33]. The fermentation of enzymatic citrus hydrolysates using cellulose-producing bacteria has also been reported [11]. Lohrasbi et al. proposed a variation of the process by first implementing the hydrolysis and then retrieving the essential oils using a flash separator; the solid

**Figure 8.** *Alternatives for the integration of the pectin production process under the biorefinery concept.*

residue is also used here to produce purified methane, and the digestate obtained from anaerobic digestion is further valorized into compost [34]. As a final option, it has been mentioned that residues from an orange waste biorefinery can be used directly as fertilizers [15].

The biorefinery concept can be associated with several relevant terms such as bioeconomy, circular economy, and industrial symbiosis. Many countries have started promoting policies and programs regarding the bioeconomy as a sustainable development strategy [7]. Circular economy and industrial symbiosis have also gained popularity among the policymakers and stakeholders of different companies. Generally speaking, these three concepts can be summarized as approaches that include the use of biomass-derived feedstocks obtained from various processes from different industries and that contribute to closing down the cycle of industrial processes by using one industry's residues as the feedstocks for another. Not only the value-added products are being produced, but a significant quantity of residues could be used as raw material, a material that would typically end up in a landfill with no further treatment. With this in mind, it is clear why incorporating the processes described above under the biorefinery concept results in a relevant field of study for the valorization of citrus residues and the sustainability of pectin production.

More studies must be performed to determine the feasibility of integrating the possible biorefinery configurations shown in **Figure 8**, the most convenient processing scale [4], and their sustainability. It would be interesting to include not only technical but also environmental, economic, and social aspects into the evaluation of the sustainability of biorefineries from citrus residues by performing an Early-Stage assessment, a methodology that allows the evaluation of multiple biorefinery pathways without the need for vast amounts of data [35–38]. However, the integrated biorefinery's isolated technical, economic, and environmental viability analysis is not enough. It is also essential to demonstrate the sustainability of those bio-based products to promote the deployment of a circular bio-based economy [39] because using residues as feedstocks does not necessarily mean that a process is sustainable. Additionally, in terms of industrial symbiosis, several strategic alliances could be built by selling some of the obtained added-value products to companies that use them as feedstocks. For example, essential oils and polyphenols are mainly used in cosmetics, toiletries, and fragrances due to their essence and benefits for the skin. Also, the market has seen a shift toward organic and natural products,

*Production of Pectin from Citrus Residues: Process Alternatives and Insights on Its Integration… DOI: http://dx.doi.org/10.5772/intechopen.100153*

increasing the popularity of essential oils both in pure form and as additives in skin care and hair products. Other products formulated using biorefinery products are jellies, jams, and frozen foods using pectin. In addition, pectin is widely used in the pharmaceutical industry to reduce blood cholesterol levels and treat gastrointestinal disorders [40]. Other applications include paper substitutes, foams, and plasticizers. Knowing this, the potential benefits of the biorefinery increase, as it would not only align with the current strategies for developing a greener industry, but other companies would also benefit from the possible sustainable-produced chemical substances, materials, and energy derived from the pectin production process.
