**2. Unit operations and process conditions for pectin extraction from citrus residues**

In the last few years, studies on developing new routes for utilizing organic citrus residues have mainly focused on pectin production. Pectin is primarily found as a component of the cell wall of plants that gives them resistance and flexibility due to its content of galacturonic acid, partially esterified with methyl ester or acetyl groups [8]. In general, the process begins by collecting citrus residues. The raw material is then washed, dried, and grinded before bioactive compound extraction. During the extraction of bio-compounds, essential oils, polyphenols, and flavonoids are removed to improve pectin's quality. After this step, pectin is retrieved from biomass by breaking down the polymer and "dissolving it" into the liquid phase. The solid phase residue contains other structural carbohydrates that could be further valorized. The liquid, rich in galacturonic acid units, is then submitted to a separation step ("precipitation"), where it is washed with alcohols or organic solvents that cause pectin to agglomerate. These solvents also eliminate remnant bioactive compounds that can alter the final pectin's organoleptic properties. Finally, solvents are evaporated from the jellified pectin to obtain the product of interest. **Figure 1** shows a diagram representing each one of the processing steps to obtain pectin.

#### **2.1 Preparation of the material**

As seen in **Figure 1**, the process begins by washing the material to eliminate excess dirt. After that, citrus residues are prepared for further processing by drying, *Production of Pectin from Citrus Residues: Process Alternatives and Insights on Its Integration… DOI: http://dx.doi.org/10.5772/intechopen.100153*

**Figure 1.**

*Process block diagram representing the unit operations to produce pectin from orange residues.*

which guarantees their storage for long periods. The material's drying process is usually carried out at temperatures around 40–60°C and drying times up to 2 days. However, the highest drying temperature reported is 95°C [9], which reduces the drying time but could cause the degradation of bioactive compounds. Also, it is desired to achieve low humidity (approximately 10%) as a way to extend the storage time of the raw material and to achieve a small particle size (< 1 mm) that generates a higher contact surface and a better performance during extraction [9].

#### **2.2 Extraction of essential oils and bioactive compounds**

It is important to remove certain bioactive compounds such as essential oils and flavonoids, besides some sugars interfering with the pectin's final quality. The purpose of removing these compounds is to improve pectin's esterification degree, galacturonic acid content and guarantee its physicochemical characteristics. At this stage, the principal compound of interest is the essential oil coming from the flavedo of the citrus peel. The essential oils from citrus fruits are conformed mostly by terpenes, which are organic substances responsible for the vegetal material's organoleptic properties. With terpenes removal, unpleasant flavors are avoided, which improves the quality of the final product [10].

Multiple methods such as vapor explosion, hydrodistillation, steam distillation, and in some cases solvent extraction can be implemented to perform essential oil extraction. The most common method used is steam distillation. In this method, the organic material is placed in a container where steam can pass and reach the sample uniformly. On the other hand, hydrodistillation works by placing the residue in direct contact with boiling water. The essential oils are retrieved once the water vapor rich in terpenes and terpenoids is condensed in both cases. Nonetheless, hydrodistillation can present agglomerations due to the direct contact of the submerged material with the liquid, which interferes with steam access to specific system zones. Another extraction method is Solid–Liquid Extraction, which can be done with various polar and non-polar solvents to retrieve the bioactive compounds selectively. However, SLE can also be assisted by heat, agitation, ultrasound, or microwaves, increasing the yields of the desired compounds.

In **Figures 2** and **3**, the yields of essential oils and the limonene content reported using different extraction methods for orange residues are shown in relationship with the pectin process. In **Figure 2**, the highest essential oil yields were obtained using Solid–Liquid Extraction with acetone (~2.2%) [1]. Nonetheless, the Solid– Liquid Extraction with acetone would require further separation of the polar and non-polar compounds due to the polarity of the solvent. For steam distillation, yields of 0.7% [11] and 0.84% [9] were obtained, which are slightly lower than those obtained by Hilali et al. with hydrodistillation and solar hydrodistillation ~1% [12]. Differences observed in yields for steam distillation could be attributed to the

**Figure 3.** *Content of limonene in the essential oils extracted.*

distribution of the sample in the system and how steam interacts with the residue. It is possible to increase steam distillation yields by increasing the pressure in the system (steam explosion) or performing double hydrodistillation [10, 13]. In the case of hydrodistillation, similar yields were obtained independently if the process is carried out with solar energy or not. As seen in **Figure 3**, the limonene content in the essential oils of orange residues is between 90% and 95% [9, 10, 12].

#### **2.3 Extraction of pectin**

Once essential oils and other bioactive compounds are removed, the extraction of pectin can be carried out. The first option is to use the liquid phase from hydrodistillation, rich in pectic substances released during heating in direct contact with water. Since pectin is heat-sensible and water-soluble, this option is attractive to perform both essential oils removal and pectin extraction. Hilali et al. reported a yield of ~12% for conventional hydrodistillation and ~ 8.3% for solar hydrodistillation [12]. Even though similar yields were obtained for essential oils using hydrodistillation, the way the heat is applied to the system may affect how much of the pectin is dissolved, resulting in lower yields. Similar behavior can be observed when pectin is retrieved from microwave-assisted hydrodistillation, with yields of around 15% [14].

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

The most common way to extract pectin from citrus residues is to employ acid hydrolysis, which consists of breaking down the bonds of pectin to obtain galacturonic acid units at high temperatures (from 80–116°C) and low pH values (1–3) with the help of dilute inorganic or organic acids. These hydrolysis reactions can also be assisted by agitation, which enhances the rate of depolymerization of pectin. **Figure 4** shows the best yields reported in recent literature for the pectin extraction process using different acids and processing conditions. From the inorganic acids in **Figure 4**, the highest yields were obtained using sulfuric acid (30.5%) [15], phosphoric acid (29.4%) [16], and hydrochloric acid with (~25%) [11]. It is important to note that the hydrolysis performed with sulfuric acid was completed at shorter times and higher temperatures (10 min and 116°C) [15] than the ones done with phosphoric acid (120 min, and 95°C) [16].

Moreover, the similar yields of pectin obtained from citrus residues using hydrochloric acid with different processing times [11, 17, 18] allow us to hypothesize that longer times could only cause a slight increase in the yield of pectin when temperatures are higher than 95°C at low pH values (1.6–1.8). On the contrary, lower temperatures (around 80°C) with hydrochloric acid reduce pectin yields. As seen in **Figure 4**, pectin yields decreased down to 16–20% [1, 19]. On the other hand, the hydrolysis of citrus residues using organic acids is mainly done with citric acid. The highest pectin yield reported using citric acid is 32.6% (160 min, at 90°C, and pH 2) [11], attributed to the long hydrolysis time. In **Figure 4**, it is possible to see that a short time of hydrolysis with citric acid results in lower yields. Once again, the use of temperatures around 80°C decreases pectin yields considerably, a behavior that was also observed when using inorganic acids. In the work of Rodsamran et al., microwave-assisted acid hydrolysis of lime residues was performed, with yields of ~16% and ~ 10% of pectin, for hydrochloric acid and citric acid, respectively [18];

#### **Figure 4.**

*Yield of pectin obtained from acid hydrolysis of citrus residues (Orange peel, \*lemon peel, \*\*lime peel) using sulfuric acid, phosphoric acid, hydrochloric acid, and citric acid.*

once again, the yields obtained with the inorganic acid resulted higher. The implementation of microwave-assisted hydrolysis has the benefit of implementing shorter process times (~5 min) but has the disadvantage of altering the final color of pectin, making it more brownish than the desired one for commercial pectin [18].

The reported data in **Figure 4** shows that the use of strong acids results in a better hydrolysis performance than organic acids due to their affinity for Ca2+ ions, which are responsible for stabilizing pectin chains [18]. However, it has been evidenced that the use of strong acids could be problematic since it causes the loss of some volatile compounds, environmental impacts such as the acidification of rain and water sources [20], and the degradation of valuable remnant substances that could have been further valorized due to their over hydrolysis. Conversely, the use of citric acid may cause lower environmental impacts than those resulting from the use of inorganic acids in the process. In addition, citric acid has been reported to cause less harsh depolymerization of pectin [18]. Also, it is easier to handle its traces during food formulations in comparison to inorganic acids.

#### **2.4 Purification of pectin**

The liquid phase that results from the hydrolysis, rich in galacturonic acid, is then retrieved and mixed with alcohols such as ethanol, methanol, 1-propanol, or its isomer isopropanol to separate pectin due to its insolubility in this type of solvents [21]. Most of the authors highlight the use of ethanol, acidified ethanol, or acetone to precipitate citrus pectin. Precipitation of pectin with ethanol is mainly done at 20–25°C, leaving the samples overnight (18 - 24 h) [17, 18, 22]. Depending on the degree of purification desired, different concentrations of ethanol can be used. At least one wash with ethanol at 96% (v/v) is made after pectin extraction. What is more, there are some cases in which the sample is washed three times or more with ethanol at different concentrations (50%, 70%, and 96%), not only to separate pectin but also to remove sugars, polyphenols, and essential oils that remain [1, 8–10, 16, 17, 22, 23]. The removal of these undesired substances helps to obtain pectin in its whitened form. In addition, ethanol could be ideal since it avoids the precipitation of other non-desired compounds [24] and can absorb water from the pectin. Ethanol could also be beneficial for the process since it can be further recovered and reused.

Moreover, since pectin requires acidic conditions for its precipitation, it is necessary to use acidified ethanol (0.5% HCl) when pectin is obtained from hot water extraction [10], as happens when doing hydrodistillation. It is also possible to remove other remnant substances from pectin and increase the organoleptic characteristic of the final product by using a final wash with acetone. For example, Rodsamran et al. used three ethanol washes and a final acetone wash to guarantee almost a complete removal of bioactive compounds and increase the purity of pectin [18].

At this point, some authors report the use of centrifugation to facilitate the separation of pectin from the solvents once they had made effect. Centrifugation has been carried out at low temperatures (4–10°C) using speeds from 4000 rpm to 9000 rpm in a time range of 10 to 20 min [9, 11–13, 22, 23]. After pectin is fully separated, it can be dried at low temperatures that guarantee the thermal stability of the polymer. It is possible to used use vacuum drying at 40°C for short periods of time (1-2 h) [1, 11, 16, 19] or convection drying at 50–55°C for 16 to 24 h [8, 9, 12, 15, 17, 18, 22, 23, 25]. It is important to highlight that pectin yields are primarily affected by other process stages, not by the drying step. However, to guarantee pectin's quality, it is recommended to avoid the exposure of the material to high temperatures for long periods.

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

#### **2.5 Quality parameters of the final product**

To evaluate the final quality of the obtained pectin after purification, the galacturonic acid content and the degree of esterification are the two main characteristics that should always be considered. The galacturonic acid content reveals how much of the retrieved sample contains the primary units to form the polymer. The degree of esterification describes how many carboxyl groups of the galacturonic acid in pectin are esterified with methanol which influences the gelling capacity of pectin. Consequently, both properties help to define the most suitable applications for the extracted pectin.

As can be seen in **Figures 5** and **6**, the highest content of galacturonic acid (~90%) and esterification degree (71–85.6%) was reported by Rodsamran et al. using hydrochloric acid and citric acid in the hydrolysis of lime peels [18]. The standalone result for the esterification degree of orange pectin obtained with phosphoric acid is also high (83.6%) [16] and suggests the necessity of further investigation of the use of this acid in the process. In orange peels, even though broad ranges of galacturonic acid content (50–75%) were reported for hydrochloric acid and citric acid, the esterification degree reported maintained a value around 65–70%. The low galacturonic acid content reported in some cases could be attributed to how the sample was washed to remove remnant phytochemicals and sugars and to the prolonged effect of temperature at low pH values. The decrease in the pH at high temperatures over long periods causes an increment in the degree of dissociation of the carboxylic acid groups [24], leading to the degradation of pectin into substances of lower molecular weight, which ethanol cannot precipitate [26].

It is possible to infer that orange pectin would have similar gelling properties no matter if it were obtained using either citric acid or hydrochloric acid at different process conditions. Since the galacturonic acid content reported in **Figure 5** is always higher than 50% and the esterification degree higher than 65%, it is possible to say that the obtained citrus pectin can be considered as high-methoxyl pectin [27, 28]. This kind of pectin forms its structure based on hydrogen bonds between hydroxyl groups, where sugars, thanks to their highly hydrophilic effect, allow the bonding between polymer chains. High-methoxyl pectin can achieve jellification in few minutes at temperatures around 95°C, suggesting the possibility of using citrus pectin in various food products. On the contrary, low-methoxyl pectin requires

#### **Figure 5.**

*Galacturonic acid content of pectin obtained from acid hydrolysis of citrus residues (Orange peel and \*lime peel) using hydrochloric acid and citric acid.*

#### **Figure 6.**

*Degree of esterification of pectin obtained from acid hydrolysis of citrus residues (Orange peel and \*lime peel) using phosphoric acid, hydrochloric acid, and citric acid.*

metallic cations (Ca2 + or Mg2+) that bond between themselves and the anionic structure of pectin to form gels due to its low degree of esterification [14].

#### **2.6 Process conditions that enhance pectin quality and recovery**

**Figure 7** shows a process diagram that suggests the most appropriate process conditions to obtain citrus pectin. In the first place, the raw material must be adequately dried to assure its preservation and milled to increase the contact surface which yields during essential oils extraction and hydrolysis. Secondly, steam distillation is preferable for essential oils extraction since it would selectively retrieve these valuable substances without affecting the material. Contrary to this, during hydrodistillation, the material is in direct contact with hot water, which causes its partial hydrolysis and the degradation of pectic substances, resulting in lower pectin yields; additionally, the use of hydrodistillation would require the acidification of ethanol during precipitation. Thirdly, the acid hydrolysis of pectin can be carried out either with hydrochloric acid or citric acid since the final pectin would always have high-methoxyl properties. Nonetheless, process conditions that tend to increase yields and galacturonic acid percentage should be employed. It is necessary

#### **Figure 7.**

*The production process of citrus pectin and suggested operational conditions.*

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

to perform a careful separation and purification during the final steps to assure high yields and purity of pectin. The last stage of pectin production will always require ethanol at 96% (v/v) for its precipitation and several washes with ethanol and acetone that remove sugars and bioactive compounds. After that, centrifugation is used to assure proper separation from the solvent (that can be later evaporated and reused) and vacuum drying to avoid the degradation of the final product. It is important to highlight that it is possible to obtain additional valuable products from the bioactive compounds extracted through steam distillation and the solids retrieved after hydrolysis rich in lignocellulose.
