Influence of Particle Size on the Bioadsorbent Behavior of Orange Peel

Currently, society is progressively committing itself to achieving a sustainable economy based on the exploitation of renewable resources by promoting waste recycling and the design of products obtained in ecologically efficient processes. From this point of view, biomass, mainly of plant origin, seems the best option to replace a high percentage of fossil fuels in their energy and chemical applications (Cherubini 2010).

However, according to the Food and Agriculture Organization of the United Nations (FAO), around 30% of global food production is lost during harvest, processing and final consumption, assuming this amount more than 1,300 million tons per year (Thi Phuong 2015).

Agri-food waste is potential raw material for various applications, which is why it is intended to develop high-value products such as cosmetics, fuels, medicines, essential oils, pectin, feed, activated carbon, pollutant adsorbents, fuels, energy, among others (Ortiz et al., 2020; Deba-Rementeria et al., 2021;). These wastes include fruit waste that is generally disposed of in landfills, composted, used as feed, incinerated or solidified; processes that may involve a high demand for energy or generate damage to the environment, which is why it is essential to reduce them and use them as an alternative source of renewable energy (Lin et al., 2021).

Of the wide variety of fruits and vegetables, whose processing generates waste suitable to be used as raw material, it is worth highlighting citrus fruits that generate large amounts of waste in the form of skin, pulp and seeds in the production process of juice and other derived foods (Mirabella et al. 2014).

Citrus fruits have various applications in the food, cosmetic and perfume industry thanks to their taste and aroma (Nateghpouret al., 2021). In the food industry approximately 26% of citrus fruits are used to make juice. During the processing of citrus fruits, the peels are the main by-product and a potential burden on the environment without additional treatment (Wang et al., 2015). Between 50-60% of the mass of the fruit remains after processing (peel, seeds and membrane residues), it is estimated that, annually, citrus waste created by food processing industries exceeds 54 million tons allover the world (Teigiserova et al., 2021). Among the variety of edible citrus fruits, it should be noted that the most abundant in the world is the orange (Citrus sinensis L.), which represents about 60% of the world total (Erukainure et al, 2016). Part of the waste generated is used to feed livestock. However, it is in such a quantity that large amounts end up being deposited in landfills causing serious environmental and economic problems (Tripodo et al., 2004).

The waste from the manufacture of orange juice is made up of pulp, seeds and skin. The skin is made up of an orange outer layer (flavedo) and a fluffy white inner layer (albedo). The pulp is very moist and rich in monosaccharides (glucose and fructose) and disaccharides (sucrose). The inner layer of albedo is rich in pectin, while the outer layer of flavedo contains a large amount of essential oils, limonene being the main component, and flavonoids (Davies1994). Its composition varies depending on the crop, the time of year and the region and technology used in the production of juice. Despite these possible variations, the orange residue is always very humid, with a variable water content between 80 and 84% (Rezzadori et al,. 2012).

Depending on the particle size, the orange peel has different water adsorption capacity. Table 1 shows the results obtained with 5g samples of orange peel of different particle size and in Figure 1 the appearance of the orange peel after being in contact with water.

TABLE 1 WATER ADSORPTION IN 5 GRAM ORANGE PEEL SAMPLES OF DIFFERENT PARTICLE SIZES AT ROOM TEMPERATURE Orange Particle size mL H 0 Medium mL / g 2 1mm> X> 500 µm 37.8 ± 0.35 7.6 500 µm> X> 250 µm 28.2 ± 0.5 5.6 X <250 µm 23.1 ± 0.8 4.6 It can be seen that the adsorption decreased as the particle size decreased. This behavior was attributed to the fact that the possibility of caking increases when there is more contact between the particles (Pietsch 2002). The powdery particles become wet, sticky and compact, finally reaching the liquefaction phase (figure 1).

On the other hand, from the extraction of albedo, citric pectin, a thickener commonly used in the food industry, is obtained by acid hydrolyzing. However, inmost processes, obtaining pectins is linked to obtaining essential oil (Cerón-Salazar 2011). FIGURE 1: Appearance of the orange peel vs particle size a) dry b) saturated with water (Garcia Raurich et al, 2020a)

The industrial production of citrus pectin has the following stages: in the first stage, the peel must be washed to remove as much soluble solids and impurities as these components make the purification process difficult. Then, the shells are subjected to a drying process, which inactivates the pectin esterase enzyme and lowers the moisture content, increasing the stabilization of the shell for storage and reducing the cost of transportation (Marti et al, 2014).

Subsequently, the dry matter is suspended in hot water with the necessary amount of a strong acid, starting the hydrolysis process. During this process, starting from the macromolecular structure formed by cellulose, hemicellulose and pectins, hemicellulose starts its degradation to glucose, galactose and fructose; cellulose to glucose and pectin to pectin monomer through a depolymerisation process (Chen et al, 2015). After a while, the resulting solution is removed from the insoluble solids by filtration. Next, it is mixed with alcohol, producing the recomposition of the pectin polymer and its corresponding precipitation. The precipitate is removed and purified by washing it with more alcohol. Finally, it is dried and ground (Claus, 2002). The material resulting from the pectin extraction is a poor food supplement for animals due to its low protein content and high sugar content (Siles et al, 2016).

On the other hand, the solid fraction presents the optimal conditions for its subsequent treatment as a bioadsorbent (Masmoudi et al, 2008). The resulting solid is treated in an alkaline medium. In this way, the saponification of the non-soluble pectin in an acid medium is achieved, as well as the solubilization of the soluble fraction of hemicellulose in an alkaline medium (Grace et al., 1996).

After chemical treatment, the resulting product has the characteristics of a cation exchanger. The optimal size for the removal of heavy metals in continuous processes has been established between 500-1000 µm (Garcia Raurich et al, 2020b). The objective of this work has been to reuse the fraction less than 200 µm obtained in the grinding of the orange peel. For this, various modifications in the chemical treatment have been studied.