Effect of Cross-Linking Agent on Mechanical and Permeation Properties of Criolla Orange Pectin

*María Guadalupe García and Martin Alberto Masuelli*

### **Abstract**

Pectin from orange peel was extracted and cross-linked, applying different cross-linking agents to visualize any effect on its mechanical and permeation properties. Calcium chloride (II) and iron chloride (III) were the cross-linking agents. Besides, commercial pectin was also used to compare its properties with neat orange pectin. Tensile testing showed mechanical stiffness of the orange pectin matrix in the presence of cross-linking agents. Calcium ions better cross-linked the polymer matrix as shown by their highest tensile strength and elastic modulus, with moderate elongation at break. Iron ions showed a weaker cross-linking effect on the pectin matrix, improving the elastic modulus but retaining almost the same tension strength. Lower elongation at break concerning neat orange pectin was observed for cross-linked samples. Water uptake (WU) and water vapor permeation (WVP) of cross-linked samples had lower values than those of neat orange pectin. However, these results are still high compared with synthetic polymers. Finally, gas permeation assays were performed using N2, O2 and CO2 gases, according to exchangeable gases in fresh fruits and vegetable packaging. Results showed a conveniently modified atmosphere effect by avoiding CO2 permeation and stabilizing N2 and O2 selectivity.

**Keywords:** polysaccharides, pectins, crosslinking agent, mechanical, permeation properties

#### **1. Introduction**

Pectin is a structural heteropolysaccharide present in the primary cell walls of terrestrial plants. It can be obtained from renewable agriculture by-products and food processing industry wastes. These natural sources of pectin make it one of the most abundant biopolymers. It consists of D-galacturonic acid residues which possess carboxylic acid groups, some of which are methyl-esterified. The degree of esterification determines the solubility of pectin and its gelling and film-forming properties. Depending on the origin of pectin, the degree of esterification can vary from high methyl (HMP, up to 50 wt. % of carboxylic acid units are esterified) to low methyl (LMP, lower than 50 wt. % of carboxylic acid units are esterified) [1]. Pectin extracted from criolla orange (*Citrus sinensis*) was first studied and characterized by Masuelli et al. [2]. These authors reviewed the extraction methods and characterization techniques of pectin in solution or film configurations. The intrinsic viscosity of orange pectin from dilute solutions was evaluated to estimate the extracted polymer's molecular weight. Results obtained ranged from 56 to 93 kDa

for basic and acid hydrolysis processes, respectively. Thermal analysis (DSC and TGA) and FTIR spectroscopy were carried out for structure characterization of pectin films. Results showed the presence of *Tg* (glassy) and *Tm* (melting) temperatures for pectin, which vary accordingly with the extraction method in the range from 57 to 69°C and 101 to 128°C, respectively. Thermograms (TGA) depicted temperature resistance up to 150°C for all types of extracted pectins. From FTIR analysis, carboxylic acid groups and esterified methyl groups were observed as signals at 1750 cm<sup>1</sup> and 1650 cm<sup>1</sup> , respectively. Morphological analysis was also carried out by taking SEM images of the film surface. All pectin films showed a homogeneous dense surface without defects. Mechanical, water absorption, and water vapor permeation tests were also performed. All pectin films showed too little resistance to tension and elongation at break compared with commercial pectin. Water absorption was a moderate but fast process, reaching 25% of absorbed water at 60 min of initiating the test. Water vapor permeation was as high as 2.62– 5.25 g<sup>m</sup><sup>2</sup> day<sup>1</sup> . Other authors [3], explored the interplay of the degree of methyl esterification (DM), pH, temperature, and concentration on the macromolecular interactions of pectin in solution. They found two levels of organizing pectin structures in solution: (i) chain clusters with a radius of gyration ranging between 100 and 200 nm and (ii) single biopolymer chains with a radius of gyration between ≈6 and 42 nm. Besides, they found that chain flexibility increases with DM and acidic pH, whereas hydrogen bonding is the responsible thermodynamic driving force for cluster formation. High methyl pectin creates structures with less efficient packing.

On the contrary, low methyl pectin at pH 7 or higher can turn into more coiled chain conformations in the presence of counterions. Furthermore, the addition of salt to pectin solutions allows the formation of complexes between positively charged ions and negatively charged carboxylic acids, which is facilitated at pH 7 or higher because of deprotonation of carboxylic acid groups. These observations explain the cross-linking effect of positively charged ions on pectin structure and gel formation at higher pH and ion concentration [4].

On the other hand, it is well known that pectin films are obtained from aqueous solutions after slow solvent evaporation [1]. High molecular weight and low pH are required to facilitate the formation of coil entanglements responsible for film formation. The chain entanglements are supported by H-bonding interactions that give strength and physical resistance to the film. Kontogiorgos et al. [5] found that the strength of interactions and conformational changes on pectin during the transition from a liquid to a glassy state are the main factors influencing the physical properties of the solid-state system. However, in contact with aqueous environments, pectin films can absorb water, first swelling the polymer matrix and then dissolving it. Several authors have probed different methods of preparing water-resistant films. Cruces et al. [6] prepared multilayer films of pectin-beeswax/colophony-pectin varying the ratio between beeswax and colophony. This method reached water vapor permeation values (56 <sup>10</sup><sup>13</sup> gmm<sup>2</sup> <sup>s</sup> <sup>1</sup> Pa<sup>1</sup> ) almost ten times higher than the WVP value of polyethylene films (LDPE 5.8 <sup>10</sup><sup>13</sup> gmm<sup>2</sup> <sup>s</sup> <sup>1</sup> Pa<sup>1</sup> ). Gharsallaoui et al. [7] prepared composite films of pectin/sodium caseinate to improve the mechanical and water barrier properties of protein-free pectin. These authors found that pectin and protein are negatively charged at neutral pH (pH higher than the isoelectric point of a protein), which favors the formation of macroscopic segregated phases. However, even at high turbidity conditions, which demonstrated phase segregation, some positively charged residues on protein might interact with negatively charged groups on pectin, improving the mechanical and water barrier properties. Other authors have prepared insoluble films by cross-linking the pectin matrix using divalent or trivalent cations [4, 8]. Besides, there exist methods of cross-linking a polymer matrix by reacting it with bifunctional molecules such as glutaraldehyde to

#### *Effect of Cross-Linking Agent on Mechanical and Permeation Properties of Criolla Orange… DOI: http://dx.doi.org/10.5772/intechopen.102976*

perform covalent cross-linking [9]. Usually, cross-linking of the polymer matrix causes chain stiffness and, consequently, detriment of mechanical properties.

Nevertheless, cross-linking might improve solvent resistance, water vapor, and gas barriers. So, it is interesting to study the proper film formation conditions in the presence of a cross-linking agent to overcome the challenge of obtaining a robust film, easy to manipulate with improved mechanical and barrier properties. In this work, pectin from criolla orange (*C. sinensis*) was cross-linked with divalent calcium ions and trivalent iron ions to improve mechanical and permeation properties. Pectin solutions at pH3.2 were used to prepare films by the "casting" method. Calcium and iron salts were separately contacted with pectin films by submerging them into ions solutions at 40°C for 24 h. The influence of positively charged ions on film properties was analyzed by uniaxial traction, water uptake, water vapor barrier, and gas permeation experiments. Conclusions about structure-properties relationships were obtained.
