**3.2 Water uptake analysis**

The hydrophilic nature of polysaccharides is a known characteristic that prevents some of these biopolymers' applications. Some strategies to reduce water uptake in films are cross-linking, blending, and mixing with other materials such as hydrophobic polymers [18], waxes [19], inorganic components [20], among others. In this study, cross-linking of the pectin matrix with calcium and iron ions was used to reduce the water uptake ability of film samples. **Table 2** shows WU results. A reduction in absorption capacity was observed in the presence of cross-linking concerning criolla orange pectin. Even when all values were significantly high, they depicted a weak cross-linking effect on water uptake. This result might be a consequence of the method used to cross-link the samples, which was ion diffusion from slightly concentrated solutions toward the matrix of the submerged film. Furthermore, this result might be evidence of superficial instead of full matrix crosslinking. It is well known that diffusion is a mass transport mechanism driven by concentration gradients and facilitated by temperature and stirring. The crosslinking procedure was performed under ambient temperature in stagnant conditions. Besides, Pec-Ca showed lower water absorption than Pec-Fe following its tighter egg-box cross-linking conformation mentioned in the previous section. On the other hand, Com-Pec showed complete solubility in water after a contact time of 24 h. This result might be explained considering the low methoxyl content of commercial pectin (galacturonic acid ≥74.0%, methoxy groups ≤6.7%) compared to criolla orange pectin [2]. Methoxy groups confer hydrophobic characteristics to the pectin backbone, which explains Pec's lower solubility in the water regarding Com-Pec.

## **3.3 Water vapor permeability**

Permeability depends on the solubility and diffusivity of water vapor molecules within the polymeric matrix [11]. When the polymer is hydrophilic, water molecules find many interacting sites and hopping by the polymer matrix through the formation of hydrogen bonds [21]. Hence, water vapor permeation is facilitated within hydrophilic polymers such as pectin. Despite this disadvantage, cross-linking reduces these interactions by blocking polar groups on pectin such as carboxylate and hydroxyl groups through interactions with divalent and trivalent ions such as Ca2+ and Fe3+. In order to study the effect of cross-linking on water vapor permeation, measurements were made gravimetrically using a modified ASTM Method E 96–95 (ASTM 96). Results are shown in **Figure 2**. A slight decrease in WVP of Pec-Ca concerning Pec was observed.

On the contrary, Pec-Fe showed an increase regarding orange pectin. This last result might be explained by an increased solubility of water vapor molecules through the polymer matrix favored by interactions with trivalent iron ions in the three folded chain conformation. Com-Pec showed the highest permeation value


**Table 2.** *Water uptake values.*

**Figure 2.** *Water vapor permeation of pectin films.*

according to the hopping mechanism of water vapor molecules proposed by Cruces et al. [21].

As mentioned before, Com-Pec has ≥74.0% of de-esterified carboxylic acid groups; hence the polymeric matrix is full of polar groups able to interact with water vapor molecules to hop through. WVP result of commercial pectin agreed with that reported by Cruces et al. (0.0361 ng�m�m�<sup>2</sup> �s �1 �Pa�<sup>1</sup> ) [21]. Other authors have studied water vapor permeation properties in polysaccharides and cross-linked polysaccharides. Values in the range from 1.5 to 0.6 ng�m�m�<sup>2</sup> �s �1 �Pa�<sup>1</sup> were reported for alginate-calcium cross-linked films [22], starch-based biopolymer with rye flour, cellulose, and citric acid as additives showed a WVP value of 0.87�ng�m�m�<sup>2</sup> �s �1 �Pa�<sup>1</sup> [23], xylan-alginate films containing bentonite, or halloysite clays showed a reduction in WVP from 0.394 for control film to 0.210 for 5 wt% for either clay [24]. Considering the reported WVP values for a variety of biopolymers, it is concluded that sensitivity to water vapor of hydrophilic polymers is still a matter of study.

#### **3.4 Gas permeation**

Gas permeation was measured in pectin films to analyze their ability to control gas exchange between internal and external sides of the packaging. According to our knowledge, O2 and CO2 are among the most important gases that take part in fruits and vegetable respiration. N2 is an inert gas representing about 78% of atmosphere content, and it might show preservation effects [25]. The three main gases used in modified atmosphere packaging (MAP) are N2, O2, and CO2. Decreasing the respiratory rate of fruits and vegetables in food packaging retards their deterioration. This effect occurs by reducing at least 5% of O2 permeability, heightening CO2 concentration, and regulating N2 exchange inside the packaging. Oxygen promotes several deteriorative reactions in food, such as fat oxidation, browning reactions, and pigment oxidation. Besides, oxygen is necessary for bacteria and fungi growth. Carbon dioxide dissolves readily in water, increasing the acidity of food surroundings which can cause pack collapse due to the reduction of headspace volume. Nitrogen does not support the growth of aerobic microbes, and it is used to balance the volume decrease caused by CO2 solubilization in water [26]. Gas permeation results and gas selectivity are shown in **Tables 3** and **4**, respectively.


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

#### **Table 3.**

*Gas permeation in pectin films.*


#### **Table 4.**

*Gas selectivity.*

Results shown in **Table 3** depict a modified atmosphere by pectin and crosslinked pectin films. Considering fruit or vegetable packed in these films, it would be possible to see that normal atmosphere content (78% N2, 21% O2, and 0.01% CO2) and its gas ratios have been modified. These results are better observed from **Table 4**, which we analyzed forwards. From **Table 3**, a reduction in N2 permeability can be observed in the case of Pec-Ca and Com-Pec regarding orange pectin. On the contrary, an increase in PN2 was observed for Pec-Fe. Oxygen permeability increased for Pec-Ca, demonstrating a detriment in its ability to reduce oxygen content inside the packaging.

On the other hand, PCO2 was lower in the case of Pec, Pec-Ca, and Pec-Fe than Com-Pec. The barrier to CO2 might represent a promising property for MAP. Differences in gas permeability between calcium and iron cross-linked films might be related to the polarity of ions concerning gases. Besides, the availability of ions within the less hydrated two-folded chains in the case of Ca2+ or more hydrated three-folded chains conformation in Fe3+ could also influence the interactions with permeate gases. These molecular conformations can also explain the increment in PCO2 for Pec-Fe. **Table 4** shows gas selectivity for selected gas pairs taking into account their abundance and gas ratio in the usual atmosphere. N2/O2 ratio in a usual atmosphere is around 3.71. From **Table 4**, it is observed that Pec-Fe has the closest value to that of the familiar atmosphere, while Pec and Pec-Ca have lower ratios and Com-Pec has the highest one. These results indicate that all films act as selective gas barriers favoring the permeance of N2 more than O2, except for Pec-Ca, in which N2/O2 selectivity is almost 1, i.e., no selectivity for N2 nor O2.

On the other hand, selectivity to O2 against CO2 was pronounced in the Pec-Ca film, followed by Pec and Pec-Fe. These results prove that an excellent barrier to CO2 is reached in cross-linked films. Furthermore, Com-Pec showed an opposite behavior concerning O2/CO2 selectivity being more permeable to CO2. Finally, N2/CO2 selectivity was excellent for Pec, and it was similar in the case of Pec-Ca and Pec-Fe. Values shown in **Table 4** indicate that N2 permeability can balance the volume decrease caused by CO2 solubilization in water as respiration and transpiration occur in fruits and vegetables. Commercial pectin showed less selective films for N2/CO2 gas pair according to its lower barrier to CO2.

According to Sandhya [26], there has been much commercial interest in developing films with high gas transmission rates. High gas transmission films are obtained by modifying the film manufacturing process so that gases such as O2, CO2, and water vapor exit or enter the package in a controlled manner such that aerobic respiration needs are met, and desirable CO2 and moisture levels are maintained. This work successfully controlled gas permeation and selectivity to obtain a modifying atmosphere inside packaging were achieved.
