of chemical substances, bacteria strains are employed for deproteinization and demineralization. For instance, Bacillus subtilis, Pseudomonas aeruginosa, and Serratia marcescens, bacterial strains that produce protease, have been tested for the deproteinization treatment and Lactobacillus plantarum has been analyzed for the demineralization process to remove calcium from chitin [106]. Moreover, for the chitin transformation into chitosan, Rhizopus japonicus, a bacteria strain that processes a high intracellular and extracellular deacetylation mechanism, has been used as a biological alternative [107]. It is worth mentioning that no depigmentation process using bacterial strains has been found.
Although chemical treatments are cheap procedures, appropriate for mass production [108], high concentrations and volumes of acid and alkali solutions are used at high temperatures during chemical treatments and, thus, biological treatments could be more suitable processes from the environmental perspective. Nevertheless, they have longer production times, are more expensive, and have not been implemented for mass production; only pilot scale processes have been reported [107].
A combination of both chemical and biological methods can also be employed. For instance, Pachapur et al. [109] proposed a combined enzymatic deproteinization (with Bacillus licheniformis) followed by a chemical demineralization process, using seawater during all steps of chitin extraction. Thereby, using large amounts of freshwater and chemicals can be avoided. Furthermore, Lopes et al. [110] compared the chitosan extraction by chemical and biological processes at pilot scale and their environmental impact. The use of a soft alkaline treatment, and the possibility of recovery NaOH, water, and fish protein hydrolysates are some of the reported improvements to decrease the environmental impact associated with the chemical treatments. Additionally, some improvements were proposed for the biological processes in order to mitigate the environmental impacts and the costs associated with the enzyme production; in particular, employing crustacean biomass as peptones for bacterial growth could be a strategy to reduce environmental loads.
Recently, a novel method for chitosan extraction has been proposed [111], using microwave technique for all chitosan extraction steps (demineralization, deproteinization and deacetylation). Although the method uses the same preparation conditions as chemical treatments, it needs less time than the conventional one. Indeed, this novel method reduced the deacetylation time from more than 6 hours to less than 30 minutes, reaching the same deacetylation degree. Furthermore, chitosan structure, morphology, and chemical composition in both methods were similar.
2.3.2 Preparation and Characterization of Chitosan Films and Coatings
Chitosan‐based materials can be prepared employing two different processes known as wet or dry techniques. The wet method is the most most employed process when using this polysaccharide because it is a versatile technique [112]. During this process, chitosan is dissolved in acidified water since it is insoluble in basic media [113, 114]. After heating and stirring the solutions, films can be obtained by solution casting and drying, or food coatings can be prepared by spraying or dipping [115, 116]. On the other hand, few works related to dry process to prepare chitosan films have been reported in the literature. Galvis‐Sánchez et al. [117] prepared chitosan films by thermal compression. First, chitosan powder was mixed with natural deep eutectic solvents, such as citric acid or lactic acid. The mixture was introduced in an oven at 80 °C for 30 minutes and then an acetic acid solution was added during manual mixing. The resultant mixture was introduced in a hydraulic press and thermo‐compressed. In another work, Guerrero et al. [118] thermo‐compressed citric acid–chitosan films at 125 °C for two minutes, using glycerol as plasticizer. Chitosan has also been compressed after blending with other polysaccharides such as starch. Valencia‐Sullca et al. [119] dispersed chitosan, starch, glycerol, and polyethylene glycol in water and the mixture was melt blended at 160 °C for 30 minutes until a homogeneous paste was obtained before processing by thermal compression.
After preparing chitosan films and coatings, material characterization must be carried out in order to know their suitability for the specific application of food packaging. Mechanical properties are one of the most important parameters of the characterization. Mechanical behavior determines whether the film or coating is suitable for the packaging purpose since it has to support mechanical loads during the logistic process to keep the product intact and avoid food deterioration [120]. Unmodified chitosan films are brittle due to the electrostatic interactions and hydrogen bonding between chitosan chains. Hence, plasticizers are used to lend chitosan better mechanical properties, to provide chitosan chains with mobility, and, thus, increase flexibility. Among plasticizers, glycerol is the most employed in chitosan films [102].
In relation to water‐related properties, these are of interest to define which kind of product can be packaged. Since chitosan films and coatings present moderate values of water resistance due to the presence of hydrophilic groups (–OH and –NH2) [113], chitosan properties are modified employing different additives. Wang et al. [121] studied chitosan films enriched with anthocyanins at different concentrations (5%, 10% and 15% by weight based on the chitosan dry basis). Regarding moisture content, this value decreased with the increase of anthocyanin concentration due to the intermolecular interactions by hydrogen bonding between anthocyanins and the hydrophilic groups of chitosan. Concerning water vapor permeability, anthocyanin‐added chitosan films exhibited lower values due to the more compact structure formed as a consequence of the interactions between anthocyanin and chitosan.
2.3.3 Food Shelf Life Extension Using Chitosan Films and Coatings
Chitosan films and coatings have been extensively assessed for non‐processed food, such as fruits, vegetables, refrigerated fish, and meat as well as for processed food such as sausages or bread. Depending on the product, different coating techniques are used: dipping, spraying, or wrapping. Leceta et al. [122] compared spraying and dipping on ready‐to‐eat baby carrots and found that both chitosan coatings were effective in maintaining the product safe against microbiological spoilage during a storage period of 15 days. A slightly better antimicrobial activity was shown for dipped samples, whereas moderately better results for weight loss and texture were presented in sprayed samples. Pea pods have also been coated by chitosan solutions, reducing the vegetable weight loss, titratable acidity, and chilling injury compared to the untreated product [123].
In order to provide the packaging with active properties that prolong the shelf life of the packed product, additives are incorporated into chitosan solutions. Some of these studies are summarized in Table 2.2. Lekjing [130] employed clove oil, an essential oil with antimicrobial and antioxidant properties, to coat pork sausages by dipping and microbiological, physical, and chemical analyses as well as sensory evaluation were carried out. It was demonstrated that the coating inhibited the microbial growth, retarded the lipid oxidation, and extended the shelf life of the product during refrigerated storage. However, negative impacts were found on odor and taste qualities. Other essential oils, such as that extracted from oregano, have also been incorporated into chitosan solutions to provide coatings with active properties. Paparella et al. [131] added oregano essential oil into chitosan solutions to coat fresh pork fillets and found that oregano had antimicrobial properties against meat spoilage bacteria, such as Brochothrix thermosphacta and Pseudomonas spp. In another study, Alsaggaf et al. [127] analyzed the effect of edible chitosan coatings applied by immersion of Nile tilapia fillets into chitosan solutions with pomegranate peel extract, an extract that has several bioactive phytochemicals for antimicrobial or food preservative applications. Composition, microbiological, antioxidant, total nitrogen volatile base value, and sensory analyses demonstrated that chitosan/pomegranate peel extract coatings were effective at extending the shelf life of fish fillets.
Table 2.2 Chitosan‐based active films and coatings.
| Food packaged | Packaging |
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