Chitosan is a linear polyamine copolymer of β-(1-4)-D-glucosamine and acetyl β-(1-4)-D-glucosamine, obtained by alkaline N-deacetylation of chitin (Park et al., 2002). In chitin, the degree of acetylation (DA) is typically 0.90, while chitosan has a typical DA of less than 0.35. Chitosan is produced mainly from an abundant waste of shellfish industry (Park et al. , 2002) and was generally recognized as safe when consumed at levels up to 3.37 grams per person per day in various foods (FDA, 2013). Chitosan, held on positive charges of the molecule, is generally recognised as safe, is an excellent material to form coatings and films which have a selective permeability to gases. Besides, chitosan exhibits antimicrobial activity against a broad range of fungal and bacterial plant pathogens mostly due to the electrostatic interaction between the positive charges of the protonated amino groups of chitosan and the negatively charged molecules at the cell surface of fungi or bacteria. Therefore, chitosan has been employed as an edible coating to extend the shelf life and maintain the nutritional quality of various fruits and vegetables and inhibit the growth of pathogenic fungi (Verlee et al., 2017; Chaudhary et al., 2020). However, chitosan coating has a poor moisture barrier property due to strongly hydrophilic characteristic (Dutta et al., 2009).
In attempt to improve a chitosan based-composite coating, ĸ-carrageenan was brought as a component. Carrageenans are water-soluble polysaccharides extracted from red seaweeds. Carrageenan structure consists of alternating copolymers of α-(1-3)-D-galactose and β-(1-4)-3,6-anhydro-D- or L-galactose (Park et al., 2001). Carrageenan is classified into six basic types, including kappa (κ)-, iota (ꚍ)-, lambda (λ)-, mu (µ)-, nu (ν)- and theta (θ). These different carrageenan types are obtained from various weed sources. They differ in number and position of ester sulphate groups on the repeating galactose units as well as the content of 3,6-anhydro-galactose (Li et al., 2014). Kappa (κ)-, iota (ꚍ)-, lambda (λ) are three important commercial types of carrageenan. Their dimers have one, two, and three sulphate ester groups, respectively (Figure 2.3). Therefore, ꚍ-carrageenan has two negative charges, and λ-carrageenan carries the average 2.7 charges, while κ-carrageenan has only one negative charge per disaccharide. The κ- and ꚍ- carrageenan contain 3,6-anhydrobridges, while lambda carrageenan contains no 3,6-anhydrobridges (Park et al. , 2001; Necas & Bartosikova, 2013; Li et al. , 2014).
The blending of chitosan and κ-carrageenan has been investigated to form a composite film based on the interaction between oppositely charged polysaccharides. Ascorbic acid could be used as a cross-linking agent between carrageenan and chitosan, and it can affect the water vapour barrier properties and mechanical properties of the composite film. Park et al. (2001) reported that ascorbic acid increased the tensile strength and reduced WVP of both chitosan and κ-carrageenan separately. However, the effect of ascorbic acid on the mechanical and barrier properties of chitosan and κ-carrageenan composite coating depends on the organic acid solvent. The weakest acid, acetic acid, did not interfere in the role of ascorbic acid due to the small molecular size and low melting point. Shahbazi et al. (2016) investigated the degradation of κ-carrageenan under thermal treatment and its influence on chitosan and κ-carrageenan blending film properties. The study showed that blending with κ-carrageenan reduced the WVP of chitosan films. Additionally, chitosan blended with thermally treated κ-carrageenan had a lower WVP compared to both the chitosan and the intact blend film.
Multilayer coating from κ-carrageenan and chitosan also has been investigated. The κ-carrageenan/chitosan nanolayered coating was characterized by (Pinheiro et al., 2012a). The nanolayers had a lower WVP compared to that of conventional chitosan-based films (WVP = (8.60 ± 0.14) × 10−11 g m−1 s−1 Pa−1, ± 50 μm of thickness). It was explained that the interactions between κ-carrageenan and chitosan layers led to an increase of the tortuosity of the material, which in turn decreased the permeability to the water molecules. The κ-carrageenan/chitosan nanolayered coating also exhibited an oxygen permeability lower compared to chitosan or ι-carrageenan film. Moreover, Pinheiro et al. (2012b) reported that the bioactive compound could be incorporated into the κ-carrageenan/chitosan nanolayered coating due to the electrostatic interaction between bioactive compound with anionic groups of κ-carrageenan not forming a bond with cationic groups of chitosan. The bioactive compound can be transported and released from the κ-carrageenan/chitosan nanolayered coating, which is beneficial for the development of bioactive compounds release systems for application in food technology, as a strategy for shelf life extension.
Due to the improved characteristic of chitosan and kappa carrageenan-based composite, they are potential to be applied for food as well as fresh produces as coating layer or packaging film. The application of the κ-carrageenan-chitosan-based composite coating on meat has been reported. Olaimat & Holley (2015) used the edible coating containing 2% chitosan and 0.2% κ-carrageenan to control Salmonella on fresh chicken breast. The κ-carrageenan-chitosan-based coating significantly reduced numbers of Salmonella on fresh chicken breast compared to uncoated pieces. Additionally, the growth of lactic acid and aerobic bacteria were significantly prevented by this coating solution. Edible 0.2% κ-carrageenan/2% chitosan-based coating containing allyl isothiocyanate or deodorized oriental mustard extract also showed the excellent potential to control Campylobacter jejuni on fresh chicken breast (Olaimat et al., 2014). However, the study on κ-carrageenan-chitosan-based composite coating for fruits and vegetables has been limited, which could be further interested to maintain the quality of fruits and vegetables during storage.
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