Food packaging films play a crucial role in protecting food from physical, chemical and biological hazards. However, the packaging films currently used in the markets are mostly made of petroleum-based polymers such as polyethylene terephthalate (PET), polyvinylchloride (PVC), polyethylene (PE) and polypropylene (PP) [ 1 ]. Although these traditional plastic packaging films are popular due to their low cost and convenient operation, the accompanying shortcomings such as non-degradability have caused significant concerns [ 2 ]. Moreover, plastic packaging films not only release harmful substances under heating conditions, but also enter the food chain in the form of microplastics and thus endanger human health. Therefore, it is necessary to use safe and biodegradable materials to prepare food packaging films instead of chemically synthesized films [ 3 ]. Generally, transparency, barrier properties and mechanical strength are essential parameters in the development of novel food packaging films.
To date, materials obtained from natural renewable sources such as proteins, lipids and polysaccharides have been used to develop biodegradable food packaging films as single or composite film formation. Among these biopolymers, carrageenan is a water-soluble polysaccharide extracted from the cell walls of various marine red algae. It is a linear sulfated polysaccharide composed of alternating units of D-galactose and 3,6-anhydrogalactose connected by α-1,3- and β-1,4-glycosidic linkages. According to the number and position of sulfate groups, carrageenan is mainly classified as κ-carrageenan, ι-carrageenan and λ-carrageenan [ 4 ]. Compared with ι-carrageenan and λ-carrageenan, the high gelling capacity of κ-carrageenan makes it an excellent film-forming material. However, the single κ-carrageenan film possesses high hydrophilicity and high brittleness, which greatly limits its application in food packaging [ 5 ]. Currently, the incorporation of plasticizers [ 6 ] and blending with other polysaccharides [ 7 ], proteins [ 8 ] or lipids [ 9 ] have been used to improve the properties of κ-carrageenan films, but the effects have not been obvious.
3) and hydroxypropyl groups (-OCH2CH(OH)-CH3) substituted for hydroxyl groups on the branch. Huang et al. [Hydroxypropyl Methylcellulose (HPMC), as a cellulose derivative, is widely used in medicine and food owing to its excellent biocompatibility, degradability and film-forming capabilities. It is a linear polysaccharide of β-(1→4)-linked D-glucopyranosyl units, with methyl (-OCH) and hydroxypropyl groups (-OCHCH(OH)-CH) substituted for hydroxyl groups on the branch. Huang et al. [ 10 ] found that polyvinyl alcohol films exhibited better mechanical and hydrophobic properties with the addition of HPMC. Brindle et al. [ 11 ] presented that the strength and stiffness of whey protein film were increased with the increased concentration of HPMC. Accordingly, HPMC has the potential to improve hydrophilicity and mechanical properties of films.
Currently, research related to κ-carrageenan and HPMC is mainly focused on the effects of plasticizer types [ 12 ], Prunus maackii extract [ 13 ] or juice [ 14 ], and cork bark extract [ 15 ] on κ-carrageenan/HPMC films. However, to the best of our knowledge, the effects of HPMC on the physicochemical properties of κ-carrageenan/HPMC films and the reasons for the improved properties have not been reported. In the present study, we aimed to investigate the impacts of adding small amounts of HPMC on the properties (rheological, barrier, water resistance, mechanical, optical and thermal properties) of κ-carrageenan film, and to speculate the possible reasons for the performance improvement by X-ray diffraction (XRD) spectra, attenuated total reflection–Fourier transform infrared (ATR-FTIR) spectra and microscopic morphological analysis.