Tuning structure and properties of gelatin edible films through pullulan dialdehyde crosslinking
Graphical abstract
Introduction
Edible films can be applied to extend shelf life of food products by providing protective layers, or can serve as carriers of food additives (Khodaei, Oltrogge, & Hamidi-Esfahani, 2020). Gelatin is one of the most widely used animal-originated proteins for preparation of edible films. As a partially denatured derivative of insoluble fibrous collagen, gelatin can be extracted from skins, bones and tissues of various animal sources (cows, pigs, fishes, chickens, and goats) and is readily soluble in hot water. Whatever its origin, the remarkable film-forming ability, biocompatibility, nontoxicity, and biodegradability of gelatin makes it ideal for food applications.
However, due to the highly hydrophilic feature of gelatin, gelatin film in its pristine form exhibits a strong tendency to swell and be dissolved when exposed to high humidity and aqueous environment (Yousuf, Qadri, & Srivastava, 2018). Though sometimes this characteristic is desirable in some applications (eg. capsules, desserts), the modification of gelatin films is still quite necessary to improve its structural stability and properties so as to accommodate changing application requirements. Among the various modification methods, crosslinking is a promising technique to improve the applicability of gelatin films (Hosseini & Gómez-Guillén, 2018). Depending on the type of crosslinking points, crosslinked gelatin films can be classified into chemical and physical categories (Lin, Regenstein, Lv, Lu, & Jiang, 2017). Most of the chemical crosslinking approaches are based on the reactive functional side groups in gelatin, mainly amino, carboxyl, and guanidine groups (Azeredo & Waldron, 2016). So far, various reactive chemicals have been used for improving properties and maintaining the stability of gelatin. Formaldehyde, glutaradehyde (Zeiger, Gollapudi, & Spencer, 2005), transglutaminase, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (Piotrowska, Sztuka, Kołodziejska, & Dobrosielska, 2008), genipin (Liu, Liu, & Brown, 2017), ferulic acid, tannin acid (Cao, Fu, & He, 2007), and proanthocyanidin (Kim, Nimni, Yang, & Han, 2005) have been reported as crosslinking agents for gelatin. However, these crosslinking system still have drawbacks of toxicity, expensiveness, easier deactivation, unsatisfactory stability, etc.
Lately, oxidized polysaccharides are being explored for their potential to stabilize the structure of gelatin-based materials. The polysaccharide that processes adjacent diols can be easily oxidized to two aldehyde groups with different oxidation systems (Leguy, Nishiyama, Jean, & Heux, 2019). The reactive aldehyde groups can react with ε-amino groups present on gelatin, which opens a promising pathway for the design of non-toxic gelatin-based materials with the potential applications in food industry. Up to now, several different polysaccharide dialdehydes have been used to improve the performance and applicability of gelatin-based films, including dialdehyde carboxymethyl cellulose (Mu, Guo, Li, Lin, & Li, 2012), dialdehyde starch (Martucci & Ruseckaite, 2009; Moreno et al., 2017, Moreno et al., 2017), dialdehyde microfibrillated cellulose (Wei et al., 2019), alginate dialdehyde (Boanini, Rubini, Panzavolta, & Bigi, 2010), and oxidized xanthan gum (Guo, Ge, Li, Mu, & Li, 2014).
Pullulan is linear polysaccharide obtained from fermentation medium of the fungus-like yeast Aureobasidium pullulans, which has been widely used in food and biomedical fields because it is colorless, tasteless, non-toxic, compatible, biodegradable and has low immunogenicity (Pattanayaiying, H-Kittikun, & Cutter, 2015; NithyaBalaSundari et al., 2020). Periodate oxidation of pullulan has been well researched and its activated products exhibited great potential for biomedical applications (Bruneel & Schacht, 1993; Li, Yi, Yu, Wang, & Wang, 2020; Zhang et al., 2019). The proposed crosslinking mechanism is also ascribed to Schiff's base formation between amino groups of gelatin and aldehyde groups of the crosslinker. Nonetheless, the application of pullulan dialdehyde (PDA) to stabilize edible protein films has not been rigorously evaluated. Moreover, detailed studies concerning the influences of molecular weight and aldehyde content of the polysaccharide aldehydes on properties of the crosslinked films are still highly needed.
In the present work, a series of PDA samples with different molecular weights and aldehyde contents were successfully prepared, and then introduced into gelatin films as crosslinking agents. The main objective of this study was to tune the structure and properties of gelatin edible films, as far as possible, through varying the oxidation degree of PDA. The structure features of cast films were characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Then the mechanical, thermal, water vapor barrier and optical properties as well as the water solubility were investigated to evaluate the effect of PDA crosslinking on properties of gelatin films.
Section snippets
Materials
Porcine skin gelatin (Type A, bloom number: ~300) and ninhydrin solution (20 g/L) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Pullulan, sodium periodate and glycerol were obtained from Macklin Biochemical Co. (Shanghai, China). All chemicals used were of analytical grade and used as received. Milli-Q deionized water (Millipore, Billerca, USA) was used for all solutions.
Preparation of PDA
Five different PDA samples were prepared by reacting pullulan with different amounts of sodium periodate (NaIO4).
Results and discussion
The PDA samples were characterized by using FTIR, XRD, size exclusion chromatography (SEC) and aldehyde content analysis (Supplementary Fig. S1).
Conclusions
Gelatin is an edible polymer with good film-forming properties and gelatin film is widely used in food packaging industry. However, gelatin film has a strong water sensitivity and tends to swell or be dissolved in high humidity environment or in water, which could further weaken its physical properties. Chemical crosslinking is a promising strategy to improve the applicability and performance of gelatin film. Pullulan dialdehyde, a kind of polysaccharide derivative containing aldehyde groups,
CRediT authorship contribution statement
Jie Liu: Supervision, Conceptualization, Investigation, Writing - review & editing. Liru Zhang: Investigation, Writing - original draft. Chang Liu: Investigation. Xuejing Zheng: Visualization, Resources. Keyong Tang: Validation, Resources, Funding acquisition.
Declaration of competing interest
The authors declared no potential conflicts of interest.
Acknowledgements
The authors would like to thank Prof. Jie Chen (JiangNan University, Wuxi, China) for SEC analysis. This work was financially supported by the Science and Technology Department of Henan Province, China [grant number 172102410022]; and National Natural Science Foundation of China [grant number 51673177].
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