The formation mechanism and thermodynamic properties of potato protein isolate–chitosan complex under dynamic high-pressure microfluidization (DHPM) treatment

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Abstract

The objective of the study was to explore the formation mechanism and thermodynamic properties of chitosan (CS)–potato protein isolate (PPI) complex under DHPM treatment. The transmission electron microscopic (TEM) results showed the formation of a complex between CS and PPI. Meanwhile, particle size and zeta-potential were shown to increase with increasing CS concentration, further confirming the formation of the complex. The surface hydrophobicity results showed CS was bound to PPI by hydrogen bond. The ultraviolet and fluorescence spectral analysis exhibited CS formed a protective mechanism against PPI destruction, preventing the exposure of tyrosine and tryptophan residues. Infrared spectrum and circular dichroism spectral analysis revealed no occurrence of chemical reaction between CS and PPI under DHPM treatment, further indicating that they are bound by hydrogen bond and hydrophobic interaction. Moreover, CS addition was shown to enhance the intermolecular interaction and promote the formation of intermolecular hydrogen bond network. Differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) revealed that CS addition could improve the thermal stability of PPI. These results have shed light on the formation mechanism and thermodynamic properties of the CS/PPI complex and facilitate its application in food industry.

Introduction

In recent years, polysaccharide and protein complexes have been widely applied in food, pharmaceutical industries and cosmetics due to their outstanding advantages such as non-toxicity, harmlessness and easy degradation [1]. Previous studies have shown that polysaccharide and protein complexes are mainly formed in the following two ways: (i) formation of a special, strong and essentially permanent structure through covalent bond interaction, and (ii) formation of a complex via physical interactions, such as electrostatic and hydrophobic interaction, spatial repulsion, and hydrogen bonding [2,3]. Polysaccharide-protein complexes not only facilitate the formation and stabilization of emulsion through their interactions, but also reduce the cost and create new functional nanoscale, microscopic or macroscopic structures in many foods [1]. In previous studies, chitosan, alginate, carboxymethyl cellulose and gum arabic were often used to form complexes with proteins [4,25]. However, proteins vary in their effect on the complex system due to the difference in amino acid sequence and spatial structure. Therefore, the selection of proteins is still a research hotspot.

Potato protein, a potential raw material for the food industry, was reported to contain a full range of amino acids and a high content of lysine with a nutritional value higher than most major plant proteins and close to egg protein [5]. Additionally, it has the functional properties to form structural networks of gels, foams and emulsions [6]. Thus, there is an increasing interest in application of plant proteins to replace animal-based proteins for the people suffering from intestinal and cardiovascular diseases [7]. However, potato protein can be denatured by industrial processing, leading to poor solubility and emulsion stability [8,9]. Previous studies have shown that the performance of potato protein can be improved by the combination of polysaccharide and protein. For instance, chitosan has been reported to improve the surface activity of soybean protein isolate, especially the emulsification activity and stability [10]. Whey protein isolate (WPI) and tremella polysaccharide complex were found to create a more stable aqueous solution and reduce gastric emptying time [11]. Niu et al. [12] mentioned that the interaction of psyllid polysaccharides with whey proteins could form a protective mechanism against protein destruction. However, rare reports are available on the interaction between chitosan (CS) and potato protein isolate (PPI), their formation mechanism and functional properties under dynamic high-pressure microfluidization (DHPM) treatment. DHPM mainly works through shear, collision and cavitation effects, leading to a reduced particle size and uniform dispersion of materials, which can change the functional properties of the complex system by altering its conformation.

The purpose of this study was to reveal the interaction between chitosan and potato protein isolate under DHPM treatment and their formation mechanism. To this end, the effects of chitosan concentration on the structure and thermodynamic properties of potato protein isolate were investigated in terms of micromorphology, surface charge, particle size, surface hydrophobicity, tertiary structure, secondary structure, rheological property and thermodynamic properties.

This study will facilitate the understanding of the mechanism underlying the formation of CS/PPI complex and provide some theoretical guidance for its application as a new material in the food industry.

Section snippets

Materials

Potato protein isolate (PPI) powder (98%) and chitosan (CS) powder (90% deacetylation) were purchased from Yuanye Biological Technology Co., Ltd. (Shanghai, China). Other reagents of analytical grade were bought from the Sinopharm Chemical Reagent Co., Ltd. (China).

Preparation of CS/PPI complex

PPI solution (1% w/v) and CS solution (1% w/v) were prepared and stored at 4 °C overnight. Briefly, PPI powder was dissolved in distilled water and CS powder was dissolved in 2% acetic acid, followed by mixing at a special weight

Micromorphology, particle size and surface charge analysis

The morphologies of PPI without DHPM treatment as well as PPI and CS/PPI complexes treated by DHPM were observed by TEM. In Fig. 1Aa, the PPI showed obvious aggregation in the acidic solution. After DHPM treatment, the PPI showed good dispersion and uniform distribution (Fig. 1Ab), while adding CS further improved the stability and uniformity of the complex solution, leading to the formation of a network structure between CS and PPI (Fig. 1Ac). With the CS concentration further increased to

Conclusion

In this study, CS/PPI complexes were prepared under DHPM treatment and their formation mechanism was investigated. CS/PPI ratio was shown to produce a significant effect on the interaction between CS and PPI and the functional properties of complex. The TEM results showed that the formation of a complex between CS and PPI, and CS addition promoted the formation of gel network structure. Meanwhile, the particle size and zeta-potential increased with increasing CS concentration, further

CRediT authorship contribution statement

Chun Hu: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Writing - review & editing. Zhouyi Xiong: Supervision, Funding acquisition. Hanguo Xiong: Resources, Project administration, Funding acquisition. Lei Chen: Writing - review & editing. Zhongli Zhang: Methodology. Qiaofeng Li: Resources.

Declaration of competing interest

All authors declared that they do not have any conflict of interest.

Acknowledgements

This work was financially supported by the Nature Science Foundation of Hubei Province (2018CFB269), National Key R&D Program of China (2018YFD0400702) and the Major technical innovation special project of Hubei (2018ABA100).

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