Mechanism of differences in characteristics of thick/thin egg whites during storage: Physicochemical, functional and molecular structure characteristics analysis

Highlights

• KEW was gradually thinning, while NEW was closer to Newtonian fluid during storage.

• Soft and flexible KEW gel gradually changed to hard and brittle NEW during storage.

• Improvement of functional properties were related to the disorder of EW protein structure.

• Differences between KEW and NEW relates the content of ovomucin and its sugar chains decomposition.

Abstract

This study systematically analyzed and compared the changes of physicochemical, functional and molecular structural characteristics between thick egg white (KEW) and thin egg white (NEW) during storage. Analysis of physicochemical properties showed that moisture content decreased significantly with the increase of pH during storage. KEW was gradually thinning, while NEW was closer to Newtonian fluid. Functional properties indicated that KEW thermal gel was gradually hard and brittle with the properties of NEW. KEW had better emulsifying property than NEW, and NEW had superior foaming ability. The α-helix and β-sheet in the FT-IR spectrum showed a downward trend, revealing secondary structure changed from order to disorder. Enhancement of fluorescence intensity indicated the structural unfolding and exposure of tryptophan residues. SDS-PAGE proved that OVO might be related to the difference between KEW and NEW characteristics. This study provided new idea and reference value for egg storage and diversified utilization of egg white.

Introduction

Egg white is rich in protein needed by human body, is a digestible protein colloid solution with a balanced proportion of amino acids (Offengenden & Wu, 2013). The structure of egg white can be divided into four layers from the outside to inside: NEW, which is close to the protein membrane, accounts for 23.2% of the total protein volume. 57.3% of the middle layer is thick protein; 16.8% of the middle layer is thin protein; and the fringe layer thick protein accounts for about 2.7 %. The proportion of KEW is closely related to the quality, storage and processing of egg products. The protein content of egg white is about 10–12%, mainly composed of ovalbumin, ovotransferrin, OVO, globulin and lysozyme. (Deng et al., 2020) have shown that OVO is the main component to maintain the gelatinous structure and high viscosity of egg white. In addition to its nutritional value, egg white also has a variety of excellent functional properties, such as gelling, foaming and emulsifying, which makes egg white has high application value in the food industry.

The current research on the properties of egg white protein thermal gels has made considerable progresses. (Jin, Chen, Zhang, & Sheng, 2021) reported the thermal gel properties of egg white protein modified by phosphorylation, the results revealed that the hardness and springiness of the modified egg white protein thermal gel were significantly improved (p < 0.05). (Ai & Jiang, 2021) obtained a thermal gel with great integrity by phosphorylated duck egg white protein. Nevertheless, the focus of the above studies was on the chemical modification of egg white protein, ignoring the variations in the functional and structural properties of egg white proteins themselves during storage.

The research progresses in this field mainly included: (Smith & Back, 1965) found that ovalbumin could be gradually converted to the more heat-resistant S-ovalbumin during storage. In addition, continued research by (Huang et al., 2012) found content of S-ovalbumin was negatively correlated with the Haugh unit. (Sheng, Huang, Wang, Xu, Hammad, & Ma, 2018) reflected the structure of ovalbumin changed from ordered to disordered during storage, and its emulsifying and foaming properties declined. (Shan et al., 2020) reported the changes of structure and function of OVO during storage, and verified that the degradation of OVO was the reason of egg white thinning. These studies focused on the changes in the structure and function of certain proteins in egg white during storage, while the variations in KEW and NEW characteristics and their comparison during storage has not been reported. (Liu et al., 2020) had fully proved that KEW and NEW show obvious differences in thermal gel properties. And from the perspective of composition-structure–function perspective, quantitative proteomics was used to reveal the mechanism of the difference. This demonstrated that it was necessary to separate the study of KEW and NEW.

Therefore, this work focused on the variations in KEW and NEW characteristics and their comparison during storage. The physicochemical, functional and molecular characteristics of KEW and NEW during storage were studied by rheology, texture profile, particle size, Fourier transform infrared (FT-IR) and fluorescence spectroscopy. It was also related and verified with the changes of emulsifying property, foaming property, and surface hydrophobicity during storage. It will be conducive to screen the egg white with high-quality functional characteristics in different storage time, and also helpful to select the KEW or NEW with specific excellent characteristics. It provides theoretical basis and reference for the efficient application of egg white.