Understanding the hydration of alkali-induced duck egg white gel at high temperature

Highlights

• The alkali-induced egg white gel will be hydration under high-temperature heating.

• The egg white gel will be liquefaction after heating 20 min for 121 °C.

• β-sheet contents decreased and β-turn contents increased obviously after heating.

• Hydrogen bonds, hydrophobicity interactions and disulfide bonds were destroyed after heating.

• The hydration of egg white gel is related to the alteration of molecular forces.

Abstract

In this paper, prepared alkali-induced egg white gel (EWG) was treated at different temperatures (100–121 °C) to explore the hydration mechanism under high temperatures (≥100 °C). The physicochemical, mechanical and rheological properties and intermolecular interactions of the EWG were investigated. Results showed that with increasing temperature the pH, free alkalinity and surface hydrophobicity of EWG increased initially but then decreased, together with gel hydration, and the browning intensity increased markedly. The mechanical properties, including hardness, puncture strength and springiness, rheological characteristics―determined via apparent viscosity and frequency sweep results―and water-holding capacity significantly decreased as temperature increased. Fourier-transform infrared (FTIR) spectroscopy revealed that the β-sheet contents decreased significantly, and the α-helical and β-turn contents improved markedly with increasing temperature. The protein fraction results showed that hydrogen bonds, hydrophobic interactions and disulphide bonds were destroyed in the heating process. This study provides deep insights into EWG destruction under high temperature and the relationship with intermolecular interactions, enabling control over EWG quality.

Introduction

A gel is defined as colloidal particles or polymers in sol or solution that are interconnected under certain conditions into a spatial network structure, with the structural gaps filled with dispersing medium (Caló & Khutoryanskiy, 2015; Zhang & Khademhosseini, 2017). Some food materials that can form gels include proteins, such as egg white protein (EWP), milk protein, whey protein, soy protein, etc.; and, polysaccharides, such as rice starch, konjac gum, guar gum, agar, algae sugar, etc. (Ai et al., 2020; Liu et al., 2019; Wang, Zhao, Liu, & Li, 2019; Wu et al., 2019). EWP can form a gel under various conditions, including upon heating, or the addition of alkali or acid. (Mine, 2007; Weijers, van de Velde, Stijnman, van de Pijpekamp, & Visschers, 2006). The egg white gel (EWG) prepared by heating is mainly through the thermal denaturation of EWP, with hydrophobic interactions and disulphide bonds promoting aggregation into a gel, while alkali-induced EWG is primarily through the denaturation of EWP, with ionic bonds, hydrogen bonds and disulphide bonds resulting in cross-linking and gel formation. EWGs prepared using different methods exhibit different properties.

The formation of EWG under alkaline conditions is mainly inspired by the production of traditional Chinese preserved eggs, which uses alkaline wood ash or the soaking of duck eggs in lye to form a gel (Ma, 2007). However, the conventional methods of preparing preserved eggs tend to fail to gel the egg whites (EWs), or the formed gel liquefies due to the excessive infiltration of NaOH, resulting in substandard speckled eggs due to uncontrolled factors (Ma, 2007). For better control of the preserved egg quality, gels prepared by adding NaOH to isolated EW can be used to visually identify and improve the gelling characteristics of EWG. However, the high protein and water content of EW provide suitable nutrients and an environment for microbial growth, causing difficulty in preserving products. Therefore, the utilisation of physical or chemical methods to inhibit microbial growth is necessary to prolong the edible quality of EWGs.

Heating is a common method used in food processing to kill microorganisms, mainly divided into pasteurisation, room-temperature boiling water sterilisation, and high-temperature steam sterilisation (Makroo, Rastogi, & Srivastava, 2020; Smelt, 1998). High-temperature steam sterilisation can kill microorganisms, and also destroys germ cells and spores; it is the most reliable and commonly used physical sterilisation method (Berk, 2018; Sastry et al., 2009). Porridge with preserved eggs is a traditional Chinese food. If we want to make it into a canned food stored at room temperature, high-temperature sterilisation is necessary. But after preliminary experiments, it was found that hydration appeared in the EWG after high-temperature treatment, which also adversely affected the preservation and taste of the EWG. Therefore, in this paper, high-temperature heating was employed to treat the prepared alkali-induced EWG. To investigate the progressive softening and even liquefaction of EWG under high-temperature treatment, we characterised its physicochemical properties and intermolecular forces. This study provides a reference for investigating the hydration mechanism of EWG and thereby informing the quality control of EWG at high temperatures and providing a theoretical basis for the industrialized production of preserved egg porridge with long shelf life.