Sitian Zhang | ALES Graduate Seminar

Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Sitian Zhang. This seminar is open to the general public to attend.

PhD with Dr. Lingyun Chen

Zoom Link: https://ualberta-ca.zoom.us/j/95729117693?pwd=giLvjoaBJyXaZbwWomBsKLRY73lzCb.1

Thesis Topic: Utilization of non-thermal processing technologies to improve pea protein gelation and gelling mechanisms study.

Abstract: 

As global trends shift towards sustainable and health-conscious diets, pea protein has become a prominent alternative to traditional animal and soy proteins, valued for its high nutritional value, affordability, hypoallergenic properties, non-GMO status, and sustainable production. However, its application in food industry is limited by gelation challenges, as it requires high temperature to open the structure and lacks cysteine for disulfide bonds, resulting in weaker gels compared to soy protein. Despite the exploration of chemical and enzymatic solutions, concerns over residues and costs persist. Consequently, non-thermal technologies like atmospheric cold plasma (ACP) and high pressure processing (HPP) have attracted increasing interest due to their short exposure time, great energy efficiency, and minimized nutrient loss. This dissertation explored the potential of ACP and HPP to enhance the gelling properties of pea protein and broaden its applications in the food area.

The first section of the thesis demonstrated that while untreated pea protein isolate (PPI) (12 wt.%) did not form gel even after heating at 90℃ for 1 h, PPI after ACP did form self-standing gels at 70-90℃ for 30 min. The gels exhibited homogeneous, interconnected macropore network structures with compressive strengths of 0.53 kPa at 70°C, 2.70 kPa at 80°C, and 6.27 kPa at 90°C, with water holding capacities between 80-90%. This enhancement allows pea protein to function effectively as a gelling agent, aligning with an internal food cooking temperature of around 75℃.

However, ACP treatment reduced the pH through the generation of reactive species like nitrites pushing it toward the protein isoelectric point and risking functionality loss. This limitation prompted further research into combining ACP with pH-shifting to address this challenge. Specifically, PPI was treated with ACP at pH 12 for 10 min, followed by pH-shifting to neutral. The treated PPI suspension (14 wt.%) formed gels with significantly improved mechanical properties (compressive strength of 2.81 kPa) at 70°C within 10-20 min. The pH-shifting partially unfolded pea protein by altering the tertiary structure. Simultaneously, ACP generated active species such as nitrites and hydrogen peroxides that reacted with the amino acid chains, facilitating aggregate formation. During heating, these aggregates served as active building blocks, forming ordered three-dimensional gel networks via hydrophobic interactions and hydrogen bonding.

HPP is another emerging non-thermal technology used for microbial inactivation and protein modification with minimal effects on food organoleptic properties. This dissertation systematically examined how variations in pressure level, treatment time, protein concentration, and sample pH impacted the microstructure and strength of PPI gels. Findings demonstrated that HPP could produce a wide range of gel textures, with compressive strengths varying from 0.11 to 130.18 kPa, enabling the simulation of jelly, pudding, and tofu textures at ambient temperatures, depending on processing conditions. Additionally, combining pH-shifting with HPP significantly enhanced freeze-thaw stability, achieving minimum syneresis of 15% after two cycles, demonstrating its suitability for frozen food products.

The gelation mechanisms of PPI with HPP, particularly when combined with polysaccharides, remain underexplored, prompting focused investigations in the final stages of research. Suspensions containing 15% PPI with 0.1-1% κ-carrageenan were subjected to HPP at 100-600 MPa for 5-30 min. The results showed that gels with 1% κ-carrageenan at 600 MPa demonstrated a 27-fold increase in compressive strength compared to PPI alone by HPP and over 5 times the strength of heat-induced counterparts. HPP facilitated protein tertiary structure unfolding, enabling phase separation and uniform submicron κ-carrageenan particle distribution in the protein matrix. This, along with a more compact protein network induced by HPP, enhanced gel strength, compensating for limited cysteine in pea protein for disulfide bonds. Conversely, heat-induced gelation caused more random protein aggregation, forming coarser and weaker gels.

Overall, both ACP and HPP effectively modified pea protein structure, significantly enhancing its gelling properties. ACP unfolded proteins and generated reactive species that promote protein aggregate formation, facilitating gel formation at lower temperatures. Similarly, HPP partially unfolded protein structures and exposed reactive sites, enabling improved protein interactions and crosslinking, which enhanced gelation. This dissertation provides innovative and energy-efficient strategies using ACP and HPP to enhance pea protein gelation, essential for developing plant-based alternatives to traditional foods like eggs, cheese, and meat. This research also revealed the gelling mechanisms of pea protein treated by ACP and HPP, fostering further innovations in plant protein applications.