Development and Characterisation of Gel-Like Sorbents from Pomelo (Citrus grandis L.) Peels as Fat Replacers in Meat Products

This PhD project investigated the conversion of pomelo (Citrus grandis L.) peel (PP), an underutilised plant-based byproduct, into various gel-like materials, including hydrogel-like, oleogel-like, emulsion-based, and aerogel-based sorbents, for potential food applications. These sorbents rapidly form gel-like structures when immersed in solvent (oil or water) at room temperature for 2–10 minutes, offering a non-thermal, efficient solution for food applications. The study specifically examined how PP powder particle size (125–500 µm) and concentration (5–40%) influence gelation mechanisms and structural properties using three approaches: direct mixing, emulsion-templating, and aerogel-templating. Additionally, the project successfully demonstrated the application of optimised PP-based sorbents as fat replacers in semi-dried chicken sausages. The investigation into pomelo peel (PP) powder (Chapter 3) revealed its unique functional properties for constructing diverse gel-like materials. Systematic characterisation showed that PP powder is rich in fibre (52.11%–59.59%), primarily composed of cellulose, hemicellulose, and lignin. PP powder also exhibited high oil sorption capacity, amphiphilic behaviour, and strong antioxidant activity, making it a promising food structuring and stabilising agent. PP powder was processed into 125, 250, and 500 μm fractions, showing that smaller particle sizes (125 and 250 μm), at concentrations of 10–40% w/w, could form self-sustaining gel-like structures. When mixed with oil, they resembled oleogel-like materials, while in water, they formed hydrogel-like substances. Textural and rheological analyses revealed that larger particle sizes and higher PP powder concentrations produced harder, more brittle gels, whereas smaller particles and lower concentrations resulted in softer and elastic gels. Increasing PP powder concentration led to denser gel networks. Principal Component Analysis suggested that chemical composition was not the primary factor driving gelation. While both 125 μm and 250 μm PP powders are fiber-dominant, their microstructural differences result in distinct gelation mechanisms. The porous structure of 250 μm PP particles enhances solvent trapping, leading to greater oil and water retention. These particles form gels primarily through direct particle aggregation, where physical entanglements and porous structures drive gel formation rather than uniform dispersion. Consequently, the PP concentration required for gelation is reduced from 15–35% to 10–25% compared to 125 μm PP particles. Furthermore, self-sustaining sorbents require less PP powder in water (10–20%) than in oil (25–35%), reinforcing its hydrophilic nature. To improve gelling efficiency and further reduce PP powder content, an emulsion-based approach was developed in Chapter 4. Emulsions with oil-to-water ratios of 1:3, 1:1, and 3:1 were prepared using 125 μm and 250 μm PP powders, without additional stabilisers. This method effectively lowered the required PP concentration to 10–25%. Gel hardness increased as PP concentration and particle size increased, with the oil-to-water ratio playing a critical role in determining textural, rheological, and oxidative stability. A higher oil content (3:1 ratio) resulted in greater hardness and improved stability, closely resembling traditional fats in elasticity and oxidation resistance. The gelation mechanism in this approach was primarily driven by PP particle hydration, which facilitated oil droplet entrapment within the gel matrix. Unlike the direct mixing method, where gelation depended on particle-to-particle interactions, the emulsion-based method introduced particle-to-droplet interactions, forming a more structured and cohesive gel network. Microstructural analysis revealed that 250 μm PP particles formed large aggregates due to stronger particle-to-particle interactions, whereas 125 μm PP particles created a more uniform, elastic gel network dominated by droplet-to-particle interactions. These findings suggest that gelation efficiency is influenced by PP concentration, particle size, and the balance between water absorption and oil entrapment. To further minimise the PP content required for gelation, a novel aerogel template method was developed in Chapter 5. Emulsion-template aerogels were produced using 5–10% PP powder via hydrothermal treatment at 95°C for 2 hours. The aerogels were subsequently immersed in rice bran oil at room temperature for 10 minutes to form aerogel-based oil sorbents (ABOS). Hydrothermal treatment modified the cell wall structure of PP particles, reducing antioxidant and soluble fibre content. The aerogel showed a binary pore system with intra particular micropores and inter particular mesopores which depends on particle size and concentration. Mesoporous exhibited superior oil sorption capabilities, with micropores becoming more dominant at higher PP concentrations but not related to oil sorption. Fourier-transform infrared spectroscopy (FTIR) analysis confirmed that oil sorption was primarily driven by physical interactions. Smaller particle (125 μm) sizes formed denser structures, increasing hardness but reducing cohesiveness. At lower PP concentrations, 250 µm aerogels exhibited superior oil absorption and favourable textural properties, making them ideal for food applications. Oral processing analysis revealed that 5–6% sorbents (125 and 250 µm) exhibited rheological and tribological properties comparable to conventional oleogels, suggesting their suitability as fat replacers. The required PP content for gelation was successfully reduced to 6–9%, aligning with traditional oleogel formulations. Key findings from Chapters 3, 4, and 5 guided the selection of 5 and 6% ABOS (125 μm and 250 μm) for fat replacement in semi-dried chicken sausages (Chapter 6). The formulation included 60% chicken mince, 20% pork fat, and 20% ice, followed by four days of drying and cooking at 75°C for 30 minutes. The ABOS replaced 20–100% of pork fat in formulations while maintaining desirable pH (~5.3) and water activity (~0.93). Compositional analysis revealed a 33–46% reduction in fat content with total fat substitution, along with a 2.8- to 3.9-fold increase in dietary fibre. Volatile profiling (SPME-GC-MS) showed that fat-reduced sausages contained terpenes and esters from PP, creating a fruity-meaty balanced aroma, whereas control sausages retained conventional lipid-derived volatiles (aldehydes and ketones). Microstructural and functional characterisation demonstrated that smaller ABOS particles (125 μm) enhanced water retention, minimised weight and cooking loss, and increased porosity, while larger particles (250 μm) reinforced matrix integrity, contributing to higher hardness and cohesiveness. These findings highlight the potential of ABOS as a functional and sustainable fat replacer in food applications.<p></p>