topic 2 - protein

• Gel structures of the food materials

  Semi-solid materials in which a molecular network entraps the liquid continuous phase (water in the case of food materials or oil by fat crystal networks)
• Gels


  • Liquid but behave like pseudo-solids due to their three-dimensional cross-linked structure entrapping the liquid
  • Do not flow in a steady state condition
  • May be weak, soft or hard depending on the extent of cross linking and the water content
  • Out-of-equilibrium systems (liquid but behave differently)
  • Mobility of their constituents is extremely reduced, these systems are frozen (or 'jammed' ) in configurations far from thermodynamic equilibrium
  • Exhibit unusually slow relaxations and aging effects, their properties continuously evolve with time
• Gels are categorised as "soft-materials"
• Jamming is a transition phenomenon from liquid to solid state below a yield stress
• Microstructure of Soft Matter

  • Inclusion with interparticle interactions (attractive / repulsive) and excluded volume effects
  • Polydisperse size of inclusion and/or aggregates
  • Anisotropic inclusion (e.g. clay platelet, rod-like vesicles, plant cells)
  • Deformable inclusion (e.g. microgel, hydrated core, polymer coated particle, liquid droplet, air/gas bubble)
  • Inclusion is made up of lamaller bilayers (sheets, gel phase), foam/cellular structures, etc.
  • Inclusion consists of polymer, peptides, worm like micelles, star polymers etc.
  • Inclusion contains a polymer with potential to form cross-links (e.g. polymer gel)
• Gel categories based on their firmness

  • Soft gel
  • Firm gel
  • Hard gel
• Food product systems


  • Soft gel: Yoghurt, soft cheese, soft soy tofu
  • Firm gel: Soy tofu, jelly (confectionery)
  • Hard gel: Cheese, paneer, jelly beans, liquorice
• Network structure in gels

  • Result of physical and/or chemical interactions
  • Cross linking of the gelling species (particulate or molecular) could be hydrophobic, ionic, van-der-waals, covalent or microcrystalline
• Gel forming mechanism


  1. Covalent bonds formed due to S-S bridges in proteins due to heating or by addition of reagents

  2. Ionic bonds formed due to the formation of bridges, such as in the presence of ions (divalent- Ca++, Mg++ or monovalent- Na+ or K+) in hydrocolloids

  3. Polymers may crystallise and form microcrystalline regions that act as cross-links

  4. Particle gels normally formed by aggregation induced by pH change or ionic strength, such as in casein micelles typical of gelation in yoghurt

  5. Particles in gels possess fractal behaviour
• Biopolymer network formation


  • Chemical gelation mechanism through point crosslinking
  • Physical gelation mechanisms through polymer chain associations
  • Particulate aggregation
• Food polymers that form gel

  • Solid fat particles
  • Casein
  • Whey protein
  • Gelatin
  • Starch
  • Alginate
  • Carrageenan
  • Protein+hydrocolloids
  • Hydrocolloids+hydrocolloids (oppositely charged)
• Fat crystal gel


  You can create a gel if you reduce the solid fat particle size to micron or nano size- large number of small crystals can form a gel
• Casein gel


  Casein micelles spherical particles, gel formed by decreasing the electric repulsion by lowering the pH
• Globular protein gel


  Heating unfolds and exposes hydrophobic group; linked by hydrophobic forces and hydrogen bonds; may form of covalent bonds (-SH and -S-S- linkages)
• Gelatin gel


  Coil-to-helix or disordered to ordered transition upon cooling, the triple helices align forming microcrystalline region
• Alginate gel


  "Egg-box" model, cation (such as Ca++) complexes between the alginate molecules forming egg-box like junctions by ionic bonding, junctions form a microcrystalline region
• Carrageenan gel


  Forms double helices between two molecules, the microcrystalline regions are formed between the double helices of different molecules, monovalent cations (e.g., K+) promote the aggregation of k-carrageenan double helices to form so-called aggregated 'domains'
• Starch gel


  Amylose and amylopectin can form single or double helices and arrange themselves to microcrystalline regions, gel formation is initiated at higher concentrations, swollen and partly deformed starch granules may interlock each other forming gels
• Composite gel matrix


  • Starch/sugar rich domains and gelatin-rich domains
  • Casein-Carrageenan composite
  • Sugar, starch and gelatin (Jelly bean formulation)
• Particle size effect on viscosity

  Smaller particles increase the number of particle-particle interactions, resulting in higher viscosity
• Oleogels


  Lipophilic liquid and solid mixtures, in which solid lipid materials (oleogelators) with lower concentrations (<10 wt.%) can entrap bulk liquid oil by ways of the formation of network of oleogelators in the bulk oil
• Oleogelators


  • Self-assembly system
  • Crystal particles system
• Ethyl cellulose oleogel


  Ethyl cellulose is heated in vegetable oil above its glass transition temperature and subsequently cooled, the EC polymers interact with each other and the oil, forming a gel network
• Milk fat olein oleogel


  Dry fractionation of melted milk fat at 21°C, olein fraction is structured with 10% monoacylglycerols (MAG) and plant sterols (PS) mixtures
• Fibre-based oleogel


  Oleogel created from refined liquid palm oil or olive oil with 1.5-2% citrus fibre or nata de coco fibre, the insoluble fibres form a network to entrap the oil