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Week 12: Dispersed Systems, Aggregating particles may form a network -…

- - - - two types
        
        soap-like substances (amphiphiles): Fairly small amphiphilic molecules, the hydrophobic (lipophobic) part being typically an aliphatic chain
        
        important characteristic: hydrophile-lipophile balance (HLB) Value
        
        HLB value of 7 indicates that the substance has about equal solubility in water and oil
        
        Surfactants with HLB >7 are generally suitable for making o/w emulsions and those with HLB< 7 for w/o emulsions
        
        Proteins often are the surfactants of choice, especially for foams (eg egg white) and o/w emulsions
        (eg milk)
        
        Surface activity increase with size of surfactant and high ionic strength
        
        polymers
      - Functions of Surfactants
        
        Creation of surface-tension gradient- essential for foam and emulsion stabilities
        
        Lowering interfacial tension - emulsion and foam formation and to avoid coalescence
        
        Aid in wetting and dispersion events
        
        Adsorption of surfactants onto particles modify interparticle forces ( enhance repulsion and stability)
    - - Most foods have a large phase boundary or interfacial area
        
        The interface can be solid or fluid
        
        An interface between two phases contains an excess of free energy, which is proportional to the interfacial area.
      - (+ Surfactant) Surface Tension Gradients
        
        If a fluid interface contains a surfactant, surface tension gradient can be created.
        
        Function of surfactant: Interface will try to become as small as possible, to minimize interfacial free energy –
        
        Immersible liquids have high interfacial tension
        
        Presence of surface tension gradients provides resistance to fluid flow
    - - dilation: real interfacial property, but difficult to measure. How the product spread out
        
        Surface dilation modulus for globular protein = 30-100mN.m-1
        
        Surface rheological parameters of protein layers naturally depend on pH, ionic strength, solvent quality, temperature and so forth
      - shear: due to the viscosity of the material in the interfacial layer
        Product will move
    - - when the surfactant lyophobic group is removed from the solvent
      - when one or both affinities are satisfied respectively at a surface or at an interface
    - - (measured by) Contact Angles
        
        When two fluids are in contact with a solid and with each other, there is a contact line between the three phases.
        
        If the angle is small (near zero), the fluid extends all over the solid surface and displaces the other fluid/ air contact
        
        If the angle is larger than 90 , the fluid is said not to wet the surface
        
        A balance between the surface forces acting in the plane of the solid surface must be achieved
        
        Young Equation:
  - - - involves both chemical and physical properties
      - Most foods are dispersed systems
      - Few are homogenous solutions e.g. cooking oil
      - Characterization
        The nature of the continuous phase determines the important properties of dispersion
        Several foods are bicontinous
        E.g. bread: gas and solid phase are continuous
        
        Continuous phase is the dominant phase
        
        Water in oil
        
        Oil in water
        
        Hydrophobic or hydrophilic
        
        can include
        
        Emulsions
        
        Colloids
        
        Foams
        
        Bicontinuous
    - - Two types
        
        Reversible (lyophilic): forms by “dissolving” a material in a suitable solvent e.g. protein, carbohydrate
        
        In foods, the difference between reversible and irreversible colloids are not always clear
        
        Oil in water containing protein as an emulsifier is a reversible colloid
        
        Oil in water emulsion is irreversible colloid
        
        Irreversible (lyophobic)
      - Food are made up of structural elements (i.e. micelles, globules etc)
        
        The size scale of structural elements in foods can vary widely
        Product properties are affected by size and scale structural elements
        
        Structural Elements Influence On Colloids :check:
        
        Surface area
        
        Surface area is increased with the presence of the particles
        
        A = 6/d
        
        1 more item...
        
        Visual appearance
        
        oil/water emulsion (EG Milk)
        
        white if 3 micro-m
        
        Transparent if droplets diameter is 0.03 micro-m,
        
        bluish white if 0.3 micro-m
        
        Pore Size
        
        The permeability (the ease with which solvent can flow through the pores) is inversely proportionally to pore size squared
        
        Hence, polymer gel is less permeable than gel made of huge particles
        
        Between particles, regions of continuous phase exist and its proportional to particle size
        
        Time scale involved: Characteristic time needed for an event to occur
        
        The larger the particles, the longer the time scales involved
        
        Eg of events: REaction of two molecules, particle to rotate, bread to be baked
        
        Ease of separation
        
        More difficult to separate small particles from a liquid than large ones
        
        Physical Stability
        
        All dispersion become inhomogeneous (Not uniform) more readily if particles are large. Huge particles are easily deformed and destroyed by external forces
        
        Interaction forces between particles are roughly proportional to their diameter and sedimentation rate to diameter squared
        
        Effect of external forces
        
        External forces acting on particles are proportional to diameter squared
        
        Internal forces within or between particles are proportional to diameter
        
        Small particles are virtually unaffected by external influences like shearing force or gravity
        
        Large particles can be deformed or even be disrupted by external forces
  - - - aggregate and stay stable
      - form sediments
    - - Steric repulsion
        
        Mechanism 2: hairy layers overlap on approach of particles
        
        causes an increased concentration of protruding hairs and thereby an increase osmotic pressure
        
        water moves into the overlap region
        
        repulsion
        
        Mechanism 1: Surface of another particle comes close, the hairs are restricted in conformation,
        
        Loss of entropy or increases free energy
        
        repulsion
      - Electric Double layers: the zone between the particle surface and the plane at which neutralisation is achieved (≤ 1 nm)
        
        Charges in most foods are predominantly negative due to absorbed ions or ionic surfactants
        
        Because the systems (i.e. food) must be electroneutral, the particles will be accompanied by a cloud of oppositely charged ions called counterions
        
        At a certain distance from the surface, the concentrations of positive and negative charges in the solution becomes equal
        
        Beyond that distance, the charge on the particle is neutralised, due to an excess of counterions in the electric double layer
      - Van der Waals Attractions
        Weaker Interaction between particles as well as between molecules
- - - - characterized by predominance of liquid and the presence of a continuous matrix of interconnected material
        :check:
        
        Particle gels: Formed due to aggregation from change in pH, ionic strength
        
        (might include) Cold set gels: Formed after heating to a temperature which the network forming material dissolves or forms a dispersion of very small particles
        
        On cooling, a gel is formed as a result of the formation of physical cross links
        
        mixtures of locust bean gum and xanthan gums, plastic fats
        
        much coarser than polymer gels
        
        E.g. triacylglycerol crystals in plastic fats, casein micelles in milk gels i.e. Yoghurt
        
        Physical cross links between polymer molecules are called junctions
        
        junction contains many individual bonds e.g. van der waals, electrostatic, hydrophobic, and hydrogen bonds
        
        Heat set gels: a solution of globular proteins is heated above its denaturation temperature, a gel may be formed if protein concentration is above a critical level.
        
        irreversible and considerably increase in firmness upon cooling
        
        E.g. egg white, soy protein isolate, meat proteins
        
        Some gels are formed by changing condition that affect molecular or colloidal interactions, such as pH, ionic strength, specific salts (e.g Ca2+ ions), or enzyme actions
        
        Polymer gels: Consists of long, linear chain molecules, which is cross-linked to other molecules
        
        Nature of cross links
        
        physical cross links
        
        Physical cross links are predominant in food gels e.g. salt bridges, microcrystalline regions or specific kinds of entanglements
        
        covalent bonds (-S-S-),
      - Functional Properties :check:
        
        To arrest the motion of the liquid (indicates the permeability of the gel)
        Stops movement of liquid to certain extent
        
        Gels are used to
        
        provide physical stability
        
        Prevent sedimentation
        
        obtain a certain consistency
        
        Texture of the product e.g. pudding
        
        Swelling and Syneresis: expulsion of liquid from the gel
        
        Lower of solvent quality, adding of salt, or increasing the number of cross links may cause syneresis
        
        Diffusion
        
        Transport of solute through the liquid in a gel
        
        The solute has to diffuse around the strands of the gel matrix
      - Food Gels
        
        Polysaccharides
        
        Most polysaccharides chains are fairly stiff
        
        Bending of chain can only occur if its length exceeds about 10 monomers
        
        Polysaccharides provide highly viscous solutions, some can form gels
        
        Gel cross links at junctions
        
        3 types of Cross links
        
        Double helices:
        Several polysaccharides (e.g. carrageenan, agar, gellan) can form double helices below a sharply defined temperature, which depends on conditions
        
        Egg box junctions: occur with some charged polysaccharides such as alginate, when divalent cations are present e.g. Ca2+
        
        Alginate typically used in the form of a hydrogel in biomedicine, including wound healing, drug delivery and tissue engineering applications.
        
        Microcrystallites: simplest type, local stacking of stretched chain segments
        
        Gelatin
        
        Contains long flexible molecular strand between cross-links
        
        Gel is very extensible
        
        Gelatin molecules tend to form triple helices upon cooling
        
        Caseinate
        
        Milk contains casein micelles
        
        Micelles can be made to aggregate by lowering pH to about 4.6 or by addition of proteolytic enzyme
        
        Gel formed is rather weak and deformable
        
        Globular Proteins
        
        Several globular proteins form irreversible heat setting gels
        
        Proteins denature, and mutually react
        
        Gel bonds formed may be S- S linkages, salt bridges, hydrophobic bonds
        
        Structure and rheological properties of these gels vary greatly with the type of proteins, pH, ionic strength, and rate of heating
    - - most vegetables and fruit tissues
    - - e.g. tomato ketchup, apple sauce, mayonnaise, beer foam
  - - - made in two ways:
        
        mechanical forces
        
        Bubbles made by beating air into the liquid
        
        Further shear breaks the air bubbles into smaller ones
        
        supersaturation
        
        A gas is dissolved in an aqueous liquid at high pressure
        
        When pressure is released, gas bubbles form
        
        Other example – bread
    - - Ostwald Ripening: Diffusion of gas from small to larger bubbles as pressure in the small bubbles is greater than in larger ones
      - Drainage of liquid from and through the foam layer, due to gravity
      - Coalescence of bubbles due to instability of the film between them
      - Enhancing Foam Stability :check:
        
        Ostwald ripening
        
        Adding surfactant e.g. egg white, protein denaturation occurs on foaming surface and the aggregation becomes irreversible absorbed like a solid particle
        
        Solid particles can stabilise e.g. bread dough
        
        Drainage
        
        Immobilise air/water interface
        
        Increase viscosity
        
        Coalescence
        
        Whipping speed i.e. not to stretch the air bubble too much
        
        Thicker films e.g. using proteins to form thick adsorbed layer
        
        Reduce extraneous particles which cause the film to rupture
        
        Use surfactants like protein to increase repulsion between films
        
        Reduce foam concentration
  - - - Type: oil/water or water/oil
      - Droplet size distribution: Smaller drops generally give more stable emulsion
      - Volume fraction of dispersed phase
      - Composition and thickness of surface layer around the droplets
        
        determines interfacial properties and colloidal interaction forces.
        
        Colloidal interaction forces greatly affect physical stability
      - Composition of continuous phase: determines solvent conditions for the surfactant, pH and ionic strength and thereby colloidal interactions
    - - Drops are formed easily but to break it up into smaller droplets, additional energy is required
        e.g. homogeniser can produce droplet size of 0.2 m
        Surfactant will have to cover the droplets sufficiently, otherwise particles may coalesce again
    - - Creaming: further enhances aggregation
        
        Prevention: agitation can disturb creaming and aggregation
        E.g. homogenisation
      - Partial Coalescence
        Part of the oil in o/w emulsion can crystallize  affect emulsion stability i.e. rupture the droplet film
        Prevention: reduce droplet size, increase repulsion, and use surfactant
      - Aggregation: enhances creaming
      - Ostwald Ripening
        Mainly in w/o emulsions and usually happens during prolonged storage
        Prevention: adding a solute e.g. NaCl
      - Coalescence
        
        Mainly with o/w emulsions
        
        Induced by rupture of thin film between close droplets
        
        Prevention: reduce droplet size, having thicker film, and greater interfacial tension
    - - Fat globules needed to cover air bubbles to ensure stability against Ostwald ripening and coalescence of air bubbles
  - - - Changes in arrangement
        
        Aggregation/ Peptization
        
        It may greatly affect susceptibility of emulsion droplets to (partial) coalescence
        
        Particles exhibit Brownian motion i.e. frequency of collision
        
        Reverse aggregation occurs with lowering osmotic pressure, lowering ionic strength, and shear
        
        Densities of dispersed and continuous phases are in balance
        
        Sedimentation
        
        Decrease with viscosity of the continuous phase
        
        Increase with particle size, temperature
        
        If the dispersed phase density is smaller than the continuous phase --> creaming
      - Changes in particle size
        
        Ostwald Ripening
        
        Coalescence
        
        Dissolve/ Grow