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Week 2: Water, Forms - Coggle Diagram

- - - - Mm focuses on microviscosity and diffusibility of chemicals in foods and is dependent on water and its properties
      - Mm decreases with decreasing temperature and/or molecular size
  - - - The solid lines depicts true equilibrium situations – thermodynamically defined
      - Heavy dashed lines represent conditions of metastable state – kinetically defined
      - Three key points
        
        Melting point line TmL
        
        Saturation line Tms
        
        Intersection point (Eutectic point) TE – temperature at which saturation of the least soluble solute is achieved
      - Tg is the glass transition curve
      - S.S solution (supersaturated solution) region – where solute is soluble and no crystallisation
      - TE to Tg’ represents the highest concentration of solute in solution (supersaturated) that can be achieved by crystallisation of ice
  - - - Frozen Foods
        Melting point curve (Tml) is of importance
        Major equilibrium curve can be easily determined and sufficient for commercial use
      - Dry and semi dry food
        Tms is the major equilibrium curve of importance
        E.g. sucrose- water to predict the behaviour of cookies during baking and storage
  - - - pH, titratable acidity, ionic strength, viscosity, freezing point, surface and interfacial tension, oxidation-reduction potential
    - - Lowering temperature decrease reaction rates
      - Freeze-concentration increases reaction rates
    - - All water converted to ice increases 9% in volume
      - Nonaqueous constituents become concentrated in unfrozen phase
    - - Most stable freezing path: A – B – C – D- E – Tg’ – F
      - State dry freezing path: A – B – C – D – E – Tg’ – F – G
    - - Further ice formation (cooling) leads to metastable super saturation of many solutes and compositional change of unfrozen fraction from path TE to E
        
        Point E is the recommended storage temperature (-20C) for most frozen foods
        
        If cooling continues below Point E, additional ice formation, freeze concentration occur, causing composition of the unfrozen fraction to change from E to that at Tg’
        
        At Tg’, most of the supersaturated unfrozen phase converts to a glass encompassing the ice crystals
        
        Further cooling caused no further freeze-concentration, simply removal of sensible heat and alteration of product temperature in the direction of point F
  - - - Starting at Point A, air drying elevate product temperature and remove moisture
        
        The product will acquire properties similar with point H (wet bulb temperature of air)
        
        Moisture removal will cause product to arrive at and pass through I, the saturation point of the dominating solute (DS) present, with little or no solute crystallisation
        
        As drying continues to point J, product temperature approaches the dry bulb temperature of air
        
        If drying is terminated at point J, the product is cooled to point K.
        
        The product will be above the glass transition curve
        
        2 more items...
        
        If drying is continued from Point J to Point L, the product is then cooled to Point G
        
        It will be below the Tg curve
        
        2 more items...
  - - - The state diagram is used to estimate relative shelf life and to show stabilities of diffusion-dependent properties of food
      - Liquid state is the least stable
      - Glass state is the most stable
      - The middle region (between the liquid and glass state) is moderately stable, and highly dependent on temperature
- - - - Hydrogen atoms are covalently bonded to oxygen atom
    - - (Reason) Two positive charge from two hydrogen (H+) atoms balanced with two negative charges from oxygen atom (O2-)
      - (BUT) Not completely neutral
        
        Has a negative pole and a positive pole making it dipolar
        
        Dipolar molecules have poles with partial charges that oppose each other
  - - - Tetrahedron configuration : Nearly tetrahedral arrangement of the orbital about the oxygen atom allows each water molecule to form hydrogen bonds with four of its neighbour
        
        Each HOH acts as a donor to two of the four water molecules and as a hydrogen acceptor for the remaining two
  - - - The crystal lattice of ice occupies more space than the same number of H2O molecules in liquid water
        
        The structure spaces molecules further apart than they are in liquid
        
        The density (g/vol) of ice is thus less than that of liquid water. Ice floats on water
      - Ice crystals are near perfect
      - Ice is not static and is homogenous
      - Water molecules in ice are stationary
    - - 3 Results
        
        Increase in distance between nearest neighbours (decreased density)
        
        Increase in average number of nearest neighbour (increased density)
        
        Highly dynamic hydrogen bonds facilitate the mobility and fluidity of water
      - Into gaseous water vapour
        
        At 100 C, one-third of the hydrogen bonds are still present
        All hydrogen bonds are broken when water changes into vapour at 100 C
- - - - Most foods are prone to non-enzymatic browning (Maillard reaction). Maillard reaction reaches its peak at aw of 0.6-0.7, reactions reduces with further increase in aw
      - Oxidation of fats or spontaneous autocatalytic breaking of fat chains increases at high aw values. Oxidation of fats increases sharply at aw values below 0.2.
- - - - Zone I
        
        Water-ion and water-dipole interactions
        
        Unfreezable at -40C
        
        Has no ability to dissolve solutes
        
        Water is strongly sorbed and least mobile
      - Zone I & II junction (grey area) = BET monolayer: Monolayer of water bound to highly polar groups of dry matter
      - Zone II : Associates with neighbouring water molecules and solute molecules primarily by H bonding
        
        Is less mobile than bulk water (free water)
        
        Mostly unfreezable at -40C
      - Zone II & Zone III junction (grey area)
        
        Enough water to form a complete hydration shell of macromolecules
      - Zone III
        
        Bulk phase water (>95%), which are freezable
        
        Bulk phase water of Zone III, either trapped or free constitutes more than 95% of total water in high moisture food
        
        Increase molecular mobility
      - Zone I and Zone II usually constitutes less than 5% of water in high moisture food materials
    - - Addition of water to high moisture content materials will not have a significant properties on water originally present.
      - Addition of water to a dry material containing a few water molecules will increase the mobility and lessen the residence time of these original water molecules
    - - At any given Aw, the water content of desorption is greater than resorption
      - Desorption is removal of water
        
        Desorption isotherms are useful for drying process e.g. milk powder manufacture
      - MSI is temperature dependent (≥ 80C no hysteresis)
      - Resorption/ adsorption is addition of water
        
        Adsorption isotherms are useful for hygroscopic products e.g. storage of powders
        
        Steeply sloping curve – food materials is hygroscopic
        
        Flat curve – food material not sensitive to moisture
      - Magnitude of hysteresis, the shape of the curve, the inception and termination points of the hysteresis loop can vary depending on
        
        5 Factors
        
        Nature of food
        
        Physical changes it undergoes when water is removed or added
        
        Temperature
        
        Rate of desorption
        
        Degree of water removal during desorption