Module 1 Water

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Module 1Water

• Food Chemistry 2
• ND Food Technology

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Table of Contents

• Introduction and uses of water in food
• Structure of water
• Water activity
• Sorption phenomena
• Types of water
• Freezing and ice structure
• Crystal growth
• Glass transition
• Water activity and reaction rate
• Water activity and food spoilage
• Water activity and packaging

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3. Water activity (aW)

• Moisture content () given on product labels does
  not indicate the ability of the water to take
  part in different reactions
• aW defined as RH, which must prevail in the
  surrounding atmosphere to avoid water exchange
  between material and air the ratio of the vapor
  pressures of pure water and a solution
• Where
• p partial pressure of water in a food
• p0 vapor pressure of water at the same
  temperature
• pequ partial pressure of water vapor in equ
  with the food at temperature T and 1 atmosphere
  total pressure
• psat saturation partial pressure of water in
  air at the same temperature and pressure

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3. Water activity (aW)

• High moisture content (exceeding solids) has aW
  close to or 1
• Moisture content lower than solids has aW lt1
• graph (Fig.1.6, p. 6)

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4. Sorption isotherms

• Sorption Isotherms presents the relationship
  between water content and relative humidity
• graph (Fig. 1.7, p. 6)
• Adsorption isotherm ? hygroscopic products (steep
  slope in graph small increase in relative
  humidity causes large increase in moisture
  content, e.g. foods with high salts or sugar
  contents)
• Desorption isotherm ? drying process
• Sorption isotherms have a sigmoid shape 3 areas
  (monolayer, additional layers, condensation on
  capillaries)
• correspond to different conditions of water
  present in the food

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5. Types of water

• Sorption isotherm indicates 3 different forms of
  water
• Langmuir (monolayer)
• Capillary
• Loosely bound
• Bound water attracted strongly (bound water)
  water unavailable as a solvent, cannot freeze
  below 0C, no vapor pressure
• Free water can extract by pressing the food
  sample between filter paper/by centrifugation
• Entrapped water (e.g. pectin gels, fruit) is not
  bound water, but water immobilized in
  capillaries/cells

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6. Freezing and Ice Structure6.1. Crystal growth

• Hexagonal crystal lattice in ice
• water molecule bound to 4 others in a
  3-dimensional tetrahedral arrangement
• loosely built
• large hollow spaces
• high specific volume (less dense than water)
• A conversion of disordered liquid structure into
  ordered crystal formation
• When ice melts, H-bonds are broken
• Phase diagram of water graph (Fig. 1.18, p.
  15)
• gas, liquid and solid
• tripple point

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6. Freezing and Ice StructurePhase diagram
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6.1. Crystal growth

• Entropy a measure of disorder in a system.
  Entropy will be lowest in the solid phase
  (ordered), become higher when heat is added to
  form the liquid phase and then highest in the
  vapor phase
• Sublimation basis for freeze drying
  (lyophilisation)

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6.1. Crystal growth6.1.1. Freeze drying
(lyophillisation)

• removing water from a product by sublimation and
  desorption
• For valuable and sensitive products
• Advantages Product shape is maintained, aroma
  is not lost, vitamins (nutritional value) are
  maintained, all unwanted processes is inhibited,
  product can be stored at room temperature in a
  sealed package.
• Disadvantages Method is very expensive in terms
  of design, energy and servicing of freeze dryers
• Food is frozen at very low temperatures (-25 to
  -30C) until all the water forms ice (frozen out)

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6.1.1. Freeze drying (lyophillisation)

• Pressure is reduced and heat supplied to the food
  in an amount sufficient for sublimation of the
  ice.
• End product has moisture content of 2-6
  (uninterrupted monomolecular film)
• Components of freeze dryer
• drying chamber condenser (remove water) cooling
  system vacum chamber
• Pressure must always be low enough for water to
  be in the gas/solid phase (pressureltvapor
  pressure)
• The temperature of the food should never rise and
  the ice should never thaw

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6.1.2. Supercooling

• Slow cooling results in large ice crystals
  found only in extracellular areas
• Supercooling To freeze water with very low
  temperatures (less than freezing point).
• causes high nuclei formation, smaller crystal
  (extra- and intracellularly) growth rate and very
  fine crystals of ice.
• The higher the rate of supercooling, the lower
  the speed of crystalisation

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6.1.3. Seeding

• Seeding After supercooling, adding nuclei in
  liquid systems
• e.g. Margarine - adding finely ground lactose to
  evaporated milk in the evaporator and
  recirculating some portion of crystallized fat in
  a heat exchanger.
• The greater the amount of nuclei, the smaller the
  size of the crystals.

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6.1.4. Freezing of food

• Water forms ice with high purity and the solute
  concentration of the unfrozen solute is
  increasing in concentration.
• Changes in pH, ionic strength, viscosity, osmotic
  pressure, vapor pressure.
• Water can expand about 9 when frozen
• Freezing can make product unacceptable
• destabilization of emulsions
• flocculation of proteins
• increase in fish flesh toughness
• loss of textural integrity
• drip loss in meat
• presence of glass forming substances may help
  prevent the above

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6.2. Glass transition

• When an aqueous system containing low molecular
  weight materials like sugars other carbs, is
  cooled to a temperature much lower than its
  melting point. This forms
• An amorphous solid glass material rather than ice
  crystals
• An undercooled liquid of high viscosity that
  exists in a metastable solid state
• Unlike crystals, in glass formation the
  disordered liquid forms a disordered glassy solid
• The reological properties of a solid, but in
  disordered form.

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6.2. Glass transition

• Moisture content in a product plays a large role
  in the transition to the glassy state
• When water is rapidly removed from foods during
  extrusion, drying or freezing, the glassy state
  may also be produced
• Water can act as a plasticizer - it increases
  plasticity and flexibility of food polymers by
  weakening the intermolecular forces
• Example When small amounts of water is added to
  dried fruit, the Tg is lowered to room
  temperature leading to structural collapse and
  stickiness.

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6.2. Glass transition

• Cryoprotectin
• The stabilization of frozen products.
• When water containing foods are cooled below
  freezing point of water, ice may form and the
  remaining water will have increased dissolved
  solids. Ice can destabilize sensitive products
  by rupturing cell walls and breaking emulsions.
• When the Tg is reached the remaining water will
  form glass, which protects the product against
  damage by ice crystals.
• Agents used for this purpose is called
  cryoprotectants, e.g. dimethyl sulphoxide,
  sorbitol
• Graph (Fig. 1.21, p. 19)

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6.3. Water Activity and Reaction Rate

• aW large effect on chemical reactions and
  microbial growth rate
• aW 0-0.2 (monolayer water) no enzyme
  activity, no microbial growth, no non-enzymatic
  browning, high lipid oxidation
• aW 0.65 osmophillic yeasts can grow
• aW 0.7-0.8 (capillary water) molds and yeasts
  start to grow
• aW 0.8 (limit of loosely bound water)
  bacterial growth starts
• aW 0.3 (min) -0.8 (max) lipid oxidation
• aW 0.3-0.8 enzyme activity slow (lipases)
• aW 0.8-1.0 (loosely bound water) enzyme
  activity rapid (amylases, phenoloxidases,
  peroxidases)
• look at graph in notes

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7. Water Activity and Food Spoilage

• Look at sorption isotherm graph
• Dried/freeze dried foods 5-15 moisture
  (monolayer category) have great storage ability
• Dried foods (e.g. dates) 20-40 moisture (mono
  multilayer category) medium storage stability
• Intermediate moisture foods moisture above 50
  (aW gt0.5) (including capillary water)
• aW can be reduced by drying or adding
  water-soluble substances (e.g. sugar to jams,
  salt to pickeled preserves)

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7. Water Activity and Food Spoilage7.1.
Reactions influenced by aw

• 1) Maillard Reaction
• A non-enzymatic browning reaction NB factor in
  food spoilage
• Dependent on aw max rate at aW 0.6-0.8 (e.g.
  milk powder _at_ 40C for 10 days (graph Fig.
  1.29), loss of lysine parallels the colour
  change)
• Browning slow at low humidities, become max in
  intermediate-moisture foods and decreases with
  high-moisture foods. Reason In the
  intermediate range all reactants are desolved,
  and further increase in moisture leads to the
  dilution of the reactants.

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7.1. Reactions influenced by aw

• 2) structural changes
• Powders (milk powder) are equilibrated at 50 RH,
  the microporous structure is destroyed and free
  fat content increased
• 3) hydrolysis of protopectin
• 4) splitting and demethylation of pectin
• 5) autocatalytic oxidation of fats
• 6) transformation of chlorophyll into pheophytin

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8. Water Activity and Packaging

• Packaging materials play large role in keeping
  quality of foods
• Sorption isotherms play NB role
• Hygroscopic products (steep isotherm) reach
  critical moisture content before reaching
  external climatic conditions need glass
  containers with moisture-proof seals/watertight
  plastic (polyvinylchloride). E.g. coffee cakes
  and loses flowability at critical RH of 50
• Other products (non-hygroscopic) have no
  reactions under normal storage conditions can
  store in polyethylene containers

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8. Water Activity and Packaging

• Where ERH is above external climatic conditions
  (e.g. processed cheese, baked goods) the
  packaging should protect the product from
  moisture loss
• Problems may arise in e.g. soup mixes where
  different ingredients are packaged together in an
  impermeable package, graph (Fig. 1.32). Starch
  (13 moisture) freeze dried veg (2 moisture) ?
  veg moistened to 9 ? Maillard. Solution
  postdry the starch first.

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8. Water Activity and Packaging

• Waterbinding is affected by the addition of salts
  (phosphates)
• decreases cooking losses, better structure and
  consistency in e.g. manufacturing sausages.
• Use of water as plastisizer in mixed food systems
  is NB. Water should be retained during shelf
  life of low fat products. Also fat-replacers
  e.g. carbs proteins is NB to react with water
  (water binding) in low fat products.
• Process requirements for foods (GMP) classifies
  foods according to aW and pH.