Physical States of Water

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Physical States of Water

• Vapor
• Liquid interaction with food components
• Hydrogen bonding
• Covalent bonding
• Salt linkages
• Van der waal interactions
• Ice

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Crystallization of Water

• Nucleation formation of crystalline nuclei
• Low temperature and fast rate are important for
  the size and number
• Crystal growth
• Small with low temperature
• Large at high temperatures
• Recrystallization
• Problems with freeze-thaw cycles (e.g., frozen
  foods)

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Problems of Crystal Growth

• Large crystals are more stable than small ones
• Small crystals melt and water used for growth of
  large crystals
• Tissues can rupture with crystal growth
• Physical changes
• Enzyme activation
• Chemical destruction
• Storage is important

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Freezing Property Changes

• Changes in pH (due to salt precipitation)
• Increase in ionic strength
• Increase in viscosity
• Increase in osmotic pressure
• Decrease in vapor pressure
• Decrease in freezing point
• Increase in surface potential
• Change in oxidation-reduction potential

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Changes in pH Upon Freezing(e.g., phosphate
buffer)

• Monobasic MH2PO4 (acidic)
• When M Na, this form precipitates first and
  the pH increases
• Dibasic M2HPO4 (basic)
• When M K, this form precipitates first and the
  pH decreases

Changes in pH, brought about by freezing can
affect biochemical and chemical reactions, and
microbial growth
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Water Activity (aw)

• The amount of water available (unbound) for
  chemical and biochemical reactions, and for
  microbial growth to occur.
• Knowledge of water activity allows us to make
  predictions about food quality.
• Processing
• freezing, dehydration, concentration, salting,
  sugaring)

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Water Activity

• Determines direction of moisture transfer
• Most reaction rates increase with increasing
  water activity
• Most rates correlate better with water activity
  than moisture content
• Moisture sorption isotherms are useful

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Water Activity (aw) Definition
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Water Activity of Selected Foods
Food Product
aw
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Moisture Sorption Isotherm
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Instruments to Measure Water Activity
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Experimental Determination of aw
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Hysteresis
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Effect of Temperature on the Sorption Isotherm
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Moisture Equilibration Between Components (Water
Migration)

• Consider products with more than one component
• Oreo cookie, Twinkies, Pizza with the works

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Factors Influencing Water Activity

• Solute interactions
• Capillary suction forces
• Surface force interactions

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Control of aw in Foods

• Understand moisture sorption isotherms
• Equilibrate with atmosphere of lower or higher
  equilibrium relative humidity (ERH)
• Formulation approaches
• Add solute(s) (e.g., humectants)
• Anticaking agents (e.g., calcium silicate)
• Remove or add water
• Packaging approaches
• Select to minimize water permeation
• Resealable packages
• Handling instructions
• Change temperature

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Solutes and Humectants

• Sodium chloride
• Sugars (e.g., sucrose, glucose, fructose)
• Sorbitol
• Glycerol
• Propylene glycol

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Halophilic- likes a salty environment Xerophilic-
likes a dry environment Osmophilic- likes high
osmotic pressure
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