Download Water Activity

Physical States of Water
• Vapor
• Liquid: interaction with food
components
– Hydrogen bonding
– Covalent bonding
– Salt linkages
– Van der waal interactions
• Ice
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)
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
Decompartmentalization
Freezing Property Changes
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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
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
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)
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
Water Activity (aw) Definition
For an ideal solution:
n1
p
ERH
aw =
=
=
n1 + n2 p0
100
n1 = moles of solvent (water)
n2 = moles of solute
p = vapor pressure of solution
p0 = vapor pressure of solvent
ERH = equilibrium relative humidity
Water Activity of Selected Foods
Food Product
aw
Moisture Sorption Isotherm
Type II: hard to
remove H2O (Hbonding)
Moisture content
Type I: tightly
bound H2O
(monolayer)
aw
Type III: loosely
bound H2O
(available)
Instruments to Measure Water Activity
Experimental Determination of aw
Hysteresis
Hysteresis: loss
of H20 binding
sites
Effect of Temperature on the Sorption Isotherm
Moisture Equilibration Between Components
(Water Migration)
Consider products with more than one component
• Oreo cookie, Twinkies, Pizza with the works
Factors Influencing Water Activity
• Solute interactions
• Capillary suction forces
• Surface force interactions
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
Solutes and Humectants
• Sodium chloride
• Sugars (e.g., sucrose, glucose,
fructose)
• Sorbitol
• Glycerol
• Propylene glycol
Halophilic- likes a salty environment
Xerophilic- likes a dry environment
Osmophilic- likes high osmotic pressure