1. Physical States of Water
Vapor
Liquid: interaction with food components
Hydrogen bonding
Covalent bonding
Salt linkages
Van der waal interactions
Ice

2. 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)

3. 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

4. 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

5. 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

6. 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)

7. 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

8. Water Activity (aw) Definition
For an ideal solution:
n p ERH
n1 + n2 p
n1 = moles of solvent (water)
n2 = moles of solute
p = vapor pressure of solution
p0 = vapor pressure of solvent
ERH = equilibrium relative humidity
aw = = =

9. Water Activity of Selected Foods
Food Product aw

10. Moisture Sorption Isotherm
Type I: tightly bound H2O (monolayer)
Type II: hard to remove H2O (H-bonding)
Type III: loosely bound H2O (available) aw
Moisture content

11. Instruments to Measure Water Activity

17. Experimental Determination of aw

20. Hysteresis
Hysteresis: loss of H20 binding sites

22. Effect of Temperature on the Sorption Isotherm

23. Moisture Equilibration Between Components
(Water Migration)
Consider products with more than one component
Oreo cookie, Twinkies, Pizza with the works

24. Factors Influencing Water Activity
Solute interactions
Capillary suction forces
Surface force interactions

25. 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

26. Solutes and Humectants
Sodium chloride
Sugars (e.g., sucrose, glucose, fructose)
Sorbitol
Glycerol
Propylene glycol

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