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Physical States of Water

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

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Physical States of Water

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  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. Decompartmentalization 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

  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. For an ideal solution: n1 p ERH 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 aw = = = Water Activity (aw) Definition

  9. Water Activity of Selected Foods Food Product aw

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

  11. Instruments to Measure Water Activity

  12. Experimental Determination of aw

  13. Hysteresis: loss of H20 binding sites Hysteresis

  14. Effect of Temperature on the Sorption Isotherm

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

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

  17. 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

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

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

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