(3) water (djm 14) (1).pptx
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WaterImportance of Studying Water in Foods
Food Safety Microorganisms need water to grow Food Quality Chemical reactions depend on water content Physical properties depend on water content Food Cost
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HHd+d+d-d-
Molecular Structure of Liquid WaterO
HHd+d+d-d- Strong Attractive Forces (Hydrogen-bonds) Highly Directional (Tetrahedral) About 4 Hydrogen-bonds per MoleculeSystem Organized to Maximize H-bondsTetrahedral structure ofwater
Molecular Interactions & Organization
Hydrogen bondsPhysicochemical PropertiesBoiling point, melting point, density, viscosity, polarityChemical Reactivity
Oxygen has strongly positive nucleus(pulls electrons)
Water: Physicochemical PropertiesUnique Properties of Water: High boiling High melting point High heat of vaporization H2OCH4NH3MW (g/mol)181617m.p. (C)0-183-78b.p. (C)100-161-33DHV (kJ/mol)40.78.223.4
Properties relatedto strong hydrogen-bonding
Types of Water in FoodsCapillary water
MgSO47H2O
Bulk waterWater of crystallization
Trapped water
Physicallybound water
Chemicallybound water
Water in different environments has different molecular properties and therefore different physicochemical propertiesPhysical StatesGas vaporLiquid waterSolid ice
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Phase Behavior: Ice, Water and Steam
SolidLiquidGasWater exists in different states (solid, liquid, and gas) depending on temperature and pressure that have different structural organizations and interactions. The equilibrium behavior of water can be described by a phase diagram.Low EntropyStrong interactionsHigh EntropyWeak interactionsMedium EntropyMedium interactions
Phase Behavior: Ice Crystallization
Liquid water-to-solid ice transitionWhy does it happen?What factors affect it?
Importance of ice formation:PreservationMicrobial, Chemical, PhysicalQualityFlavor, Texture, Appearance
Ice Crystallization: ThermodynamicsThermodynamics The thermodynamically favorable physical state of water at a particular temperature and pressure is governed by the free energies of the states in question (which are determined by molecular interactions and entropy) Thermodynamics determines the maximum amount of crystallization that can occur under a particular set of conditions if the system can come to equilibrium.
IceWaterPhaseTransition(DG)
DG = 0
WaterIce
DGDG > 0T < TmT = TmMeltingCrystallization
IceWater
DGDG < 0T > Tm
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Ice Crystallization: Kinetics
Liquid water-to-solid ice transition:(1) Supercooling - Liquid water can be cooled appreciably below its melting point before crystallization occurs: DT = T - Tm(2) Nucleation Small clusters of water molecules, called nuclei, need to form and be stable before crystals can grow(3) Growth Water molecules join the existing crystal surfacesNucleiFormationCrystal Growth
WaterIce
DG*Transition is thermodynamicallyfavorable below Tm
NucleationGrowth
Ice Crystallization: Effect of Kinetics on Ice Crystal SizeFactors Affecting Crystal SizeThe size of the crystals formed depends on the relative rates of nucleation and growth. Faster Nucleation Rate Many nuclei are initially formed that grow slowly, which results in the formation of many small crystals. Faster Growth Rate A few nuclei are initially formed that grow quickly, which results in the formation of few large crystalsHigh viscosity slows diffusion processes at very low temperaturesLarge ice crystals cause quality problems, such as grittiness and iciness
Need to Rapidly Cool To Particular Temperature to Avoid Large Crystals
Solute-Water Interactions: Nature, Effects and ImportanceWater acts as a solvent for many solutesA solute is a substance that can be dispersed in a solvent (in this case water)The are many different kinds of solutes in foods, including carbohydrates, proteins, salts, acids, bases, surfactants
Importance:SafetyMicrobial contaminationQualityFlavor, Texture, AppearanceStabilityChemical & Physical
Molecular interactions:Water acts with solutes differently depending on their molecular characteristics, e.g., polarity, charge, shape.Effects:Water-solute interactions determine many of the physical and chemical properties of foods
Dissolution: ThermodynamicsEntropy of Mixing S > 0 - Always FAVORS MIXING (increases with higher T)
Enthalpy of Mixing If H < 0 - FAVORS MIXING If solvent-solute bonds are stronger than bonds within separate phases. If H > 0 - OPPOSES MIXING If solvent-solute bonds are weaker than bonds within separate phases.
Free Energy of Mixing The overall free energy is made of entropy and enthalpy termsIf G < 0 - FAVORS MIXING If G > 0 - OPPOSES MIXING
DG = DH-TDS
Separate phasesSolutionDissolution
PhaseSeparation
SoluteSolventWill a solute dissolve or not?MixedUnmixed
Functional Groups: Polar molecules have regions that have a partial charge, e.g., alcohols (-OH), amines (-NH2), & thiols (-SH) Examples: Water, Sugars, Alcohols, Amino acids, Aldehydes, KetonesMolecular interactions: The dominant interactions are:Fundamental: Dipole-dipoleCompound: Hydrogen bondsDissolution in Water: Polar Solutes
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HHd+d+d-d-
Dissolution in Water: Polar Solutes
Polar solutes normally have good solubility in water because solute-water interactions are fairly similar in strength to water-water interactions. Solubility depends on strength of interactions and solute compatibility with tetrahedral structure of water Molecular dimensions Bond orientations
SugarWater
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Solubilities & Water activities (aw) of saturated sugar solutions at 25C(Bussiere and Serpelloni, 1985)Dissolution in Water: Polar SolutesIngredient Solubility (%) aw
Sucrose67.4 0.844
Glucose51.0 0.891
Fructose 80.0 0.634
Lactose 18.7 0.931
Sorbitol 73.0 0.725
Mannitol 18.0 0.977
Sugars can have different solubilities in water because of different structures
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Dissolution in Water: Polar Solutes
SugarWater
d+d+d-d-Cavity in WaterTetrahedral structure
d+d+d-d-
d+d+d-d-
d+d+d-d-Correct Shape & Charge DistributionCorrect Shape; WrongCharge DistributionWrong Shape; CorrectCharge DistributionHigh SolubilityLow SolubilityLow Solubility
Sugar molecules vary in their shape, dimensions & bond orientations
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Dissolution in Water: Ionic Solutes
Cl-
d+2d-d+
Na+
Many ionic solutes have good solubility in water Ions can form strong ion dipole bonds with water Water close to ion is bound & therefore has different properties than bulk water Ions: Sign Magnitude Dimensions
Ion
Ion-orderedregion
Intermediatedisordered region
Water-orderedregion
StructureBreakerStructureMaker
Dissolution in Water: Ionic Solutes
Dissolution in Water: Ionic Solutes The Hofmeister Series Some ions alter water structure more effectively than others due to differences in their size and charge (which determines their charge density). Small ions with high charges are most effective since they have the highest charge density The interaction of water with ionic solutes alters the functionality of other ingredients in water, e.g., (NH4)2SO4 is used to precipitate proteinsIncreasesalting outIncreasesalting inHigh ChargeDensityLow ChargeDensityHigh ChargeDensityLow ChargeDensity-2-2++
Periodic Table
Proteins precipitate at high salt concentrations: The amount of salt required depends on protein type and salt type. Some salts are more effective at strongly binding water than others e.g. (NH4)2SO4.
HydratedProtein
No SaltLow SaltHigh SaltDissolution in Water: Ionic Solutes Salting Out
LimitedFree water
ProteinAggregation
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Non-polarsoluteWater moleculeshighly organized intetrahedral structure
Dissolution in Water: Non-Polar Solutes - The Hydrophobic Effect Most non-polar solutes have poor solubility in water Origin water molecules form strong hydrogen bonds with each other, but only weak VDW bonds with non-polar solutes
V. StrongWeakV. WeakDipole-dipoleVDWVDWMagnitudeType
Oil
Water
Hydrophobic Effect: Transfer of Oil Molecule to Water
Transfer Oil Molecule to WaterOverall: Replace strong hydrogen bonds with weak van der Waals bonds, which is thermodynamically unfavorableDGtransfer
Cavity FormationBreak strongHydrogenbondsForm weak VDV bondsIntroduce non-polar molecule into water
Cavity FormationBreak weak VDWbonds
Bulk Water Molecular Interactions: 3-3.5 H-bonds Entropy: Some disorder
Bound Water Molecular Interactions: 4 H-bonds Entropy: Highly ordered
Hydrophobic Effect: OriginEntropy change always unfavorableEnthalpy change depends on temperatureDG is positive (unfavorable) overall
Change molecular interactions and entropy
Hydrophobic Effect: Origin of Hydrophobic InteractionsDG = Free energy change due to hydrophobic effect (J)DA = Change in the contact area between non-polar groups and water (m2)g = Interfacial tension (J m-2)Reduced contact area between non-polar groups and water- Thermodynamically favorableAssociation of Non-polar groupsDG = gDA
Non-polar groupsWater
Hydrophobic Effect: Importance
SurfaceactivityProteinConformationBinding
Solubility
Structure formation
Lipid membranes and surfactant micellesStructure and transitions of globular proteinsBinding of polar lipids to starch helicesImmiscibility of oil and waterAdsorption of emulsifiers to oil droplets and air bubbles
Adding a solute to water changes its phase behaviorDissolution in Water: Influence on Physicochemical Properties of Water
Freezing point depressionBoiling point elevation
Greater possible disorder (entropy) of molecules in solution, than in pure liquid, therefore driving force for solidification or vaporazation is lessDG = DH - TDSLower disorderHigher disorder
GasSolution
S
WaterIn Solution:
Lower disorderHigher disorder
One phase(aqueous sucrose solution)Two phase(sucrose crystals + saturated aqueous sucrose solution)
Dissolution in Water: Phase behavior of sucrose-water
Room temp77%
x = mass fraction of crystalsCT = Total [solute]CS [solute] in saturated solutionCC = [solute] in crystal (=100%)Mass balance:The phase behavior of a solute-water mixture can be described by a phase diagram, which specifies the type of system formed under different conditions, such as composition and temperature
Pure ice
50% sucrose solution
Dissolution in Water: Influence of sucrose on ice formationCool
20% sucrosesolutionOne phase(aqueous sucrose solution)Two phase(Ice crystals + aqueous sucrose solution)
The phase behavior of a solute-water mixture can be described by a phase diagram, which specifies the type of system formed under different conditions, such as composition and temperature
Use of Sugars as Cryoprotectants: Freezing & Thawing0 wt% sucrose20 wt% sucroseHydrogenated palm oil-in-water emulsions stabilized by WPI (-40 C/40C) sucrose modifies ice crystal formation
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Water Activity: A parameter to characterize influence of water on food stability and propertiesProblem: Water is known to play an important role in determining food properties However, there is not a good correlation between total water content and food properties: Chemical reaction rates Microbial growth rates Physical properties A new parameter was needed to describe waters behavior Water Activity
UMASS Pilot Plant: 1988-1990>130 million pounds todayMicrobial stability: aw < 0.62& Moisture migration control
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Moisture content versus water activityCake: MC= 30%MC= 40% Will water move from the cake to the icing ?The answer is not sure30- because the moisture content does not predict water movement
Icing: MC= 15%Lili He
What can be used to predict water movement ?What cause the water movement?30
Water Activity: Thermodynamic Definition
P0P
Thermodynamic Definition: Ideal Situation (Equilibrium) aw = water activity fw = fugacity (escaping tendency) pw = partial vapor pressure (head space concentration)
Problem: Most foods are not at equilibrium!
FoodPure Water
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Water Activity: Practical Definition
PoP
Practical Definition: Real Situation (Non-Equilibrium) RVP = Relative Vapor Pressure pw = partial vapor pressureFoodPure Water
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Water Activity: Raoults LawParametersXwater = Mole fraction of water
nwater = Number of moles of water
nsolute= Number of moles of solute
AssumptionsIdeal Mixture All molecular interactions are equalSolution - Modify by activity coefficient: aw = gsXwater
SoluteSolventHow does solute concentration affect water activity?
Raoults Law
Water Activity: Moisture Sorption Isotherm
A moisture sorption isotherm provides information about how water interacts with a material, and how much available water is present
Moisture Sorption Isotherm
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Water Activity: Moisture Sorption Isotherm Influence of Solute Molecular WeightThe above graph shows the relationship between Raoults law approach and the moisture sorption isotherm approach (i.e. it ignores molecular interaction effects)The moisture sorption isotherm depends on the molecular weight of the solutes involved (since there are different moles of solute per 100 g of material)
Same massMore molesSame massLess moles
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Water Activity: Moisture Sorption Isotherm Influence of Molecular Interactions
A moisture sorption isotherm is highly dependent on the material being tested due to differences in the molecular weights of solutes, as well molecular interactions between water and the solute components.If it is assumed that the A, B & C have similar molecular weights, then the water solute interactions would be: A > B > C (since at the same water content, the water activity is much lower for A, which means the water is bound more tightly)Moisture Sorption Isotherms
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Water Activity: Moisture Sorption Isotherm ShapesMoisture sorption isotherms can usually be divided into three regions:Low water activity: Monolayer binding of water to molecular surfaces, e.g., potato chips, crackers, cookiesIntermediate water activity: Multilayer binding of water to molecular surfaces, e.g., breakfast cereals, rice, pasta, hard candy, chewing gum, raisonsHigh water activity: Free water due to saturation of molecular surfaces, e.g., jams and jellies, bread, milk, meat, yogurt, fruits
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Water Activity: Moisture Sorption Isotherm HysteresisA moisture sorption isotherm often depends on whether water is added to a material (adsorption) or removed (desorption) leading to hysteresisThermodynamics: The two curves should be the same.Kinetics: Hysteresis occurs due to kinetic phenomenon such as super saturation, crust formation or capillary formationWater ActivityMoisture Content
Water Activity: Approaches to controlling water migrationRaisin aw = 0.55Cereal aw = 0.1Water will tend to flow from raisins to cereal. To prevent: (i) Change driving force: e.g., add glycerol to lower aw of raisin.(ii) Create kinetic energy barrier: e.g., coat raisins with a material that prevents water flow (e.g., fat).
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DG*
DGThermodynamicallyFavorable StateTo prevent water-migration: (i) Thermodynamic approach: Change driving force by equaling aw. (DG 0)(ii) Kinetic approach: create an activation energy (energy barrier, DG*) to inhibit movement.
Water Activity: Influence on Chemical, Biochemical and Microbial Reaction RatesThe water activity of a food influences many important kinetic processes in foods:Chemical Reaction RatesMicroorganism growthEnzyme Activity
Water Activity
Water Activity: Influence on Chemical Reaction RatesThe chemical reactivity of water-soluble reactants depends on the water activity:Concentration decreases distance between reactantsHigh solute concentrations causes restricted molecular diffusion
Concentration closer togetherRestricted mobility slower movement
Water Activity: Influence on Physical Properties
Candy Floss
Cookies & Crackers
Potato & Tortilla Chips
CerealsWater activity plays a major role in determining their physical properties, such as texture (crispiness, crunchiness)
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Crystalline and Amorphous Solids: Small Molecules
Glassy stateMetastableLow molecular mobilityDisordered packingJammed Highly BrittleCrystalline stateThermodynamically stableLow molecular mobilityHighly ordered packingElastic, strong
Crystalline and Amorphous Solids: Polymers
Rubbery stateHigher molecular mobilityDisordered packingPliable (Rubbery)Glassy stateLow molecular mobilityDisordered packingJammedBrittle (Glassy)Crystalline stateLow molecular mobilityHighly ordered packingElastic, strong
Glass-Rubbery Transitions: Temperature and Water
Rubbery stateGlassy state
Rubbery stateWaterHeat
Water Activity: Glass TransitionsRubbery state(Soggy)
Water Activity: Glass Transitions
glassy state
rubbery state
Acids, Bases, & Buffers
Importance of pH in foodsInfluences flavor tartness, sourness Influences stability and reactivity Physical, chemical & microbial Influences texture & appearanceAggregation, gelation
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Acid-base equilibria: Water
H2OH3O+ + OH-
H2OH+ + OH-Actual:Simplified:Dissociation Equilibrium Constant: Kw = 10-14
Acid-base equilibria: pH
H2OH+ + OH-
pH[H+][OH-][OH-][H+]110-110-1310-14210-210-1210-14310-310-1110-14410-410-1010-14510-510-910-14610-610-810-14710-710-710-14810-810-610-14
Acid-base equilibria: Buffers
AH + H2OA- + H3O+B + H+BH+Acid:Base:What fraction of a weak acid or base dissociates in water?A strong acid/base fully dissociates (e.g., HCl or NaOH)A weak acid/base partially dissociates (e.g., -COOH or -NH2)How does this change the pH of the resulting solution?
UCDavis, ChemWiki
AHA- + H+Actual:Simplified:[conjugated acid]1[conjugated base]2[conjugated acid]2[conjugated base]1
How does charge change with pH?
AHA- + H+Deprotonated formProtonated form
Property of the molecule
Property of the solution
Henderson-HasselbalchInformation of fraction of molecule protonated or deprotonated
AHA- + H+How does charge change with pH?
Acid-base equilibria: Buffers
A buffer is a weak acid or base that can retard the change in pH when acid or base is added
The buffer capacity (in the alkali direction) is defined as the number of moles of OH- that must be added to one liter of buffer in order to increase the pH by 1 unit.b=1/Slope
Titration of a strong acidTitration of a weak acidpKa = 5
Acid-base equilibria: Buffer Capacity
The buffer capacity (in the alkali direction) is defined as the number of moles of OH- that must be added to one liter of buffer in order to increase the pH by 1 unit.A buffer works best at pH values close to its pKa value.
Food Acids, Bases & Buffers AcidSteppKaAcidSteppKaOrganic AcidsInorganic AcidsAcetic14.75Carbonic16.37Citric13.14210.2524.77o-Phosphoric12.1236.3927.21Fumeric13.03312.6724.44Pyrophosphoric10.85Lactic13.0821.49Malic13.4035.7725.1048.22Propionic14.87Sulfuric1-3.0Succinic14.1621.9225.61BaseSteppKaTartaric13.22Ca(OH)211.424.8222.43NaOH10.2
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ElectrostaticRepulsion(Soluble)Acid-base equilibria: Effect on FunctionalityProtein solubility+
+--VDW + Attraction(Insoluble)-NH3+-NH2-CO2--CO2HElectrostaticRepulsion(Soluble)
Milk Curds
Acid-base equilibria: Effect on Functionality
Filamentous High WHC Transparent ElasticParticulate Low WHC Opaque RubberyElectron MicroscopypH