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Water Importance 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 O H H d+ d+ d- d-

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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

O

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