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Molecular Structure of Liquid Water

OH

H

δ+

δ+

δ−

δ−

•  Strong Attractive Forces (Hydrogen-bonds•  Highly !irectional ("etrahedral•  Abo#t $ Hydrogen-bonds per Molec#le System Organized to Maximize H!onds

"etrahedral

str#ct#re o% 

water 

Molec#lar &nteractions ' OrganiationHydrogen

 bonds

"#ysicoc#emical

"roperties• )oiling point* melting

 point* density* viscosity*

 polarity

• Chemical +eactivity

Oxygen has

 strongly positive

nucleus

(pulls electrons)

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Water: PhysicochemicalProperties$nique "roperties of Water%•  High boiling•  High melting point•  High heat o% vaporiation

H&O CH' (H)

M, (gmol ./ .0 .1

m2p2 (3C 4 -./5 -1/ b2p2 (3C .44 -.0. -55

∆H6 (78mol $421 /29 952$

Properties related

to strong hydrogen- bonding

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ypes o a er nFoods

Capillary water 

MgSO$:1H

9O

)#l7 water 

,ater o% crystalliation

"rapped water 

Physically

 bo#nd water Chemically

 bo#nd water 

,ater in di%%erent environments has di%%erent molecular properties and there%ore di%%erent

p#ysicoc#emical properties

"#ysical States• Gas – vapor 

•  Liquid – water 

•  Solid – ice

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Phase Behavior:Ice, Water and Steam

Solid

Liquid

*as

,ater e;ists in di%%erent states (solid* li

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Phase Behavior: IceCrystallization

Liquid +atertosolid ice transition

,hy does it happen?

,hat %actors a%%ect it?

Importance of ice formation%

• "reser,ation•  Microbial, Chemical, Physical 

•Quality•  Flavor, Texture, Appearance

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Ice Crystallization% -#ermodynamics

-#ermodynamics

•  "he thermodynamically %avorable physical state o% water at a partic#lar temperat#re and press#re is

governed by the %ree energies o% the states in 0 T < Tm

 T = Tm

 Melting 

Crystallization

Ice

Water

∆G   ∆G < 0

 T > Tm

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Solute-Water Interactions:

Nature, Efects an I!"ortance

Water acts as a sol,ent for many solutes

• A solute is a s#bstance that can be dispersed in a

solvent (in this case water

• "he are many di%%erent 7inds o% sol#tes in %oods*

incl#ding carbohydrates* proteins* salts* acids*

 bases* s#r%actants

Importance%• Safety

•  Microbial contamination

• Quality

•  Flavor, Texture, Appearance

• Sta!ility

• Chemical Physical 

Molecular interactions%

• ,ater acts with sol#tes di%%erently

depending on their molec#lar

characteristics* e!g!, polarity*

charge* shape2

7ffects%

• ,ater-sol#te interactions

determine many o% the physical

and chemical properties o% %oods

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8issolution% "hermodynamics

7ntropy of Mixing

9S : ; Always F3 ; F3 ; F3

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Functional *roups@ Polar molec#les have regions thathave a partial charge* e!g!, alcohols (-OH* amines (-H9*

' thiols (-SH

7xamples% ,ater* S#gars* Alcohols* Amino acids*Aldehydes* etones

Molecular interactions% "he dominant interactions are@Fundamental@ !ipole-dipole

Compound@ Hydrogen bonds

8issolution in Water% Polar Sol#tes

OH

H

δ+

δ+

δ−

δ−

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8issolution in Water% Polar Sol#tes

•  "olar solutes normally have good solu!ility in water beca#se sol#te-

water interactions are %airly similar in strength to water-water

interactions2•  Sol#bility depends on strength o% interactions and sol#te compatibility

with tetrahedral str#ct#re o% water •  Molecular dimensions

•  ?ond orientations

S#gar ,ater 

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Solu!ilities @ Water acti,ities .aw/ of saturated sugar solutions at &ABC

!"ssiere and Serpelloni, #$%&'

#issolution in Water: Polar Sol"tes

In$reient Solu%ility &'( aw 

Sucrose 012$ 42/$$

*lucose G.24 42/.

Fructose /424 4205$

Lactose ./21 425.

Sor!itol 1524 4219G

Mannitol ./24 4211•  S#gars can have

di%%erent

sol#bilities in water

 beca#se o% di%%erentstr#ct#res

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#issolution in Water: Polar Sol"tes

S#gar 

,ater 

δI   δI

δ-δ-

Ca,ity in Water-etra#edral structure

δI   δI

δ-δ-

δI

δI   δ-

δ-δI   δI

δ-δ-

Correct S#ape @

C#arge 8istri!ution

Correct S#ape Wrong

C#arge 8istri!ution

Wrong S#ape Correct

C#arge 8istri!ution

Hig# Solu!ility Lo+ Solu!ility Lo+ Solu!ility

S#gar molec#les vary in

their shape* dimensions '

 bond orientations

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8issolution in Water% &onic Sol#tes

Cl-

δI

2δ-δI

 aI

•  Many ionic solutes #a,e good solu!ility in +ater

•  Ions can form strong ion1 dipole !onds +it# +ater

•  Water close to ion is 5!ound6 @ t#erefore #as different properties t#an !ulD

+ater

 Ions%

•  Sign

• Magnit#de

•  !imensions

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&on

&on-ordered

region

&ntermediate

disordered region

,ater-ordered

region

Structure

?reaDer

StructureMaDer

8issolution in Water% &onic Sol#tes

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•  "roteins precipitate at #ig# salt concentrations%

"he amo#nt o% salt re

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 on-polar 

sol#te

,ater molec#les

highly organied in

tetrahedral str#ct#re

8issolution in Water% (on"olar Solutes

-#e Hydrop#o!ic 7ffect

•  Most non-polar sol#tes have poor sol#bility in water 

•  Origin – water molec#les %orm strong hydrogen bonds with each

other* b#t only wea7 6!, bonds with non-polar sol#tes

! $trong 

ea# 

! ea# 

 &ipole*+ipole

& 

& 

Magnitude -ype

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Oil

Water

Hydrop#o!ic 7ffect%

"rans%er o% Oil Molec#le to ,ater 

Transer Oil Molecule

to ater 

O,erall% +eplace strong

hydrogen bonds with wea7 van

der ,aals bonds* which is

thermodynamically #n%avorable

∆Gtransfer

Cavity

 Formation

 -rea# strong  %y+rogen

bon+s

 Form weak

& bon+s

 Introduce non-polar

molecule into water 

Cavity

 Formation

 -rea#  weak

& 

bon+s

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?ulD Water•  Molecular Interactions%• .*.!/ %*bon+s

•  7ntropy% $ome +isor+er 

5?ound6 Water•  Molecular Interactions%• 0 %*bon+s

•

 7ntropy%  %ighly or+ere+ 

Hydrop#o!ic 7ffect% Origin

• >ntropy change always #n%avorable• >nthalpy change depends on temperat#re

G is positi(e "nfa(ora)le' o(erall

Change

molecular

interactions an+

entropy

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Hydrop#o!ic 7ffect% Origin o% Hydrophobic

&nteractions

•   ∆G = Free energy change d#e to hydrophobic e%%ect (8•   ∆* = Change in the contact area between non-polar gro#ps and water (m9•

  γ  = &nter%acial tension (8 m-9

 1e+uce+ contact area

bet"een non*polar groups an+ "ater 

* Thermo+ynamically

 avorable

 Association o

 2on*polar groups

∆

G )γ∆

*

 2on*polar

 groups

ater 

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*ddin+ a sol"te to ater chan+es its phase )eha(ior

8issolution in Water% Influence on "#ysicoc#emical"roperties of Water

Freezing point depression ?oiling point ele,ation

*reater possi!le disorder

.entropy/ of molecules in

solution4 t#an in pure liquid4

t#erefore dri,ing force for

solidification or ,aporazation

is less

∆4  ∆ %  - T ∆$ 

Lo+er

disorder

Hig#er

disorder

*as

Solution

S

Water

&nSol#tion@

Lo+erdisorder

Hig#erdisorder

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One p#ase

(a

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P#re ice G4J s#crose

sol#tion

#issolution in Water: In+uenceo sucrose on ice or!ation

Cool

94J s#crose

sol#tion

One p#ase

(a

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se o Su$ars asCryo"rotectants:

reezin$ . /ha0in$

; +tJ sucrose &; +tJ sucrose

Hydrogenated palm oilin+ater emulsions sta!ilized !y W"I .';

KC';KC/ 1 sucrose modifies ice crystal formation

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Water *ctivity:* parameter to characterie in1"ence of ater on food sta)ility and properties

"ro!lem%

•  ,ater is 7nown to play an important role in determining %ood properties•  However* there is not a good correlation between total water content and

%ood properties@•  Chemical reaction rates•  Microbial gro"th rates•

 Physical properties•  A new parameter was needed to describe waters behavior 

•  Water 3cti,ity$M3SS "ilot "lant% 0G0GG;

.54 million po#nds today

Microbial stability@ aw N 4209' Moist#re migration control

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1oisture content versus 0ater activity

Ca7e@ MC 54JMC $4J

 ill "ater move rom the ca#e to the icing 5

"he answer is not s#re

54

- beca#se the moist#re content does not predict

water movement

&cing@ MC .GJ

=ili He

 sample

"ater 

m

m MC    ×=.44

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Water *cti(ity2 Thermodynamic 3e.nition

P4P

-#ermodynamic 8efinition%•   &deal Sit#ation (>

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Water *cti(ity2 Practical

3e.nition

PoP

"ractical 8efinition%•  +eal Sit#ation (on->

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Water *cti(ity2 4ao"lt5s 6a

"arameters  X +ater  Mole fraction of +ater

n+ater  (um!er of moles of +ater

nsolute (um!er of moles of solute

3ssumptions

Ideal Mixture 1 All molec#lar interactions are e

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Water *ctivity: 7oist"re SorptionIsotherm

* !oisture sor"tion isother! pro(idesinformation a)o"t ho ater interacts ith amaterial, and ho m"ch a(aila)le ater is present

Moisture Sorption Isot#erm

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Water *ctivity: 1oisture Sor"tion Isother! 2In1"ence of Sol"te 7olec"lar Wei+ht

"he above graph shows the relationship between +ao#lts law approach and themoist#re sorption isotherm approach (i!e! it ignores molec#lar interaction e%%ects

"he moisture sorption isot#erm depends on the molec#lar weight o% the sol#tesinvolved (since there are di%%erent moles o% sol#te per .44 g o% material

• Same mass

•More moles

• Same mass

• =ess moles

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Water *ctivity: 1oisture Sor"tionIsother! 8 In1"ence of 7olec"lar Interactions

* !oisture sor"tion isother! is hi+hly dependent on the material)ein+ tested d"e to di/erences in the molec"lar ei+hts of sol"tes, asell molec"lar interactions )eteen ater and the sol"te components9

If it is ass"med that the *, ! : ; ha(e similar molec"lar ei+hts, then theater 8 sol"te interactions o"ld )e2 * > ! > ; since at the same atercontent, the ater acti(ity is m"ch loer for *, hich means the ater is)o"nd more ti+htly'

Moisture Sorption Isot#erms

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Water *ctivity: 7oist"re SorptionIsotherm Shapes

1oisture sor"tion isother!s can "s"ally )e di(ided into three re+ions2I3 4o0 0ater activity2 7onolayer )indin+ of ater to molec"lar s"rfaces, e9+9,

potato chips, cracers, cooiesII3 Inter!eiate 0ater activity2 7"ltilayer )indin+ of ater to molec"lar

s"rfaces, e9+9, )reafast cereals, rice, pasta, hard candy, chein+ +"m, raisons

III3 5i$h 0ater activity2 Free ater d"e to sat"ration of molec"lar s"rfaces, e.g., ams and ellies, )read, mil, meat, yo+"rt, fr"its

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Water *ctivity: 7oist"re Sorption

Isotherm Hysteresis

* !oisture sor"tion isother! often depends onhether ater is added to a material adsorption'or remo(ed desorption' leadin+ to hysteresis Thermodynamics2 The to c"r(es sho"ld )e the same9 Kinetics2 Hysteresis occ"rs d"e to inetic phenomenon

s"ch as s"per sat"ration, cr"st formation or capillaryformation

Water 3cti,ity

   M  o   i  s   t  u  r  e

   C  o  n   t  e  n   t

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,ater Activity@ Approaches to controlling

water migration+aisin aw  42GG

Cereal aw  42.

Water +ill tend to flo+ from raisins to

cerealE -o pre,ent%

(i Change driving %orce@ e!g!, add

glycerol to lower aw o% raisin2

(ii Create 7inetic energy barrier@ e!g!,

coat raisins with a material that prevents

water %low (e!g 2* %at2

5

∆G*

∆G

Thermo+ynamically

 Favorable $tate

-o pre,ent +atermigration%

(i -#ermodynamic approac#% Change

driving %orce by e

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Water *ctivity: In1"ence on ;hemical,

!iochemical and 7icro)ial 4eaction 4ates

/he 0ater activity o a oo in+uences !anyi!"ortant 6inetic "rocesses in oos: ;hemical 4eaction 4ates7icroor+anism +roth nyme *cti(ity

Water 3cti,ity

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Water *ctivity: In1"ence on;hemical 4eaction 4ates

/he che!ical reactivity o 0ater-solu%le reactantse"ens on the 0ater activity: ;oncentration 8 decreases distance )eteen reactants

Hi+h sol"te concentrations ca"ses restricted molec"lardi/"sion

Concentration

 – closer together 

 1estricte+ mobility

 – slower movement

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Water *cti(ity2In1"ence on Physical Properties

Candy Floss CooDies @ CracDers

"otato @ -ortilla

C#ips

Cereals

Water acti,ity plays a maPor role in determining t#eir

p#ysical properties4 suc# as texture .crispiness4

crunc#iness/

1oisture Gain

Elastic 1oulus

*lassyState

.Crispy/

7u%%er

y

State

&So$$y(

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Crystalline an *!or"hous

Solis: S!all 1olecules

*lassy state•

Metastable• =ow molec#lar mobility

• !isordered pac7ing

• 8ammedQ

• Highly )rittle

Crystalline state•

"hermodynamically stable• =ow molec#lar mobility

• Highly ordered pac7ing

• >lastic* strong

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Crystalline an *!or"hous

Solis: Poly!ers

2u!!ery state• Higher molec#lar mobility

• !isordered pac7ing

• Pliable (+#bberyQ

*lassy state• =ow molec#lar mobility

• !isordered pac7ing

• 8ammedQ

• )rittle (DlassyQ

Crystalline state• =ow molec#lar mobility

• Highly ordered pac7ing

• >lastic* strong

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Glass-7u%%ery /ransitions:

/e!"erature an Water

2u!!ery

state

*lassy

state

2u!!ery

state

WaterHeat

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Water *cti(ity2 Glass Transitions

1oisture Gain

/e!"erature Increase

*lassy

State.Crispy/

2u!!ery state

.Soggy/

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Water *cti(ity2 Glass Transitions

VVglassy state VVr#bbery state

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*ci %ase e8uili%ria:

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*ci-%ase e8uili%ria:pH

H9O HI I OH-

"5 95; 9

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*ci-%ase e8uili%ria2 !"/ers

AH I H9O A-  I H5O

I

) I HI)HI

Acid@

)ase@

,hat %raction o% a wea7 acid or base dissociates in water?•  A 9strong: aci+;base ully +issociates (e!g!, %Cl or 2aO%)

•  A 9"ea#: aci+;base partially +issociates (e!g!, *COO% or *2% 

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Ho does char+e chan+e ithpHD

AH A-  I HI

!eprotonated %orm

Protonated %ormProperty o% the

molec#le

Property o% the

sol#tion

Henderson

Hassel!alc#

&n%ormation o% %raction o% molec#le

 protonated or deprotonated

LKLK

LK

LK

LLKK   +−+−

×=

=  %  A%  A

 A% 

 %  A

 8 

LKLKlog.4

 A%  A p%  p8 

−

−=

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AH A-  I HI

. 9 5 $ G 0 1 / 4

94

$4

04

/4

.44

Charged

pH

Concentration of Species

AH

 pK a = 5

Ho does char+e chan+e ith

pHD

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*ci-%ase e8uili%ria2

!"/er ;apacity

• "he buer capacity (in the al7ali direction is de%ined as the n#mber o% moles o% OH- that m#st be added to one

liter o% b#%%er in order to increase the pH by . #nit2

• A b#%%er wor7s best at pH val#es close to its p a val#e2

[ ]( )( ) 9.4.

.4529

 p%  p8a

 p%  p8aC 

−

−

+=β 

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Food *cids, !ases : !"/ers3cid Step pa 3cid Step pa

Organic Aci+s 3norganic Aci+s

3cetic . $21G Car!onic . 0251

Citric . 52.$ 9 .429G

9 $211 o"#osp#oric . 92.9

5 025 9 129.

Fumeric . 5245 5 .9201

9 $2$$ "yrop#osp#oric . 42/G

Lactic . 524/ 9 .2$

Malic . 52$4 5 G211

9 G2.4 $ /299

"ropionic . $2/1 Sulfuric . -524

Succinic . $2.0 9 .29

9 G20. ?ase Step pa

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7lectrostatic

2epulsion

.Solu!le/

*ci-%ase e8uili%ria: /ect on F"nctionality

Protein sol#bility

E

E

-

-

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*ci-%ase e8uili%ria: /ecton F"nctionality

Filamento#s•  High ,HC•  "ransparent•  >lastic

Partic#late

•  =ow ,HC•  Opalectron Microscopy

 pH NN p&

−−− −− −

ati(eProteins

Heat

 pH p&

• ,HC water holding capacity Protein Del "ype

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*ci-%ase e8uili%ria: /ect onF"nctionality

!enoic acid is only "sed as a preser(ati(ein acid foods e9+9, fr"it "ices, pH A-?')eca"se it )ecomes non-ionied at lo pH

and can enter the lipid mem)ranes of cells p a $29

Antimicrobial Activity

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*ci-%ase e8uili%ria: /ect onF"nctionality

;itral is a 1a(or molec"le in many)e(era+es9 Its de+radation rate is hi+hlydependent on pH