Post on 01-Jun-2018
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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