week 1 h2o properties, solutes interactions & types of h2o
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Week 1: Water Chemistry (Part I)
Properties, Solutes Interactions &Types of Water
Mohd Nazri Abdul Rahman
BFoodSc. (Hons) (UMS), MSc. (UKM)
1/13/2013 NT20903 Food Chemistry & Biochemistry 1
NT20903 Food Chemistry and Biochemistry
Academic Session 2011/2012
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Chapter Objectives
At the end of this chapter, you will be able to:
1. Understand the nature and properties of waterand aqueous solutions.
2. Identify the types of water in food systems3. Consider many roles played by water in food
systems
4. Understand the central role of water in food
chemistry.
5. Discuss the important of water activity inprolonging of food shelf life
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1/13/2013 NT20903 Food Chemistry & Biochemistry 3
Most abundant compound
found on earth, food
component that is given less
attention.
Major chemical component
of the earths surface.
Is the primary solvent in
which metabolic/life
processes.Only liquid that most
organisms ever encounter.
Water
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Water
Organisms are 70 90% water.
Normal metabolic activity can occur only whencells are at least 65% H2O.
All food contains water(water content varies infood) a substantial component of most foodscomposition.
E.g.
No. Food Type % Water
1. Peanut 2
2. Corn Flakes 3
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Water
Dependency of life on H2O is not a simple matter,
but it can be grasped by considering the unusual
chemical and physical properties of H2O.
Water & its ionization pcdts, H+ ions and
hydroxide ions, are critical determinants of the
structure & function of many biomolecules,including AAs & CHONs, nucleotides & nucleic
acids, & even phospholipids & membranes.
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Water
Water is an indirect participant a difference
in the conc. ofH+ ions on opposite sides of a
membrane represents an energized condition
essential to biological mechanisms of energy
transformation.
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Water
A molecule consist of 2 atoms of hydrogenand 1 atom of oxygen (H2O) which is
chemically bonded on covalent.
O
H H
Covalent
bond
H2O1/13/2013 7NT20903 Food Chemistry & Biochemistry
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Water
A unique molecule because H2O has polarbonds, has unequal distributions of+ve and -vecharges.
Water molecules are dipoles(dwikutub).
O -
H + H +
Dipoles
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Water
Every molecule bonds to anotherforming ahydrogen bonds (strong attractive forces
among molecules).O -
O - H + H + O -
O -
H +
O -
H + H +
Hydrogen
bond
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Importance of water in life
1. As a reactant and reaction medium, has solvent
properties.
2. Stabilizes many biopolymer conformations.
3. Facilitation of dynamic behavior of
macromolecules.
4. Catalystic (enzymatic) properties.
5. Carries nutrients and waste materials.
6. Stabilize / governs / maintain body temperature.
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Water Has Unusual Properties
Water has a substantially higher boiling point, melting
point, heat of vaporization, & surface tension. Suggest that intermolecular forces of attraction between
H2O molecules are high.
Thus, the internal cohesion of this substance is high.
Water has an unusually high dielectric constant, itsmaximum density is found in the liquid (not the solid)state, and it has a negative volume of melting (that is,the solid form, ice, occupies more space than does the
liquid form, water).
Compare water with chemical cpds of similar atomic organization &
molecular size such as hydride of oxygen, with hydrides of oxygens nearestneighbors in the periodic table, namely, ammonia (NH3) and hydrogen
fluoride (HF), or with the hydride of its nearest congener, sulfur (H2S).
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Physical Properties of H2O
Waters ability to engage in 3-dimensional H+
bonding unusual properties:
High melting and boiling point
Surface tension and enthalpies of various phasetransitions (extra energy to break internal H+ bonds)
Boiling point was broken the only hydrogen, not
the covalent bond.
Covalent bond Hydrogen bond
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Physical Properties of H2O
On the basis of comparison waters properties withthose of molecules of similar MW & atomiccomposition (CH4, NH3, H2S, H2Se, and HF), water isseen;1. To have unusually high melting and boiling point
temperatures2. To exhibit unusually large value for surface energy,
permittivity (dielectric constant), heat capacity, heats ofphase transformation (fusion = pelakuran, vaporization= pengewapan, and sublimation = pemejalwapan)
3. To have a somewhat lower than expected density4. To exhibit the unusual property of expansion upon
solidification
5. To have a viscosity that is quite normal
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Table 1.1 Physical Properties of Water and Ice
Property Value
Molecular weight 18.0153
Melting point (at 101.3 kPa) 0.00C
Boiling point (at 101.3 kPa) 100.00C
Critical temperature 373.99C
Critical pressure 22.064 Mpa
Triple point temperature 0.01C
Triple point pressure 611.73 Pa
Hvap at 100C (haba pengewapan) 40.647 kJ/mol
Hsub at 0C (haba pemejalwapan) 50.91 kJ/mol
Hfus at 0C (haba pelakuran) 6.002 kJ/mol1/13/2013 14NT20903 Food Chemistry & Biochemistry
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Table 1.2 Properties of Related Small Molecules
CH4 NH3 H2O H2S H2Se HF
MW 16.04 17.0 18.01 34.08 80.9 20.01
mp (C) - 182.6 - 77.7 0 -86 -60 - 83.1
bp (C) - 161.4 - 33.3 100 -61 - 41 19.5
Hvap(kJ/mol)
8.16 23.26 40.71 18.66
Sebatian ini boleh dibandingkan dgn air krn kesemuanyamempunyai proton (+ve) yg terikat kpd oksigen atau bbrpa atom
elektronegatif yg lain. Ini dpt dilihat, H2O mempunyai;
Takat didih paling tinggi
Haba tentu pengewapan yg paling tinggi
Takat lebur paling tinggi
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a. Thermal Conductivity (TC)
Thermal conductivity ofH2O is large as compared withmost other liquids.
Thermal conductivity ofice is larger than might beexpected for a nonmetallic solid.
Thermal conductivity of ice at 0C is approximatelyquadruple that of liquid H2O at the same T, indicatingthat ice will conduct thermal energy at a much greaterrate than will immobilized (e.g., tissue) water.
Since the heat capacity of H2O is approximately 2x thatof ice, the thermal diffusivities of H2O and ice differ byabout a factor of 9.
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b. Thermal Diffusivity (TD)
Since TD is indicative of the rate at which amaterial will undergo a change in T, we wouldexpect that ice, in a given thermal environment,
will undergo T change at a rate 9x greaterthanthat for liquid H2O.
These differences in TD & diffusivity values for
H2O and iceprovide a good basis for u/standingwhy tissues freeze more rapidly than they thawunder symmetrically applied T differentials.
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Hydrogen Bonding in Water Is Key to Its
Properties
2 hydrogen atoms of water are linked
covalently to oxygen, each sharing an electron
pair, to give a nonlinear arrangement.
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This bent structure of the H2O molecule hasenormous influence on its properties.
If H2O were linear, it would be a nonpolar substance.
In the bent configuration, however, the electronegativeO atom and the two H atoms form a dipole thatrenders the molecule distinctly polar.
Furthermore, this structure is ideally suited to H-bondformation.
Water can serve as both and H donor and an Hacceptor in H-bond formation.
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The potential to form four H bonds per water moleculeis the source of the strong intermolecular attractionsthat endow this substance with its anomalously highbp, mp, heat of vap., & surface tension.
In ordinary ice, the common crystalline form of water,each H2O molecule has four nearest neighbors to whichit is hydrogen bonded: Each H atom donates and Hbond to the O of a neighbor, and the O atom serves as
an H-bond acceptor form H atoms bound to two diff.water molecules results a local tetrahedralsymmetry (figure below).
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Structure of normal ice (hexagonal ice six H-bonded molecules)
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Hydrogen bonding in water is cooperative. an H-bonded water molecule serving as an acceptor is a better
H-bond donor than an unbonded molecule (and an H2Omolecule serving as an H-bond donor becomes a better H-bondacceptor).
Thus, participation in H bonding by H2O molecules is aphenomenon of mutual reinforcement.
The H bonds between neighboring molecules are weak (23kJ/mol each) relative situated asymmetrically between thetwo oxygen atoms along the O-O axis.
There is never any ambiguity about which O atom the Hatom is chemically bound to, nor to which O it is H bonded.
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Structure of Ice Is Based on H-Bond
Formation
In ice, the hydrogen bonds form a space-filling, 3Dnetwork.
These bonds are: directional and straight; that is, the Hatom lies on a direct line bet. the two O atoms.
This linearity & directionality mean that the H bonds in iceare strong.
Directional preference of the H bonds leads to an openlattice structure.
Example: if the water molecules are approximated as rigid
spheres centered at the positions of the O atoms in thelattice, then the observed density of ice is actually only 57%of that expected for a tightly packed arrangement of suchspheres.
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The H bonds in ice hold the water moleculesapart.
Melting involves breaking some of the H bonds
that maintain the crystal structure of ice so thatthe molecules of water (now liquid) can actuallypack closer together.
Thus, the density of ice is slightly less than that of
water. Ice floats, a property of great importance to
aquatic organism in cold climates.
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In liquid water, the rigidity of ice is replaced byfluidity and the crystalline periodicity of ice givesway to spatial homogeneity.
The H2O molecules in liquid waterform a disorderedH-bonded network, with each molecule having anaverage of 4.4 close neighbors situated within acenter-to-center distance of 0.284 nm (2.84 A).
At least half of the hydrogen bonds have nonidealorientation (that is, they are not perfectly straight);consequently, liquid H2O lacks the regular latticelikestructure of ice.
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Molecular Interactions in Liquid
Water Are Based on H Bonds
The participation of each water moleculesin an average state of H bonding to itsneighbors means that each molecule isconnected to every other in a fluid
network of H bonds. The average lifetime of an H-bonded
connection between two H2O molecules inwater is 9.5 psec (picoseconds, where 1psec = 10-12 sec).
Thus, about every 10psec, the average H2Omolecule moves, reorients, and interactswith new neighbors (figure @ right side).
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Solvent Properties of Water Derive from Its
Polar Nature
Because of its highly polar nature, water is an
excellent solvent for:
ionic substance such as salts;
nonionic but polar substances such as sugars,
simple alcohols, and amine; and
carbonyl-containing molecules such as aldehydes
and ketones.
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Example:
Sodium chloride isdissolved becausedipolar watermolecules participate
in strong electrostaticinteractions with theNa+ and Cl- ions,leading to the
formation ofhydrationshells surroundingthese ions.
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Water Solute Interactions
1. Macroscopic level
Water binding and hydration
Water holding capacity (WHC)
2. Molecular level
Interaction with;
a) Ions and ionic group
b) Neutral groups (hydrophilic solutes)
c) Non-polar substances
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1. Macroscopic Level
a) Water binding and hydration = tendency ofwater to associate with hydrophilic (suka air)substances.
b) Water holding capacity = the ability of a matrixof molecules (macromolecules) at low come to
physical entrap large amount of waterin orderto inhibit exudation, e.g. pectin, starch, etc.
Example: Sponge = absorb more water = WHC
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Bulk flow is restrictedbut movement of
individual molecule is possible.
WHC has profound effect on food quality:
syneresis of gel,
thaw exudates,
inferior performance of animal tissue due to
decline in pH during post-mortem.
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2. Molecular Level
a) Interaction with ions and ionic groups Hinder mobility of water (disekat daripada
bergerak); covalent bond > H2O-ion bond >
H2O H2O H2O and simple inorganic ions undergo dipole-
ion interactions (Figure 1.1).
Strongly interact with H2O molecules, causingmobile and densely packed.
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Fig. 1.1 Likely arrangement of water molecules adjacent to
sodium chloride ion pair. Only water molecules in the plane of
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Large ion and monovalent (K+, Rb+, NH4+, Cl-,Br, NO3, etc) have weak electron field; and arenot structure breakers. Disrupt the normal
structure of H2O. Varying abilities to hydrate (compete for H2O),
alter H2O struct. etc.
Conformation of proof and stability of colloids,greatly influenced by the kings and amountsof ions present.
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b) Interactions with Neutral Groups (hydrophilicSolutes)
Interaction H2O ion > H2Ononionic/hydrophilic = H2OH2O/depends on the internal strength, may or may not
reduce mobility and alter other properties.
Hydrogen bonding of H2O with various potentiallyeligible groups (e.g. hydroxyl, amino, carbonyl, amide,imino, etc.) results in water bridges where 1 H2Omolecule interacts with 2 eligible H-bonding sites on 1or > solutes.
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c) Interaction with nonpolar substances
Mixing H2O with hydrophilic substances suchas hydrocarbons, rare gases, and the apolar
groups of fatty acids, amino acids, and CHONsis considered as thermodynamicallyunfavorable event (hydrophobic hydration).
E.g. H2O + oil = unmixed due to nonpolar
H2O tends to minimize its association withnonpolar entities that are present association.
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Fig. 1.2 Schematic depiction of (a) hydrophobic hydrationand (b) hydrophobic association. Open circles arehydrophobic groups. Hatched areas are water.
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(a) Hydrophobic hydration = a process of reduction
in entropy, considered an indicator of increased
order, is thought to occur bec. of special structures
in water that form in the vicinity of these
incompatible apolar entities.
(b) Hydrophobic interaction = process of a partial
reversal of hydrophobic hydration.
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Hydrophobic hydration = a process of reduction in
entropy, considered an indicator of increased
order, is thought to occur bec. of special structures
in water that form in the vicinity of these
incompatible apolar entities.
Hydrophobic interaction = process of a partialreversal of hydrophobic hydration.
R (hydrated) + R(hydrated) R2
(hydrated) + H2O
Where R = apolar group (Fig. 1.2b) above
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Type of Water
1. Free water (air bebas) as a solvent forspreadibility; as a medium for colloidaldispersions (agen penyebar)
2. Immobilized water (air pegun) in tissue cell
(either extra- or intracellular fluids)3. Hydrated water (air dihidratan) combines in
the molecules
4. Absorbed water (air serapan) in gel.
5. Adsorbed water (air jerapan) on the surface ofthe solid.
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Water also categorized into vicinal water (air
visinal) and multilayer (air lapisan berbilang).
Type I (Monolayer, vicinal water)
Type II (Multilayer water)
Type III (Entrapped water)
Type IV (Free water)
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Water Binding in Foods
Water in food and biological systems exists in 3 typesor degrees of boundness.
1. Pure waterwith full activity can be considered asfree water (Type IV) and is not found in biological
systems.2. Entrapped water or Type III water involves physically
entrapped water in tissues and membranes and issimilar in its properties to water in dilute solutions. The freezing point is reduced only to a slight extent and
the normal solvent capacity is exhibited by entrappedwater.
It is easily removed during evaporation, concentration ordrying operation.
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Type III of water constitutes the major proportionof water in plant and animal food tissues and isreadily available for chemical reactions and thegrowth and activity of microorganisms.
As it is removed the remaining water graduallyassumes a lower activity.
When all the Type III water is removed the moisturecontent is about 12 to 25% by weight and the wateractivity is lowered to 0.8.
The rates of chemical reactions such as sugar-aminereactions (Maillard reaction) increase as the wateractivity is lowered to about 0.8 and less.
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3. Multilayer water or Type II water involves water that is H-bonded in water-solute and water-water clusters, andwater in micro-capillaries. It has limited solvent capacity and does not freeze completely
even at -40C.
It is more difficult to remove this type of water than Type IIIwater.
Partial removal of Type II water eliminates the last possibility ofmicrobial growth and greatly reduces most kind of chemicalreactions.
Complete removal of Type II water leaves 3 to 7% of moisturelevel in the food (water activity is about 0.25) and correspondsapproximately to optimum stability of dry pcdts that containsignificant amount of oxidizable lipids.
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This small amount of water is required to inhibit
oxidative ranciditythrough several mechanisms.
The small amount of waterfacilitates destruction
of free radicals, H-bonds to hydroperoxides and
slows the rate of their conversion to other pcdts,
and hydrates or coordinates with metals, therebyreducing their ability to catalyze oxidation.
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4. Monolayer, vicinal water or Type I water involveswater adsorbed to solutes as a monolayer and waterin chemical hydrates. This corresponds to a moisture content in the range of 0 to
0.7% by weight and can be partially removed bydehydration but not by freezing even at -40C.
Type I water is tightly boundand is considered to be truebound water.
It is in this region where acceleration of lipid phasereactions (oxidative rancidity) occurs as lipids becomemore exposed.
However, the rates of enzyme catalyzed reactions such ashydrolysis of lipids or proteins decrease.
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Fig. 1.3 Rates of chemical and enzymatic reactions and
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Fig. 1.3 shows the effect of moisture content and water
activity of foods on the growth rates of m.o and of variouschemical reactions that occur in foods.
It also indicates the shelf life or storage stability of foods as afunction of water activity.
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Fig. 1.3 shows the effect of moisture content
and water activity of foods on the growth
rates of m.o and of various chemical reactions
that occur in foods.
It also indicates the shelf life or storage
stability of foods as a function of water
activity.
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The mc-aw relationship istemp. dependent & a intemp. shifts the boundaries
of zones I and II to points of> moisture and lower aw.
Water adsorption anddesorption isotherms do
not coincide, i.e. theyexhibit hysteresis as shownin Fig. 1.4
Hysteresis = lack of
superimposability on anisotherm prepared bydesorption.
Fig. 1.4 Hysteresis of moisture
sorption isotherm (MSI).
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At any given mc, the aw during
resorption (erapan semula)(addition of water to dry
sample) is less than that
during adsorption (jerapan)
and at any given aw thecontent during desorption
(penyerapan) is greater than
that during adsorption.
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At any given mc, the aw during resorption
(erapan semula)(addition of water to dry
sample) is less than that during adsorption
(jerapan) and at any given aw the contentduring desorption(penyerapan) is greater
than that during adsorption.
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Sifat-sifat Air Visinal & Air Lapisan Berbilang
Sifat-sifat Air Visinal Air Lapisan Berbilang
Keterangan
am
Air yang berinteraksi kuat
dengan tapak hidrofilik khusus
pada juzuk bukan akueus
melalui sekutuan air-ion dan
air-dwikutub; apabila air jenisini berada pada aras
maksimum, air ini cukup untuk
memberikan penutupan satu
lapisan kumpulan yang sangathidrofilik dan boleh sampai
daripada juzuk bukan akueus;
juga merangkumi air di dalam
mikrokapilari (garis pusat < 0.1
um)
Air yang memenuhi baki
tapak lapisan pertama
dan membentuk
beberapa lapisan
tambahan di sekelilingkumpulan hidrofilik
juzuk-juzuk bukan
akueus; ikatan hidrogen
air-air dan air-bahanlarut paling banyak.
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Sifat-sifat Air Visinal Air Lapisan
Berbilang
Takat Sejuk beku
dibandingkan
dengan air tulen
Tak boleh disejuk
beku pada suhu -
40C (terikat)
Kebanyakannya tak
boleh disejuk beku
pada suhu -40C
(terikat); bakinya
boleh disejuk bekudengan takat
penyejukbekuan
yang amat
direndahkan
Kemampuan pelarut Tiada Sedikit hingga
sederhana
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Sifat-sifat Air Visinal Air Lapisan
Berbilang
Kemobilan translasi
(aras molekul)dibandingkan
dengan air tulen
Amat dikurangkan Dikurangkan sedikit
hingga banyak
Entalpi pengewapan
dibandingkandengan air tulen
Amat ditingkatkan Ditingkatkan sedikit
hingga sederhana
% Air jumlah di
dalam makanan
berkelembapantinggi (90 H2O atau
9 g H2O per g jirim
kering)
0.5 0.4% 3 2%
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Sifat-sifat Air Visinal Air Lapisan Berbilang
Hubungan
denganisoterma erapan
Air di dalam zon I
isoterma terdiridaripada sedikit sahaja
air juzuk dengan
bakinya ialah air
visinal; sempadan ataszon I tidak jelas dan
agak berubah-ubah
mengikut hasil
keluaran dan suhu
Air di dalam zon II
isoterma terdiridaripada air yang
terdapat di dalam zon I
campur air yang
ditambah ataudisingkirkan di dalam
kawasan zon II; air yang
kedua ini semata-mata
air lapisan berbilang;
sempadan zon II tidak
jelas dan agak berubah-
ubah mengikut jenis
makanan dan suhu
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Sifat-sifat Air Visinal Air Lapisan
Berbilang
Kesan pemerosotan
yang biasa berlaku
Kestabilan
keseluruhan
optimum pada nilaimonolapisan (0.2
0.3 aw)
Apabila kandungan
air meningkat
melampauibahagian bawah zon
ini, kadar hampir
semua jenis
tindakbalasmeningkat
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