week 1 h2o properties, solutes interactions & types of h2o

7/30/2019 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

1/13/2013 2NT20903 Food Chemistry & Biochemistry


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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

the paper are represented.1/13/2013 34NT20903 Food Chemistry & Biochemistry


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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.


(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

microbial growth as a function of water activity in foods.1/13/2013 47NT20903 Food Chemistry & Biochemistry

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).


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


if if i i i l i i


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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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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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week 1 h2o properties, solutes interactions & types of h2o

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