water 1. water's importance 2 1. solvent ◦ most molecules dissolved in water 2. reactant ◦...

WATERWATER

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WATER'S IMPORTANCEWATER'S IMPORTANCE

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1. Solvent◦ Most molecules dissolved in water

2. Reactant◦ Water's involvement in hydrolysis

reactions

3. Product◦ Water's involvement in condensation

reactions

4. Heat transfer medium◦ boiling, steaming, cooling

WATER'S IMPORTANCEWATER'S IMPORTANCE5. Texture

◦ Juiciness, mouthfeel Snack foods Vegetables Meat

6. Preservation◦ Highly perishable foods usually have high water

activity E.g. bread vs. cracker or cereal

7. Economics◦ More water added = more $

UNDERSTANDING THE PHYSICAL AND CHEMICAL PROPERTIES OF WATER IS IMPORTANT IN THE STUDY OF FOOD AND PROCESSING

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PHYSICAL & CHEMICAL PROPERTIES PHYSICAL & CHEMICAL PROPERTIES OF WATEROF WATER

Compound Melting point Boiling point

H2O 0ºC 100ºC

H2S -83ºC -60ºC

NH3 -78ºC -33ºC

Methanol -98ºC 65ºC

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Water has very unique properties not shared by other similar hydrogen compounds or compounds of similar weight

Why? – this is explained by the unique structure of H2O

STRUCTURE OF WATERSTRUCTURE OF WATER

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Tetrahedral arrangement Two free electrons of O

act as H-bond acceptors while H acts as donor

Highly electronegative O pulls electrons from H, making H behave like a bare proton

Forms a dipole because of the electronegative O

STRUCTURE OF WATERSTRUCTURE OF WATER

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Because of the DIPOLE and TETRAHEDRAL structure we can get strong H-bonding

Water capable of bonding to 4 other water molecules

Unique properties of water from other hydrides

H-bond NOT a static phenomenon◦ T dependent

PHASE CHANGES OF PHASE CHANGES OF WATERWATER

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Pre

ssu

re

Temperature

WATER VAPORWATER VAPOR

Water is “free” and devoid of any H-bonds◦Large input of energy needed

endothermic process

◦Large dissipation of same energy needed to make water lose kinetic energy exothermic process

Waters latent heat of vaporization is unusually high◦ to change 1 L from liquid to vapor need

539.4 kcal

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LIQUID WATERLIQUID WATER

T (ºC) Density (kg/m3) Viscosity (m2/s)

0 999.9 1.7895

5 1000.0 1.535

25 997.1 0.884

100 958.4 0.294

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Extensive H-bondingH-bond formation dependent on T

◦ With increasing T get more mobility and increased fluidity

ICEICE

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Forms when exactly 4 H-bonds are formed between water molecules◦ 2.78 A vs. 2.85 A in liquid◦ To get this order a lot of energy

needs to be adsorbed by the environment

The strong H-bonding in ice forms an orderly hexagonal crystal lattice◦ 6 H2O molecules

Has 4X more thermal conductivity than water at same temperature

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Can go from ICE to GASCan go from ICE to GAS

Pre

ssu

re

Temperature

Basis for Freeze Drying

Sublimation

PROPERTIES OF ICEPROPERTIES OF ICECrystallization

◦ Crystal growth occurs at freezing point◦ Rate of crystal growth decreases with decreasing

temperature◦ Solutes slow ice crystal growth

Nucleation - affects ice crystal size.◦ Slow freezing results in few nucleation sites and

large, coarse crystals◦ Fast freezing results in many nucleation sites and

small, fine crystals◦ Heterogeneous nucleation

usually caused by a foreign particle, such as salt, protein, fat, etc.

◦ Homogeneous nucleation very rare, mainly occurs in pure systems

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PROPERTIES OF ICEPROPERTIES OF ICESUPERCOOLING

◦Water can be cooled to temperatures below its freezing point without crystallization

◦When an ice crystal is added to supercooled water, temperature increases and ice formation occurs

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1. http://www.youtube.com/watch?v=czmQ2_ymaOo2. http://www.youtube.com/watch?v=gGpNhBPYNfs&feature=related3. http://www.youtube.com/watch?v=DpiUZI_3o8s

PROPERTIES OF ICEPROPERTIES OF ICEFreezing induced

changes in foods (examples)

Destabilization of emulsions

Flocculation of proteins

Increased lipid oxidation

Meat toughening Cellular damage Loss of water holding

capacity

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Example: Effect of freezing on seafoods

WATER SOLUTE WATER SOLUTE INTERACTIONSINTERACTIONSAssociation of water to hydrophilic substances◦Bound water - occurs in vicinity of

solutes Water with highly reduced mobility Water that usually won't freeze even at -

40ºC Water that is unavailable as a solvent

◦“Trapped” water Water holding capacity Hydrophilic substances are able to entrap

large amounts of water Jellies, jams, yogurt, jello, meat

Yogurt - often see loss of water holding as whey is released at the top of the yogurt

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WATER SOLUTE WATER SOLUTE INTERACTIONSINTERACTIONS

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Ionic polar solutes◦ React readily with water and most are

usually soluble in water◦ Water HYDRATES the ions◦ Charge interactions due to waters high

DIELECTRIC CONSTANT Can easily neutralize charges due to its

high dipole moment Large ions can break water structure

◦ Have weak electric fields Small ions can induce more structure

in water◦ Have strong electric fields


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Nonionic polar solutes◦ Weaker than water-ion bonds◦ Major factor here is H-bonding to the polar site◦ Example: SUCROSE

4-6 H2O per sucrose Concentration dependent

>30-40% sucrose all H2O is bound

T dependent solubility

◦ C=O, OH, NH2 can also interact with each other and therefore water can compete with these groups

◦ H-bond disrupters urea - disrupts water

Water bridge


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Nonpolar◦ Unfavorable interaction with

water◦ Water around non-polar

substance is forced into an ordered state Water affinity for water high

compared to non-polar compound Water forms a shell

Tries to minimize contact

◦ Hydrophobic interactions Caused because water interacts

with other water molecules while hydrophobic groups interact with other hydrophobic groups

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATER

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Boiling point Vapor pressure is equal to

atmospheric pressure Strongly influenced by water

- solute interaction◦ Solutes decrease vapor

pressure and thus increase boiling point Sucrose +0.52ºC/mol NaCl +1.04ºC/mol

ATMOSPHERIC PRESSURE

VAPOR PRESSURE


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Freezing point lowering Freezing point can get

extensive depression via solutes

Alter ability of water to form crystals due to H-bond disruption

Sucrose -1.86ºC/mol NaCl -3.72ºC/mol

◦ Eutectic pt - temp. Where “all” water is frozen -

usually around -50ºC

◦ In most cases small amounts of water remains unfrozen (-20ºC) These small patches of water

can promote chemical reactions and damage

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERWhat explains all this?Raoult's law

P = P*/X1

orP*-P/P*= x/55.5M

P = vapor pressure of solution; P* = vapor pressure pure solvent; X1 = mole fraction of solute; x = grams solutes in solution; 55.5M = moles of water per liter

This relationship is not only important for explaining the concepts of depressing freezing point and elevating boiling point◦ Also explains the concept of water activity

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EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATEROsmotic pressure of solutions There is a tendency for a system containing

water and a solution separated with a membrane to be at equilibrium

The pressure needed to bring the two solutions at equilibrium is called OSMOTIC PRESSURE

The more the solution has of dissolved solutes (e.g. salt) the higher its osmotic pressure

Can use this in food processing and preparation◦ E.g. Crisping salad items

Increase turgor

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Surface tensionWater surface

behaves differently than bulk phase◦ Like an elastic film◦ Due to unequal

inward force◦ Resist formation of a

new surface thus forming surface tension

1. http://www.youtube.com/watch?v=45yabrnryXk&feature=fvw2. http://www.youtube.com/watch?v=76CNkxizQuc&feature=related

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERWater has high surface tension

◦72.75 dynes/cm (20ºC)Because of the high surface

tension special considerations are needed in food processing

To affect it one can:◦Increase T (more energy) reduces

surface tension◦Add solutes

NaCl and sugars increase surface tension

Amphipathic molecules reduce surface tension

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PhotoFrost®

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERIonization of water Water can ionize into hydronium (H3O+) and hydroxyl

(OH-) ions◦ Transfer of one proton to the unshared sp3 orbital of

another water molecule

Pure water: Keq = Equilibrium (or ionization) constant

Keq = [H3O]+ [OH]-

[H2O]

[H3O]+ [OH]- = Keq = Kw (Water dissociation constant)

[10-7] [10-7] = [10-14]

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EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERAcids and bases in food systems

◦Acid - proton donor NH3 + H2O NH4

+ + OH-

◦Base - proton acceptor CH3COOH + H2O CH3COO- + H3O+

◦Weak acids and bases Most foods are weak acids These constituents are responsible for

buffering of food systems◦Some examples

Acetic, citric, lactic, phosphoric, etc.

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EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERAcids and bases in food systems

◦ Is there a difference between weak and strong acids? Strong acids

When placed in solution, 100% ionized

Weak acids When placed in solutions weak acids form an

equilibrium

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HCl = H+ + Cl-

pH = -log [acid] = -log [H]+

HOAC H+ + OAC-

pKa = -log Ka

Keq = [H]+ [OAC]-

[HOAC]

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERWeak acids and bases

◦ One cannot relate pH to concentration for weak acids and bases because of this equilibrium

◦ One must understand how the acid behaves in solution

◦ Knowing the dissociation constant of the acid is important to determine the effect on the pH of the system

◦ The relationship of pH for weak acids and bases relies on the Henderson - Hasselbach equation:

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pH = pKa + log [salt] [acid]


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Weak acids◦ Graphically

behave like the figure when titrated with a strong base. The reverse holds true for weak bases


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Buffering◦ Buffers resist

changes in pH when acids and bases are added

◦ Characteristics of a buffer Maximum

when pH = pKa or when [acid] = [salt]

Rule of thumb: pH = pKa ± 1

What is this point and its significance to food systems?


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Let’s return to Henderson-HasselbachpH = pKa + log [salt] [acid]

K1 = 4.6 x 10-3 ; K2 = 2.04 x 10-10

pH

Equivalents OH-

O

NH2

CH3 OH


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Let’s return to Henderson-HasselbachpH = pKa + log [salt] [acid]

K1 = 4.6 x 10-3 ; K2 = 2.04 x 10-10

pH

Equivalents OH-

O

NH2

CH3 OH

O

NH3+

CH3 OH

2.5 meq

O

NH3+

CH3 O-

O

NH3+

CH3 OH

1.25:1.25 meq

O

NH3+

CH3 O-

2.5 meq

O

NH2

CH3 O-

O

NH3+

CH3 O-

1.25:1.25 meq

O

NH2

CH3 O-

2.5 meq

O

NH3+

CH3 OH

O

NH3+

CH3 O-

O

NH2

CH3 O-

pK1 pK2

OH-

OH-


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Examples of natural pH control◦ Fruits - citric, malic, acetic, etc

Microbial control Flavoring

◦ Milk – pH around 6.5 Controlled by three components

Phosphate, citrate, carbonate

◦ Eggs Fresh eggs - pH = 7.6 After storage for several weeks - pH = 9-9.7

Due to loss of CO2

Problem - Loss of carbohydrate groups on proteins. Loss of protein functionality, causing decreased viscosity and poor foaming properties

EFFECT OF SOLUTES ON EFFECT OF SOLUTES ON WATERWATERExamples of “man made” pH control

◦Food additives - ACIDULANTS Citric acid - pectin jellies

pH must be around 2.9-3.0 Also provides balance between tartness and

sweetness Yogurt and cottage cheese

Fermentation - glucose or lactose to lactic acid pH reduction to around 4.6 will cause the gelation Can add acidulants to imitate dairy yogurts - lactic,

citric, phosphoric, HCl Cheese

Alkaline salts of phosphoric acid to get good protein dispersion

Thermal process control pH below 4.5 usually hinders C. botulinum growth Less severe heat treatment required for these Acidulants used to lower pH below 4.5 for some fruit

and tomato products

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◦Acidulants - leavening agents Used in the baking industry to give rise

(release of CO2) - alternative to yeast

When HCO3- becomes acidic (pH < 6), CO2

forms, CO2 not very soluble so released as a gas

Overall eq: H+ + HCO3- H2O + CO2

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◦Leavening systems Bicarbonate (NaHCO3) - source of HCO3 and CO2

Leavening acids Drive bicarbonate (HCO3) to CO2

Rate of acid release varies and therefore CO2 release

Phosphate - rapid release of CO2

Sulfate – slow release of CO2

Pyrophosphate - can be cleaved by phosphatases becoming more soluble - used in refrigerated doughs

-Glucono-lactone - used in refrigerated doughs

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◦ Acidulants - antimicrobials pH is important for two reasons: 1. Solubility and 2.

Activity The salt is more soluble in aqueous systems The acid is more active in its antimicrobial efficiency

Benzoic acid (0.05-0.1%) Found naturally in prunes, cranberries, cinnamon and cloves Active below pH 4 (active acidic form of the salt) Highly soluble in the form of sodium salt Effective - yeasts and bacteria, less for molds Uses in acid foods - soft drinks, juices, pickles, dressings etc.

Parabens or r-hydroxybenzoate esters (0.05-0.1%) Broader pH range (active at higher pH) Mainly use methyl and propyl esters Uses in baked goods, wines, pickles, jams, syrups, etc.

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◦ Acidulants - antimicrobials Sorbic acid (Na+ and K+ salt forms) (0.02-0.3%)

Max activity at pH 6.5; active at acid pH values Most effective for yeast and molds

Inhibit, not inactivate Uses in cheese, juices, wines, baked goods, etc.

Proprionic acid (proprionate) Ca2+ salt Active up to pH 5 Uses in breads (retards Bacillus) which causes ropiness in

breads Ropiness - thick yellow patches that can be formed into a

rope-like structure making the bread inedible Acetic acid Nitrites and Nitrates Sulfites

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WATER ACTIVITYWATER ACTIVITYWhat is meant by water activity?

◦Water has different levels of binding and thus activity or availability in a food sample

◦Simply put, Water activity (aw) helps to explain the relationship between perishability and moisture content Greater moisture content faster spoilage

(normally) Why are there some perishable foods at the

same moisture content that don't spoil at the same rate?

There is a correlation found between aw and various different spoilage and safety patterns

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WATER ACTIVITYWATER ACTIVITY

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Water has different levels of binding and thus activity or availability in a food sample

Food companies and regulatory agencies (e.g. FDA) rely on aw as an indicator of how fast and in what fashion a food product will deteriorate or become unsafe, and it also helps them set regulatory levels of aw for different foods

Highly perishable foods aw > 0.9

Intermediate moist foods aw = 0.6-0.9

Shelf stable foods aw < 0.6

WATER ACTIVITYWATER ACTIVITYThermodynamic definition of aw

◦The tendency of water molecules to escape the food product from liquid to vapor defines the aw

aw = p/pO=%RH/100

◦Water activity is a measure of relative vapor pressure of water molecules in the head space above a food vs. vapor pressure above pure water

◦Scale is from 0 (no water) to 1 (pure water)

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Sorption isotherms◦ Help relate moisture

content to aw

◦ Each food has their own sorption isotherm

◦ It is interesting that when water is added to a dry product, the adsorption is not identical to desorption

◦ Some reasons Metastable local domains Diffusion barriers Capillary phenomena Time dependent

equilibrium

Temp. dependent

WATER ACTIVITYWATER ACTIVITYWater sorption of a mixture

◦ A mixture of two different food components with different aw leads to moisture migration from one food to another which can create problems

◦ This is one reason why it is important to know the aw of a food product or ingredient

◦ Examples: Caramel, marshmallows and mints – all similar

%moisture but very different aw

Fudge (aw = 0.65-0.75) covered with caramel (aw = 0.4-0.5) – what happens?

Granola bar with soft chewy matrix (aw = 0.6) and sugar coat (aw = 0.3)?

Hard candy (aw = 0.2-0.35) on a humid day?

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So, knowing the aw of a food component one can select the proper ingredients for a particular food product

For example, it is possible to create a multi-textured food product if components are added at the same aw


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Temperature dependency of the sorption isotherm can be a major problem and often overlooked

Example:

Crackers that experience a temperature rise during transportation

At the same moisture content which would spoil faster?


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Sorption isotherms also explain the level of water binding in a food (i.e. types of water) ◦ Type I: Tightly “bound” water

(monolayer) Unavailable/Unfreezable (at -40C) Water - ion; water - dipole

interactions◦ Type II: additional water layer

(Vicinal water) Slightly more mobility Some solvent capacity

◦ Type III: Water condensating in capillaries and pores (Multilayer Bulk-phase water) More available (like dilute salt

solution) Can be entrapped in gels Supports biological and chemical

rections Freezable

Monolayer

True monolayer


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Importance of aw in foods ◦ Food stability

directly related to aw

◦ Influences storage, microbial growth, chemical & enzymatic deteriorations, etc.


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A. Microbial stability◦ Foods with aw > 0.9 require refrigeration because of

bacteria spoilage Exception: Very low pH Foods

◦ Can control by making intermediate moisture foods (IMF) Food with low aw to prevent microbial spoilage at room temp.

But which can be eaten w/o hydration Aw = 0.7 - 0.9 (20 -50% water) - achieved by drying or using

solutes (sugar, salt) dried fruits, jelly and jam, pet foods, fruity cakes, dry sausage,

marshmallow, bread, country style hams Minimal processing however preferred over IMF Special problems

May need mold inhibitor Lipid oxidation - may need antioxidant or inert packaging

◦ Important in grains to prevent mold growth & possibly mycotoxin development Must be below 0.8

WATER ACTIVITYWATER ACTIVITYB. Chemical stability

◦Maillard browning Doesn't occur below type II water Increases in type II water - water

becomes a better solvent while reactants become more mobile

Reduced in type III - dilution or water is an inhibitor

Depends on food product (aw 0.53-0.55 in apple juice vs. 0.93 in anchovy)

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◦Lipid oxidation Low aw, lipid oxidation high - due to

instability of hydroperoxides (HP)- unstable w/o water, no H-bonding

Slightly more addition of water stabilizes the HP and catalysts

Above type II water, water promotes the lipid oxidation rate because it helps to dissolve the catalysts for the reaction

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◦Vitamin and pigment stability Ascorbic acid very unstable at high aw

Stability best in dehydrated foods - type II water Problem with intermediate to high moisture

foods Must consider packaging for these foods

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WATER ACTIVITYWATER ACTIVITYC. Enzyme stability

◦ Hydration of enzyme◦ Diffusion of substrate (solubility)◦ Not significant in dehydrated foods◦ Little enzyme activity below type II water◦ Exceptions: in some cases we get activity

at ↓aw Frozen foods Lipases (work in a lipid environment)

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water 1. water's importance 2 1. solvent ◦ most molecules dissolved in water 2. reactant ◦...

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