Lecture 1 Water (1) Printed

8/2/2019 Lecture 1 Water (1) Printed

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“We acknowledge

the Traditional

Owners of this

Land, the Nyungar

people, and pay

respect to the Elders

of their community.”

Water in Food

Water in Food Systems• Solvent - Salts, sugars, starch & proteins

• Dispersion medium - Colloids

• Reaction medium - Hydrolysis

• Plasticizer & lubricant

• Stabilizer of macromolecule conformation - Formation of H bonds

• Cost - Dehydration / transportation / wilting

• Other – Spoilage and microbial growth

Structure of Water

• Oxygen has 6 and hydrogen has one electrons in outer

shell

• Hydrogen and oxygen share an electron pair, forming a

covalent bond• These 4 electron pairs try to arrange themselves as far

away from each other as possible

10405’



Structure of Water

• The positive and negative charges in water

are not evenly distributed

• Molecule is partial positive and partialnegative (water is a POLAR molecule)

• Participates readily in hydrogen bonding

Hydrogen Bonding

• Weak bonding• Can occur in polar

molecules such as water

• In water, each molecule is

attached to 3.5 others

molecules

• In ice, attached to 4 other molecules

Principles of food chemistry

DeMan, 1999

Water • Unique

– Only compound abundant in three physical states

– Solid/liquid/gas

• Essential for life – Metabolic processes

• Major component in foods

• Influences on the structure, appearance and taste of foods

– Dried vs. fresh

• Influence the susceptibility to spoilage

Moisture Contents of Various Foods

FoodWater Content (%)

Pork (raw) 53-60Beef (raw) 50-70

Berries 80-85

Apples 90

Strawberries 90-95

Avocados 74-80

Potatoes 85-90Lettuce 90-95



Physical Properties

• Compared to other similar molecules,water has very high values for

– Melting point

– Boiling point

– Surface tension

– Dielectric constant [permittivity]

– Heat capacity

Physical Properties• Compared to other similar molecules, water has

very high values for

– Heats of phase change

• The heat of fusion – Amount of energy given off when a

substance freezes

– H fusion 80 cal/gram = 1.44 kcal/mole

• The heat of vaporization

– Amount of energy given off when a gasliquefies

–H vaporization = 560 cal/g = 10.5kcal/mole expect 8-16 Kcal/mole

• A lot of these anomalies are due to the extent of hydrogen bonding

Molecular Weight

BoilingPoint °C

SurfaceTension

Water (H2O) 18 100 73Hydrogen Sulphide(H2S)

34 -60 NA

Ammonia (NH3) 17 -33 NAMethanol (CH3OH) 32 65 22Ethanol (C2H5OH) 46 78 22Ether (C2H5OC2H5) 74 34 17



Boiling Points of Chemical Compounds Physical Properties

• Moderately low density

– Less dense as solid than as liquid

• Normal viscosity

• Result of strong intermolecular forces in water

Molecular Structure Molecular Structure - Ice• Ice has a well defined structure

– Each water molecule is H-bonded to 4surrounding water molecules

– Two are H-bonded to the central oxygen; Thetwo H atoms are bonded to neighbouring Oatoms on neighbouring H2Os

Water Ice



Molecular Structure - Water

• Partial retention of hydrogen-bonded tetrahedral

structure of ice.

– Thermodynamically favoured

• Rapid breaking and reforming of bonds to allow

movement

Molecular Motion

• Vibrational

– Around own bonds

– Ice, water and steam

• Rotational

– Spin on the spot

– Water and steam

• Translational – Move between regions

– Steam

Melting

Freezing

Sublimation

Deposition

Condensation

Vaporization

Triplepoint of

water =0.01 C and0.006 atm

Chemical Bonding

• Several

common bond

types inmolecules

– Hydrogen

– Hydrophobic

– Ionic

– Covalent



Bonding - Ionic

– Interacts strongly with charged ions

– Increases solubility of polar/chargedsubstances

• Eliminate ionic attraction

– Ions (Na+, Cl-)

• Alter the structure and mobility of water

©1999 by Stephen K. Lower; all rights reserved. November 10, 1999

Chem1 General Chemistry Reference Text States of matter

Bonding - Ionic

Bonding - Hydrogen

• Water competes for thehydrogen bonding sites

• Important in proteins – H bonds between side

chains and amide groupsstabilise structure

Interactions - Hydrophobic

• Hydrophobic groups

prefer a lipophillic or

non-aqueous

environment.

– Move away from

water, toward

each other



Water in Foods

• “Water Binding” and “hydration” refer to the

ability of water to associate with hydrophilicsubstances in food.

• Even though this water may be associated or

even trapped with the food the individual water

molecules may be free or not associated.

Water in Foods

• The interactions of individual water molecules

with various solutes produces what is termed“bound water”.

• The amount of water present in a food that is in

the “bound” form or associated with bound water

is also important in determining the shelf life of

the food.

Forms of Water in Foods

1. Bound

2. Capillary or monolayer

– Monolayer surrounding macromolecules

3. Free water

– Bulk water

Forms of Water - Bound• Strongly bound

– Constitutional water

• Not available as a solvent or to support enzymeactivities – Molecular level

• Un-freezable water



Forms of Water - Capillary

– Successive layers over the macromolecule

– Mostly not freezable at - 40C

– Influences on the physical properties of

solutes

– Increases the rate of most reactions

Forms of Water – Free Water

– Fills capillaries and pores

– Retains all the properties of “water”

• Increases molecular mobility

• Decreases viscosity

– Freezable

– Physically constrained within the

macromolecular matrix

Freezing of Water

• Freezing itself is not lethal to micro-organisms

• Increase the solute concentration

– Osmotic pressure

– pH

• Freezing is not an effective means of destroying

micro-organisms

Moisture Content

• Wet basis or ‘as is’

Weight of H2O / Weight of sample

• Dry basis

Weight of H2O/ Weight of sample (dry)



Water Activity

• Generally foods with a greater moisture content

are more perishable

• But some foods with the same water content

differ significantly in perishability

– Due to the difference in extent that water

associates with nonaqueous constituents

– Intermediate moisture foods

Water Activity

• Water activity is defined as the ratio of the

vapour pressures of pure water and a solution at

a given temperature:

aw =

o

where aw - water activity

- partial pressure of water in foodo - vapour pressure of water at the

same temperature

The Concept of Water Activity

• The availability of water for

– Physical

– Chemical and – Biochemical reactions

• Physical properties of biological materials aremodified by the presence, amount and activity of water

• Critical factor in the stability of foods

Water Activity

• NOT the total moisture content which includes“bound water”

• It is a measure of the water available to microbesfor growth.



Determination of Water Activity

• %ERH is the measured variable.

Range of Water Activity Values

• 0 (bone dry) - 1.00 (pure water)

• expressed as a decimal fraction of the

equilibrium relative humidity (ERH) of a

particular material.

• A sample containing a water activity reading of

0.91 is equivalent to an (ERH) reading of 91%.

Water activity is not directlyrelated to the moisture

content?

Critical Aw



Bound and Free Water Temperature Dependence of Water Activity

Aw of Common Foods

• >0.95 Fresh fruit, meat, milk

• 0.95-9 Cheese

• 0.9-0.85 Margarine,• 0.85-0.8 Salted meats

• 0.8-0.75 Jam

• 0.75-0.65 Nuts

• 0.65-0.60 Honey

• 0.5 Pasta

• 0.3 Cookies

• 0.2 Dried vegetables, crackers

• Gram –ve 0.97

• Gram +ve 0.90

• Yeasts 0.88

• Moulds 0.80

• Halophiles 0.75

• Xerophiles 0.65

Aw and Changes in Foods



Aw and Changes in Foods

• Mobility of components depend on the available

water

• Bring components together

• Dilution may be a factor – at high Aw levels or

being a product of the reaction

Physical Preservation and Aw

• Preservation without the addition of antimicrobial preservatives

• Utilised in a wide variety of food preservationprocedures

– Evaporation

– Drying

– Salting

– Brining

– Freezing

• Reduce the amount of free water

Physical Preservation and Aw

• Absence of chemical preservatives,

– Aw determines the ability of a system to resistmicrobial growth.

– More tightly water is bound, the lower itschemical potential and Aw.

• Ideal ingredient would have a high moisturecontent with a low water activity.

• Low Aw does not prevent the presence of

microorganisms, it only limit the micro-organismgrowth.

Control of Water Activity

• Include materials with lower water activity

– Salt - Calcium and sodium

– Glycols (PEG’s, propylene glycol) – Alcohols

– Sugars (sorbitol, high fructose corn syrup)

– Glycerin

– Gums (xanthan)

– Proteins



Control of Water Activity

• Substances bind water

– Hydroxyl groups of sugars

– Carbonyl and amino groups of proteins

– Other polar sites that hold water by hydrogen

bonding

– Dipole-dipole forces

– Van der Waals forces

– Ionic bonds

Processing

S e n s o r y q u a l i t y / s h e l f - l i f e

Formula / Aw

Fq Fs

Shelf life

Water activity

Fs = Formula that gives the required shelf-life

Fq = Formula that gives the required quality

The Moisture Sorption Isotherm• The relationship between the equilibrium moisture

content of a material (g of water/g of dry matter) and

water activity, at a given temperature.

• Measured by addition of water to a dry sample

– absorption

• Or by removal of water from a wet sample

– desorption

Region A / I

Region B /II

Region C / III

Aw of a product



The Moisture Sorption Isotherm

• The differences ‘hysteresis’

• Hysteresis could be due to change in theavailability of active polar sites after drying

• During drying

– The polar sites draw more closely together tosatisfy each other

– Reduces the water holding of the material onsubsequent hydration.

The Moisture Sorption Isotherms

Significance

• For concentration and dehydration

processes• To avoid moisture transfer among the

ingredients

• To determine the moisture barrier properties needed in a packaging material

• To determine what moisture content willcurtail growth of microorganisms of

interest• To predict the chemical and physical

stability of food as a function of water content

Hydroscopic?

Composition Effects



Composition Effects – 20% fatSorption Isotherm for Mixtures

Lecture 1 Water (1) Printed

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