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Week 3  – Water

FST 1800Fundamentals of Food Science

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• To understand physical state of water

• To identify factors influencing boiling andfreezing point of water

• To differentiate between moisture content and

water activity

Learning objectives

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Lecture outline

• States of water

• Factors influencing boiling & freezing point

of water

• Moisture content

• Water activity

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States/phases of water

• Solid, liquid, gaseous

• liquid • steam

• 4th physical state: amorphous glass

• ice

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Phase transition

First order

• Govern changes in physical state betweensolid, liquid and gaseous states

• Examples - melting, crystallization,condensation & evaporation

• Changes occur isothermally

• A certain amount of heat is either released orrequired as the latent heat (H) for the transition

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First order phase transition

• Latent heat of fusion – energy required toconvert 1 g of ice to water at 0C

• Melting – energy is absorbed

• Crystallization – energy is released

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Phase transition cont.

Second order• Occur in amorphous materials

• Transformed from solid “glassy” state to liquid-like rubbery

state at glass transition temperatures, Tg

• No latent heat is involved

• Change in specific heat/heat capacity (energy to raise thetemperature of 1 g of water by 1C)

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Phase transition cont.

(Roos, 2002)

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Vapour pressure

• Pressure that is exerted by vapour molecules that haveescaped from liquid

• High vapour pressure – many water molecules in the vapourstate (low boiling point)

• Low vapour pressure – water molecules do not vapourizeeasily & a few molecules exist in vapour state (high boilingpoint)

• with temperature

• with solute addition (sugar, salt)

(Vaclavik & Christian, 2008)

Na+ Cl+

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Factors influencing freezing point

• At the freezing point, water in its liquid state is inequilibrium with solid state, ice

• Addition of solute (salt, sugar) – depresses the

freezing point because the addition of solute reduces thevapour pressure

(McWilliams, 2008)

Mass of 

sodium

chloride

(NaCl) (%)

Depression

of freezing

point

Actual

freezing

temperature

(C)

2 1.19 -1.19

4 2.41 -2.41

6 3.70 -3.70

8 5.08 -5.08

(Modified from CRC Handbook of Chemistry and Physics, 2004)

Mass of 

sucrose

(%)

Depression

of freezing

point

Actual

freezing

temperature

(C)

2 0.11 -0.11

4 0.23 -0.23

6 0.35 -0.35

8 0.49 -0.49

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Factors influencing boiling point

• Temperature at which vapourpressure has just exceededatmospheric pressure

• At sea level, water boils at

100C

• Altitude – atmospheric pressuredecreases with an increase inaltitude

• Vapour pressure that will exceedatmospheric pressure drops asaltitude rises – boiling point

(McWilliams, 2008)

Elevation (ft) Temperature of  

boiling water (C)

Sea level 100.0

1000 98.9

2000 97.8

4000 95.6

5000 94.4

6000 93.3

7000 92.2

8000 91.1

10,000 88.9

(Modified from McWilliams, 2008)

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Sucrose (%) Temperature of  

boiling water (C)

0 100.0

10 100.4

20 100.6

40 101.550 102.0

60 103.0

70 106.5

80 112.0

90 130.0

Factors influencing boiling point cont.

• Addition of solute – vapourpressure is lowered, increasein boiling point

(Modified from McWilliams, 2008)

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Free & bound water

Free water• Extractable (cutting/squeezing)• Able to dissolve & dilute many substances

Bound water• Cannot be extracted• Cannot flow• Cannot serve as solvent

• Can be frozen at very low temperatures (belowfreezing point of water)

• Exhibit no vapour pressure

(McWilliams, 2008)

(Vaclavik & Christian, 2008)

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Moisture content & water activity

Moisture content

• Quantitative amount of water in a sample on wet or drybasis

• Depends on the amount of material

Water activity (aw)

• Comparison (ratio) of the vapour pressure of waterin a food sample with the vapour pressure of purewater at the same temperature

• Measure free/unbound water

(McWilliams, 2008)

(Vaclavik & Christian, 2008)

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Water activity

(McWilliams, 2008)

Water activity = vapour pressure of water in samplevapour pressure of pure water

aw = p/p0

= ERH (%)/100• if water activity = 0.7,the equilibrium relative humidity (ERH) = 70%

(Decagon services)

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Water activity cont.

(McWilliams, 2008)

(Decagon services) Which way does water move?

20% Moisture 30% Moisture

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Moisture content & water activity cont.

Initial Final

Moisture Content 4% 20%

Water Activity 0.30 0.75

Decagon services

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Type of food product Water activity (aw)

Fresh meat & fish 0.99Bread 0.95

Aged cheddar 0.85

Jams & jellies 0.8

Plum pudding 0.8

Dried fruit 0.6

Biscuits 0.3

Milk powder 0.2

Instant coffee 0.2

Water activity cont.

Water activity (aw)• Water activity scale: 0 (bone dry) to 1 (pure water)

http://www.foodscience.csiro.au/water_fs.htm

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Application of water activity

• Predict growth of bacteria, molds or yeasts

• Monitor shelf life of food

http://www.foodscience.csiro.au/water_fs.htm

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Alter the water activity

• Temperature• Drying• Freezing• Addition of salt/sugar

Food Water activity at

10 (C) 20 (C) 30 (C) 40 (C)

Distilled

water

1.000 1.000 1.000 1.000

Dry soup

mix

0.191 0.239 0.292 0.302

Beef jerky 0.694 0.697 0.693 0.698

Sausage 0.942 0.943 0.944 0.938

(Modified from McWilliams, 2008)(McWilliams, 2008)

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Summary

• Water exists in liquid, crystalline, gas or glassy states

• The physical states/phases of water are interchangeablethrough either absorption/release of heat

• Freezing & boiling points are affected by the addition ofsolutes

• The freezing point of water is depressed whilst theboiling point of water is elevated in the presence ofsolutes

• Altitude influences the boiling point of water

• Water activity is an important parameter in determiningshelf life of food compared to moisture content as it

measures ONLY the free/unbound water