topic 1 water (part 1)

8/18/2019 Topic 1 Water (Part 1)

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CHM2204

Food Chemistry

Topic 1 Water

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Content

1. Role of water

2. Structure

3. Physical function

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Learning Outcome

• Describe the interaction of water with food

components.

• Evaluate water activity and its importance in

dehydrated food products.

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4

Foods % Water

apple 84

orange 87

grapes

81

strawberry

92

broccoli 91

cucumber

96

peppers 92

potato 79

beef, raw

73

chicken, raw 69

beef, cooked

62

chicken, cooked 62

salami, beef 60

bread, commercially prepared

36

dried fruit 31

jams/preserves 30

beef jerky

23

wheat flour 11

cookies/biscuits

6

peanut butter

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Water Contentin Foods


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Role of water

• Affects texture

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Crisp & Turgor

Perception of

tenderness

Role of water

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• Quality

Role of water

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• Quality & shelf life

• Support growth of microorganisms

Role of water

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• Capable of

ionizing (H3O+,

OH

-

)

Solvent

Stabilizing

colloids

Role of water

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• Heating and

cooling

• Cleaning agent

Role of water

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Chemistry of water

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0.096 nm


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Hydrogen bond

• A hydrogen bond is a weak bond between polar

compounds where a hydrogen atom of one molecule

is attracted to an electronegative atom of another

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Hydrogenbonds

Hydrogen bonding of water molecules in a tetrahedralconfiguration.

Each water molecule is able to hydrogen-bond with

four others allowing the formation of an extensive 3D

structure13

Association of water molecules


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• Floating of ice• Coordination number is higher for water than for ice

Liquid water

Ice

Coordination number

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Coordination

number

O-H- - - -O

Ice (0 °C) 4 0.276 nm

Water (1.5 °C) 4.4 0.290 nm

Water (83 °C) 4.9 0.305 nm

Coordination number and distance between

two water molecules


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Water Chemistry


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• The specific heat of water is the energy (in calories or in joules)

required to raise the temperature of 1 g of water by 1ºC, and is

the same whether heating water or ice.

• The latent heat of fusion is the energy required to convert 1 g

of ice to water at 0 ºC and is 80 cal; that is, 1 g of ice at the

freezing point absorbs approximately 80 cal as it changes to the

liquid state.

• The latent heat of vaporization is the energy required to

convert 1 g of water into vapor at 100 ºC and is 540 cal; that is,

1 g of water at the boiling point absorbs approximately 540 cal

as it becomes steam.

Specific heat and latent heat of water

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Specific Heat Values of Selected Food and

Other Materials

Material Phase Cp in

J/(g • °C)

Air Gas 1.01

Aluminum Solid 0.90

Ammonia Liquid 4.70

Apples

(ambient)

Solid 3.59

Copper Solid 0.38

Diamond Solid 0.51

Ethanol Liquid 2.44

Glass Fluid 0.84

Material Phase Cp in

J/(g • °C)

Gold Solid 0.13

Graphite Solid 0.71

Ice Solid 2.04

Lamb

(ambient)

Solid 2.80

Oxygen Gas 0.92

Potatoes

(ambient)

Solid 3.43

Water vapor Gas 2.08

Water Liquid 4.18 21


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Water as a solvent/dispersing medium

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SolutionColloidal

Dispersion

Suspension


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• Water-solute interactions.

• < 1 millimicron (mµ).

• “Water binding” and “Hydration” – refers to general

tendency for water to associate with hydrophilic

substances.

• Dissolving charged molecules e.g. NaCl

• Dissolving polar molecules e.g. sugar

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True Solution


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• Molecules that are too big to form true solutions may be

dispersed in water.

• 0.001 – 1 millimicron.

• Contains dispersed phase and continuous phase.

• Colloidal dispersions are often unstable.

• Salad dressing, milk, mayonnaise and egg white foam are

some examples of colloidal systems in foods.

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Colloidal Dispersion


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• Particles that are > 1 µm are too large to form a

colloidal dispersion.

•These form a suspension when mixed with water.

• The particles in a suspension separate out over a

period, whereas no such separation is observed with

colloidal dispersions.

• Examples of coarse suspension include cellulose,

cooked starch, pectic substances, gums, and

some food proteins in water.

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Coarse Suspension


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• Monolayer Water (~ 0.1-0.9% wet basis of moisture content in

food)

– Bound water in food – water that exists in the vicinity of

solutes. – Strongly acts with specific hydrophilic sites of nonaqueous

constituents.

– Restricted in its movement due to charges, hydrogen bond,

physical entrapment.

– Water-ion and water-dipole bonds. – Hard to remove from food.

– Never be able to remove water completely.

– Does not freeze at -40°C or lower.

– Unavailable as solvent for additional solutes.

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Type of water


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• Multilayer Water (Bulk water) - (~1-5% wet basis of moisture

content in food)

– Additional layer of water around food particle (the hydrophilic

groups). – Water-water and water-solute hydrogen bonds.

– Not as hard to remove as the monolayer.

• Mobile or Free Water - (~ 5-96% wet basis of moisture content

in food) – Flow is unimpeded.

– Properties close to dilute salt solutions.

– Water-water bonds predominate

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Type of water


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How much is the amount of un-freezable

water?

Based on protein content it varies:

about 8-10% in animal tissues

0.4 g/g dry protein in egg and fish


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Thank You...

topic 1 water (part 1)

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