Water class lecture

1. 9/8/2012
1
Food Chemistry
Water
Present By: GM. Rabiul Islam
Assistant Professor
Department of Food Engineering and Tea Technology
Shahjalal University of Science and Technology, Sylhet
Water
• Interactions with water
•macroscopical : waterbinding/waterholding
capacity
– molecular level : kinds of water
>bound water
– constitutional
– vicinal or monolayer
– multilayer
• free water
(Vimrul)
16

2. 9/8/2012
2
Water content of some food
Water activity
aw = P/P0 = ERH/100
P = partial vapor pressure of food moisture at
temperature T
P0 = saturation vapor pressure of pure water at T
ERH = equilibrium relative humidity at T
aw=N, n'/n'+n''.. this is ERH eqn.

3. 9/8/2012
3
Properties of bound water
Property Constitutional
water
Vicinal water Multi layer water
Description Water as a part of Water that interact Water is first layerDescription Water as a part of
no aqueous systems
proteins
Water that interact
with hydrophilic
sites also water in
capillaries ( <
0.1μm)
Water is first layer
sides and in
additional layers
Freezing point Un‐freezable at ‐40
o
C
Un‐freezable at ‐40
o
C
Mostly un‐
freezable; remains
freezable with
greatly reducinggreatly reducing
freezing point
Solvent capacity None None Slightly to
moderate
Evaporation
enthalpy
Increased greatly Increased greatly Increased slightly to
moderately
Properties of bound water
Property Constitutional
water
Vicinal water Multi layer water
Water in high
moisture food ( 40
%)
< 0.03 %) 0.05 ± 0.4% 3% ± 2
Relation to
sopration isother
Zero aw left of zone
1
Zone 1 consists of
constitutional and
vicinal water zone 2
mainly
Deteriorative
reaction
Auto oxidation Optimum stability
at 0.2‐0.3 aw
Increase reaction
rate

4. 9/8/2012
4
Water activity of sugar and salt
Water content vs aw

5. 9/8/2012
5
Sorption isotherm
•link water activity and water content (g water/100g dry mater)
•link with kinds of water
Sorption isotherm

6. 9/8/2012
6
Sorption isotherm (hysteresis)
Sorption isotherm‐influence of
temperature

7. 9/8/2012
7
.
Sorption isotherm‐modeling
BET equation
m= moisture content H2O/g dry mater
m1 = monomolecular moisture content
e= constant

8. 9/8/2012
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Sorption isotherm‐modeling
Sorption isotherm‐modeling

9. 9/8/2012
9
Sorption isotherm‐influence of
temperature
Characterization of water
• kinds of water –matrix interaction
diffusion measurements
di‐electrical measurements
– dielectric relaxation (principle microwave)
– depending upon kind of water

10. 9/8/2012
10
Characterization of water
• diffusion measurements
Characterization of water (di‐electrical
measurements)•di‐electrical measurements: The mobility of water depend on the degree of
binding and can be measured by di‐electricl analysis
>dielectric relaxation (principle microwave)
> depending upon kind of water> depending upon kind of water

11. 9/8/2012
11
Characterization of water (di‐electrical
measurements)
Characterization of water (NMR: Nuclear
magnetic resonance measurement)

12. 9/8/2012
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Characterization of water (NMR
measurement)
There are two relaxation mechanism have been
id tifi didentified
1. Cross relaxation
2. Proton exchange
Characterization of water (NMR
measurement: Cross relaxation)

13. 9/8/2012
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Characterization of water (NMR
measurement: Proton exchange)
Characterization of water ( DSC
measurement)
DSC (differential scanning calorimetry): measurement as a function of
heating and cooling

14. 9/8/2012
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Relationship water activity and
stability
• physical stability
• chemical stability
• biochemical stability
• microbiological stability
Relationship water activity and
stability
Relative reaction ratee

15. 9/8/2012
15
Relationship water activity and food
stability
Some time the relation is not valid !
‣eg. hurdle theory (microbiological stability)
‣water activity = equilibrium, but foodstuffs are
metastable
‣frozen foodstuffs:
Relationship between water activity and temperature:
above and below freezing

16. 9/8/2012
16
Glass transition
Viscosity as a function of reduced temperature Tm/T for glassy and
particularly for
Glass transition (Fringed micelle
structure model)
Composed of microcrystalliates with a melting T=Tm and
amorphous regions with glass transsion T=Tg

17. 9/8/2012
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Glass transition ( state diagram)
Glass transition (State diagram)

18. 9/8/2012
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Glass transition
Tg depend upon?
Glass transition and mobility
WLF kinetics versus Arrhenius kinetics
i fl f t tinfluence of temperature on
– mobility
– viscosity
– reaction speed

19. 9/8/2012
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Glass transition and mobility
Relationship between aw, glass
transition and food stability

20. 9/8/2012
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Relationship between aw, glass
transition and food stability
Relationship between aw, glass
transition and food stability