1. Water

2. Water Most abundant substance in living systems
Makes up 70 % or more of the weight of most forms of life
Importance of water:
Carries nutrients and wastes
Solvent
Reaction medium
Lubricant
Influences food structure, appearance, taste and susceptibility to spoilage

3. Physical properties of water & ice

4. Physical properties of water & ice
Temperature dependent parameters
ice
water
-20 oC
0 oC
0oC
20 oC
Density (g/cm3)
0,9193
0,9169
0,9998
0,9982
Vapour pressure (kPa)
0,103
0,6113
06113
2,3388
Heat capacity (J/g/K
1,9544
2,1009
4,2176
4,1818
Thermal diffusivity (m2/s)
2,433
2,240
0,561
0,598
Thermal conductivity (W/m/K)
11,8*10-7
11,7 *10-7
1,3*10-7
11,4*10-7
Vapour pressure: the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate
The heat capacity is the amount of heat required to raise the temperature of an object or substance by one degree.
the thermal conductivity of a substance divided by the product of its density and its specific heat capacity.
Thermal conductivity: the rate at which heat passes through a specified material, expressed as the amount of heat that flows per unit time through a unit area with a temperature gradient of one degree per unit distance.

5. Physical properties of water & ice
Physical properties have a number of important practical considerations:
Slow thawing properties of frozen foods
Use of steam as a heat medium in the food industry

6. Water contents of some foods
Raw pork
Raw beef
Chicken
53-60
50-70 74
Berries, cherries, pears
Apples, peaches, oranges
Strawberries, tomatoes
80-85
85-90
90-95
Green peas
Carrots, potatoes, broccoli
Cabbage, cauliflower, lettuce
74-80
80-90

7. The water molecule V- shaped molecule Partial –ve charge
Partial +ve charges
Polar molecule thus intermolecular attractive forces
Multiple hydrogen bonding (3D) (require much heat to break)

9. The ice molecule Water crystallizes in a open low density structure
Interstitial spaces
Quantity, size and structure influenced by amount and kind of solutes eg sucrose, gelatin, glycerol (gelatin causes formation of more disordered forms of crystals)
Some properties of ice different from those of water (conductivity)

10. The Ice molecule

11. Interactions of water with food stuffs
Two levels of water interactions
Macroscopic and molecular level
Interactions of water at molecular level important to understand chemical and microbiological stability of foods

12. Macroscopic interactions
Water binding and hydration: tendency of water to associate with hydrophillic substances
Water holding capacity: ability of a matrix of molecules to physically entrap large amounts of water such that exudation is prevented eg starch (bulk flow restricted but behaves as free water)
Both concepts refer to the capacity of a food stuff to contain large amounts of water without making the food itself a liquid

13. Molecular interactions: Interaction with charged groups
Interaction between a dipole (water) and a charged group
Interactions are typically stronger than H bonds ( kJ/mol vs kJ/mol)
Charged particles attract water molecules resulting a hydratation layer
4-6 water molecules can be attracted around charged particle because of strong polarity of particle

14. Molecular interactions: Interaction with charged groups
Disruption of normal structure of water, Alter mobility of water
Eg with NaCl, there is hydration of Na and Cl ions (stronger bonds formed)
Structure makers or cosmotropes (reduce availability of water for dissolved molecules, induce formation of hydrophobic clusters)
Increase surface tension of water as dipole-charge interactions dominate vs. H bonds
Surface tension

15. Interaction with charged groups
Impact of several salts on the surface tension of water

16. Interaction with charged groups
Larger ions have a weaker electric field and are less polarizing
Not able to interact with larger quantities of water molecules
Structure breakers or chaotropes (break interaction between hydrophobic molecules)
Improve solubility of proteins, reduce surface tension

17. Interaction with non-polar groups
Thermodynamically unfavorable
Eg reaction with hydrocarbons
Increase in water to water hydrogen bonding causing hydrophobic hydratation
Hydrophobic interactions ( formation of tertiary structure of proteins)
Formation of clathrate hydrates (H atoms and two non bonding orbitals of Oxygen are oriented to the outside of a cage like structure
Important with respect to protein structure

18. Interactions with non polar groups

19. Interactions with polar groups
Interactions between polar, non charged groups is based on H bonds
Strength of H bonds comparable to strength of H bonds between water molecule themselves
H bonds can also be formed between two non-water molecules

21. Different kinds of water
Free water: behaves physicochemical as pure water and can be subjected to macroscopic flow or not
Bound water: can be further differentiated according to strength of interactions with food components
Constitutional water: part of the molecular structure of non water molecules present in the food. Strongly bound to food matrix and not available
Vicinal water: mostly bound to H bonds, can be present in narrow capillaries, strongly reduced availability and known as the monolayer water

22. Different kinds of water
Hydration layer formed around food components attracts other water molecules via H bonds
Expanding hydration layer is formed consisting of several water layers- multi layer water with reduced availability
Transition zone towards the free water
Water content generally poor predictor of food stability
Stability of foods depend on extent to which water is available as a solvent, reactant or to support microbiological development

23. Water activity
Aw = p/po (partial pressure of water above the sample divided by the partial pressure of pure water at the same temp)
Refers to the intensity with which water associates with non aqueous constituents
Used to predict food stability and safety
Influences quality of foods

24. Moisture Sorption Isotherms
Graphical representation of the relationship between the Aw and water content of a food at a constant temp
relationship is complex and unique for each product due to different interactions between the water and food components at different moisture contents.
An increase in Aw is almost always accompanied by an increase in the water content, but in a nonlinear fashion
Difference (called hysteresis) in the adsorption and desorption isotherm curves.

25. Moisture Sorption Isotherms

26. Types of sorption isotherms
Typical of anticaking agents
Once water absorption sites are occupied, no additional uptake is possible and a plateau in moisture content is reached
An anticaking agent is an additive placed in powdered or granulated materials, such as table salt, to prevent the formation of lumps (caking) and for easing packaging, transport, and consumption.

27. Types of sorption isotherms
Sigmoidal shape or S shaped
Typical of foods with more complex food components such as polymeric proteins and Carbohydrates

28. Types of sorption isotherms
J shaped isotherm
Typical for foods rich in crystalline low MW components such as sugars and salts

30. Food stability map

31. Influenze of Aw on important reactions
Enzymatic reactions
Non enzymatic browning reactions
Lipid oxidation
Microbial deterioration of foods

32. Tutorial questions
What is the importance of Aw on the quality of foods
Define Aw and explain why it is used in place of water content in determining perishability of foods
Describe the 3 types of moisture sorption isotherms including the types of water found in food systems
Explain the interactions of water with charged, neutral and non polar groups