1. Water as one of the food components
It is an essential compound of many foods.
It can be found as an intracellular or extracellular constituent in vegetables and animal products.
It may occur as a dispersing medium (e.g. butter) or solvent (e.g. orange juice) in a variety of foods while it may be found as a minor constituent (e.g. milk powder) in some foods.
FDE 101-BCFE
10/8/2019

2. Water content of some foods
Tomato 95
Beer 90
Orange 87
Banana 75
Meat 65
White bread 35
Honey 20
Rice 12
Milk powder 4
FDE 101-BCFE
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3. Structure of the water molecule (H2O)
(-)
Partially negative charges
It is a dipolar molecule which means it has positive and negative poles that are positioned opposite to each other
The distance between the nuclei of hydrogen and oxygen is nm
Partially positive charges
Partially positive charges
The angel between two hydrogen atoms is 104.5°
(+)
FDE 101-BCFE
10/8/2019

4. Types of water Bound water Free water
It is incorporated into the chemical structure of other components, such as proteins, lipids, carbohydrates, as a monolayer/multilayer bounded or capillary absorbed.
Bound water does not freeze and remain unfrozen below the freezing point of water (usually at -20°C)
It is not available to act as a solvent for vitamins, acids, sugars etc…
It is resistant to drying and is not removed.
It is the largest amount of water present in foods.
It is easily removed from the foods by drying, centrifugating and pressing.
It occurs dispersing medium for salts, acids, sugars etc…
It can freeze at temperatures under freezing point.
It triggers reactions since it is reactive.
It can be found in food as a free (e.g. water in fruits) or trapped form (e.g. starch gel).
FDE 101-BCFE
10/8/2019

5. Sorption isotherm of water
Relative humidity can be simply defined as the ratio of the actual amount of water in the air compared to how much the air can hold at a specific temperature
When the water content of food and relative humidity of the enviroment are not equal, the food gains or loses water.
Desorption: Lowering water content of moist foods to reach equilibrium with its surroundings
Adsorption: Increasing water content of dry foods to reach equilibrium with its surroundings
The water content of food and enviroment are same at equilibrium stage. After the equilibrium is reached, the water content of food does not change.
FDE 101-BCFE
10/8/2019

6. Three phase diagram or water
The blue area is a low temperature-high pressure zone in which the water is in solid phase (ice form)
The green area is a high temperature-low pressure zone in which the water is in gas phase (vapor form)
The yellow area is a high temperature-high pressure zone in which the water is in liquid phase (water form)
Triple point is specific condition in which solid, liquid and gas phases are in equilibrium.
Critical point is another specific condition. When the temperature and pressure increase, the density of the water decrease where the pressure of the water increase. At this point, its form is neither liquid nor gas. It gains the new phase, which is called supercritical fluid.
Supercritical fluid
Critical point
(ice)
(water)
Triple point
(vapor)
FDE 101-BCFE
10/8/2019

7. Ice formation When water freezes, it expands nearly 9%.
It means that volume of ice is higher than the volume of water at the same weight.
D = M / V (D: density, M: mass; V: Volume)
The density of ice is lower than the density of water at the same weight.
Rearrangement of hydrogen bonds and the large spaces in the ice molecule could be considered as a results of the increase in volume.
FDE 101-BCFE
10/8/2019

8. The effect of freezing on food quality
The main effect is mechanical damage of cell wall.
According to this damage, some other problems may arise from food to food.
Destabilization of emulsions
Increase in toughness of fish flesh
Loss of textural integrity
Softening in vegetable tissues
Increase in drip loss in meat
FDE 101-BCFE
10/8/2019

9. Water activity (aw)
It is defined as the ratio of the water vapor pressure of food (P) to that of vapor pressure of pure water (Po) at the same temperature.
Water activity ranges from zero (water absent) to 1.0 (pure water).
Free water is responsible for water activity of any food items.
Water activity is a measure of how efficiently (the amount of water and the degree of its availability) the water present can take part in a chemical reaction, microbiological growth, and enzyme activity.
aw: water activity
P: partial pressur eof water in a food
Po: vapor pressure of water at the same temperature
FDE 101-BCFE
10/8/2019

10. Water activity and foods
aw values
Definition of food
Examples
>0.85
Moist foods (moisture content is >50%)
Cheese (aw: )
Fresh fruits, meat, milk (aw: 0.95)
Intermediate moisture foods (moisture content is 10-50%)
Honey (aw: )
Jam (aw: )
Flour, rice (aw: 0.80)
<0.6
Low moisture foods (moisture content is <10%)
Pasta (aw: 0.5)
Dried vegetables (aw: 0.3)
Crackers, cookies (aw: )
As the moisture content increases, perishability increases
FDE 101-BCFE
10/8/2019

11. Reference
deMan, J.M. (1999) Principles of food chemistry. (3rd ed.) Chapter 1: Water (page 1-30) An aspen publication, Maryland, USA.
FDE 101-BCFE
10/8/2019