1. WATER ACTIVITY μwfood= μwvapor
Consider a food system enclosed in a container. All the components in the food are in thermodynamic equilibrium with each other.
Considering moisture within the food system and the vapor in the headspace, their chemical potentials are equal to each other:
μwfood= μwvapor
The activity of water in foods can be expressed as:

2. Water activity in foods can be expressed as:
Water activity can be defined as the ratio of the vapor pressure of water in the system to the vapor pressure of pure water at the same temperature.
It can also be expressed as the equilibrium relative humidity (ERH) of the air surrounding the food at the same temperature.
Water activity is an important property in food systems. Most chemical reactions and microbiological activity are controlled directly by the water activity.
In food science, it is very useful as a measure of the potential reactivity of water molecules with solutes.

3. PREDICTION OF WATER ACTIVITY
Raoult’s law is the basic equation for determination of water activity of ideal solutions. According to Raoult’s law, water activity is equal to the mole fraction of water in the solution:
where Xw is the mass fraction and M is the molecular weight. The subscript w is for water and s for solute
Raoult’s law is not valid for macromolecular solute due to the very low molecular weight ratio of water and solute. If the solute ionizes in solution Raoult’s law can be written as:
where ψ is the degree of ionization.

4. Exercise 1
NaCl, sucrose or the NaCl-sucrose solutions are commonly used for osmotic dehydration of potatoes.
Estimate the water activity of 20% sucrose solution.
Estimate the water activity of 20% NaCl solution
Estimate the water activity of solution containing 10% NaCl and 10% sucrose.
Which solution do you think will be more efficient for osmotic dehydration of potatoes having water activity of 0.942?
Data:
Molecular weight of water: 18 kg/kg-mole
Molecular weight of NaCl: kg/kg-mole
Molecular weight of sucrose: 342 kg/kg-mole
NaCl ionizes and its maximum degree of ionization, ψ: 2.

7. WATER ACTIVITY MEASUREMENT METHODS Will be based on
Colligative Properties
Isopiestic Transfer
Hydrometers
Hyrdroscopicity Of Salts

8. EFFECTS OF TEMPERATURE ON WATER ACTIVITY
The temperature dependence of water activity can be described by the Clasius-Clapeyron equation if the isosteric heat and water activity values at one temperature are known
where qst is net isosteric heat of sorption or excess heat of sorption, and aw1 and aw2 are the water activities at temperatures T1 and T2.

9. EFFECTS OF PRESSURE ON WATER ACTIVITY
The effects of pressure on adsorption isotherm are relatively small and negligible at reasonable pressure levels. The variation of water activity with pressure at constant water activity is given as:
where aw1 and aw2 are water activities at total pressures P1 and P2 (Pa), R is the gas constant (8314 m3 Pa/kg-mole K), and T is temperature (K).

10. ADJUSTMENT OF WATER ACTIVITY
The simplest method to obtain the sorption data of foods is storing a weighed sample in an enclosed container maintained at a certain relative humidity, at constant temperature, and reweighing it after equilibrium is reached.
Theoretically, at equilibrium water activity of the sample is the same as that of the surrounding environment. However, in practice a true equilibrium is never attained because that would require an infinitely long period of time.
Therefore, the sample is weighed from time to time during equilibration. When the difference between successive weights of the sample becomes less than the sensitivity of the balance being used, it is accepted that equilibrium is reached. The moisture content of the sample is then determined.

11. The desired relative humidity environments can be generated by using saturated salt solutions, sulfuric acid, or glycerol.
Table below shows the water activity of selected saturated salt solutions at different temperatures. Although saturated salt solutions are commonly used, they provide only discrete water activity values at any given temperature. Water activity of most salt solutions decreases with an increase in temperature.

12. SORPTION ISOTERM
A moisture sorption isotherm describes the relationship between water activity and the equilibrium moisture content of a food products at constant temperature. It is also called the equilibrium moisture content curve.
Desiccators can be used for preparation of sorption isotherms. In the desiccator method, saturated salt solutions, sulfuric acid or glycerol solutions are put into the bottom of desiccators .

13. Although the desiccator method is very commonly used for water activity determination and preparation of sorption isotherms, there are some errors arising from this method. The error comes from the disturbance of equilibrium caused by opening the desiccators, taking the sample, and closing it again.
These disturbances cause adsorption of water from the surrounding air by samples with low water activities and desorption of water from samples having high water activities.
If desorption occurs, the results are not affected significantly since desorption occurs slowly. However, if adsorption occurs, water activity is affected significantly since adsorption is a fast process.

14. Equilibrium moisture content (X
Equilibrium moisture content (X*) is the moisture content of a substance at equilibrium with a given partial pressure of the vapor. It is used to describe the final moisture content that will be reached during drying.
Free moisture content is the moisture content in a substance in excess of equilibrium moisture content (X − X*). Free moisture can be removed by drying under the given percent relative humidity.
The moisture content data can be given in a dry or wet basis. Moisture content is in a dry basis if it is expressed as the ratio of the amount of moisture in the food to the amount of dry solid (kg of moisture/kg of dry solid). If the moisture content of a sample is described as the ratio of the amount of water in the food to the total amount of wet solid (kg of moisture/kg of wet solid), it is in a wet basis.
Moisture content is usually given in a wet basis to describe the composition of the food material. It is more common to use moisture content in a dry basis to describe moisture changes during drying.

15. The sorption isotherm is useful to determine the shelf life and to assess the background of operations such as drying, conditioning, mixing, packaging, and storage.
The isotherm also gives information about the specific interaction between water and the product since it directly relates the thermodynamic potential (Gibbs free energy) of water in the system to its mass fraction
The two principal techniques used for the adjustment of water activity are the integral and differential methods.
In the integral method, several samples are prepared and each is placed under a controlled relative humidity environment simultaneously. The moisture contents of the samples are measured after constant weight is attained.
In differential method, a single sample is placed under successively increasing or decreasing relative humidity environments. Moisture content is measured after each equilibration. The differential method has the advantage of using only a single sample. As a result, the error coming from the sample variation is eliminated.
However, since equilibration can take several days, the sample may undergo various degenerative changes. The integral method avoids this problem because each sample is discarded after appropriate measurement is made.

16. Classification of standard moisture sorption isotherms
Sorption isotherms of food materials are generally in sigmoid shape (type II). The effects of Raoult’s law, capillary effects, and surface–water interactions are important in sorption curves and they are additive.

17. Type I isotherm is observed in pure crystalline sugar
Type I isotherm is observed in pure crystalline sugar. It shows very little moisture gain up to a water activity of 0.7 to 0.8 since the only effect of water is hydrogen bonding to the –OH groups present on the surface of the crystal
Thus, surface effect is important, which means grinding the sugar to smaller particles will increase the moisture content at low water activity values. As the water activity is increased, water begins to penetrate into the crystal, causing dissociation of sugar–sugar interactions and a solution is obtained. At this stage, the effect of Raoult’s law is important.
The type III isotherm is observed in the case of anticaking agents. In these types of materials, binding energy is so large that water activity is depressed while water is absorbed. When all the binding sites are filled, the increase in moisture content causes water activity to increase drastically.

18. Hysteresis
Sorption isotherms can be generated from an adsorption process (starting from a dry system having a zero water activity) or a desorption process (starting with a wet system having a water activity value of 1). The difference between these curves is defined as hysteresis (see figure below).
Hysteresis is observed in most hygroscopic foods. In the figure, in region A, water is tightly bound. In region B, the water is less tightly held and usually present in small capillaries, and in region C, water is free or loosely held in large capillaries.
Desorption isotherms usually give higher moisture content than adsorption isotherms.