Water Activity

Water Activity A remarkable compound with

exceptional properties Life depends on the presence of water

in liquid state Cell cytoplasm is an aqueous

environment where many reactions takes place

A permeable cell membrane allows water to move in and out

Water ActivityDynamic of the steady state may be

influence by: Net flow out is higher (plasmolysis) Net flow into the cell (rupture of

membrane) – prevented by the existence of cell wall

Both situation will lead to a “stressed cell/living organisms”

Water Activity Freezing point (0oC) and boiling point

(100oC) Is this liquid state the maximum and

minimum temperatures for growth of microbes?

Freezing points is affected by presence of solutes

Boiling points affected by hydrostatic pressure

Water Activity

Water Activity Micro-organisms can grow a subzero

temperature due to the presence of certain compounds in their cytoplasm – eg. Polyol (an antifreeze)

Bacteria are known to grow at boiling temperature in volcanic vent under sea

Water Activity Cytoplasm needs to be in liquid phase

for active growth Growth and survival? Water exist in the environment in

liquid phase (pure water/solution), gaseous phase in the atmosphere, and solid phase

Water Activity

Water Activity, aw

Movement of water between the cytoplasm and its environment

Water activity of a substrate Define as the ratio of the partial pressure of water in the

atmosphere in equilibrium with the substrate, P, compared with the partial pressure of the atmosphere in equilibrium with pure water at the same temperature, Po

Equilibrium Relative Humidity (ERH) expressed in fraction rather than percentage:

Water Activity, aw

Depends on the number of molecules or ions present in solution rather than their size

Sodium chloride dissociates into two ions in solution is more effective in reducing water activity compared to sucrose on a mole-to-mole basis

Water Activity, aw – Physical Chemist’s view

Prefer chemical potential of water which is a complex parameter made up of:

A reference state A water activity term A pressure term A gravitational term

Water Activity, aw – Physical Chemist’s view In everyday life’s situation, it is possible to ignore the

pressure and gravity terms Therefore the relationship between the water potential

and water activity is given as:

R (gas constant) = 0.08205 dm3 atm K-1 mol-1; Vm (the molar volume of water) = 0.018 dm3 mol-1

At 25oC (298oK) a water activity of 0.9 correspond to a water potential of -143 atm or -14.5 MPa

Water potential Osmotic component – associated with

the effect of solutes in solution Matrix component – associated with

the interaction of water molecules with surfaces

Eg. Rise of water in capillary tube and availability of water in a complex matrix such as a cake

Osmotic pressure Force per unit area required to stop

net flow of water molecules feom a high to a low water activity region

What happens to a micro-organisms placed in pure water?

How does bacteria, fungi and algae cope with this situation?

Osmotic pressureAs water activity is decreased, or osmotic pressure is

increased, in the environment it is essential that the water activity of the cytoplasm is even lower, or its

osmotic pressure even higher How does micro-organisms manage the situation above?

Water activity

Water activity Exact range is influence by physico-

chemical and nutritional conditions

Water activity

Water activity

Water activity – important terms Halotolerant – able to grow in the

presence of high concentrations of salt

Osmotolerant – able to grow in the presence of high concentrations of unionized organic compounds

Xerotolerant – able to grow on dry foods

Water activity and food spoilage The limiting value of water activity for the

growth of any micro-organism is about 0.6 – below this value the spoilage of foods is not microbiological but may be due to insect damage or chemical reactions such as oxidation

At water activity of 0.6 or water potential of -68 MPa, cytoplasm need to have high concentrations of compatible solute to prevent macromolecules such as the DNA to malfunction and thus active growth is affected

Water activity and survival At low water activity with no active

growth, many microbes can still survive

This is the basis of culture collections storage

Water activity and water content Water content does not reflect well on how available

that water is, i.e. what the water activity is – needs to establish a relationship between these two parameters

E.g. oil-rich nuts (4-9% water content), protein rich legumes (9-13% water content), and sucrose rich dried fruits (18-25% water content) all have water activity of 0.7 – susceptible to spoilage by most micro-organisms

The relationship between water activity and water content is very sensitive to temperature