Moisture Requirement/water Activity

by

One of man’s oldest methods of preserving foods is drying or desiccation. The
preservation of foods by drying is a direct result of removal of moisture.
Microorganisms need water for growth. Without water no growth can occur.
The exact amount of water needed for growth of microorganisms varies. This
water requirement of microorganisms is best expressed in terms of available
water or water activity aw, the vapour pressure of the solution (of solutes in
water in most foods) divided by the vapour pressure of the solvent (usually
water).

Thus aw, for pure water would be 1.00. The water activity depends on
the number of molecules and ions present in solution, rather than their size.
Thus a compound like sodium chloride, which dissociates into two ions in
solution, is more effective at reducing the water activity than a compound like
sucrose on mole-to-mole basis.

Effect on Microbial Growth and Activity

Bacteria require higher values of aw for growth than fungi. Gram-negative
bacteria have higher requirements than gram positives. Most spoilage bacteria
do not grow below aw 0.91, while spoilage molds can grow as low as 0.80.
However, food-poisoning bacteria like Staphylococcus aureus can grow at aw
as low as 0.86, while Clostridium botulinum does not grow below 0.94. 

Yeasts
and molds can grow over a wider aw range than bacteria. The lowest aw values
for bacteria is 0.75 for halophilic (meaning salt-loving) bacteria, while xerophilic (dry-loving) molds and osmophilic (preferring high osmotic
pressures) yeasts can grow at aw values of 0.65 and 0.60, respectively. 

The
limiting value of water activity for the growth of microorganisms is about 0.6
and below this value the spoilage of foods is not due to microorganisms but
may be due to insect damage or chemical reaction such as oxidation. 

At a
water activity of 0.6, corresponding to a water potential of -68MPa (Mega
Pascals), the cytoplasm would need to contain very high concentrations of an
appropriate compatible solute and it is probable that the macromolecules such
as DNA would no longer function properly and active growth may stop.

Most bacteria grow well in a medium with a water activity aw approaching
1.00 (at 0.995 to 0.998), i.e., they grow best in low concentrations of sugar or
salt. Culture media for most bacteria contain not more than 1 per cent of sugar
and 0.85 per cent of sodium chloride (physiological salt solution). 

As little as
3 to 4 percent sugar and 1 to 2 percent salt may inhibit some bacteria. The
optimal aw and the lower limit of aw for growth vary with the bacterium, as
well as with food, temperature, pH, and the presence of oxygen, carbon
dioxide, and inhibitors. 

The optimal aw and the lower limit of aw for growth is
lower for bacteria which are able to grow in high concentrations of sugar or
salt. Some examples of lower limits of aw for growth of some food bacteria are
given in Table 2.2. These figures would vary depending on conditions used for
growth of the microorganisms as mentioned above.

Minimum aw values for growth of microorganisms of
importance in food

Molds differ considerably in optimal aw and range of aw for the germination of
asexual spores. The minimal aw for spore germination is as low as 0.62 for
some molds and as high as 0.93 for others (e.g., Mucor, Rhizopus, and
Botrytis). Each mold also has an optimal aw and range of aw for growth.
Examples of optimal aw are 0.98 for Aspergillus sp., 0.995 to 0.98 for
Rhizopus sp., and 0.9935 for Penicillium sp. 
The aw would have to be below
0.62 to stop all chances for mold growth, although aw below 0.70 inhibits most
molds that cause food spoilage. The reduction of the aw below the optimum
for a mold delays spore germination and reduces the rate of growth and
therefore is an important factor in food preservation. Many of the molds can
grow in foods with aw approaching 1.00 (pure water).
With a reduction of water activity of food, the number of microorganisms capable of maintaining active growth in it decreases. On the other hand, there
are microorganisms that grow better at reduced aw. These microorganisms are
generally associated with foods having low water activity. 
Since low water
activities are associated with three distinct types of food, the following three
terms are used to describe the microorganisms especially associated with these
foods:  

i) Halotolerant – able to grow in the presence of high concentrations of salt 

ii) Osmotolerant – able to grow in the presence of high concentrations of
nonionized organic compounds such as sugars. 
iii) Xerotolerant – able to grow on dry foods.
The halobacteria are obligately halophilic and cannot grow in the absence
of high concentration of salt.

Ways of Reducing Water Activity

Water is made unavailable in various ways: 
1. Solutes and ions tie up water in solution. Solutes lower aw and this
reduction in aw depends on the total concentration of dissolved molecules
and ions. Since these bind to water molecules, there is reduction in aw. 
Therefore, an increase in the concentration of dissolved substances such as
sugars and salts is in effect a drying of the material (Graph 2.1). Not only
is water tied up by solutes, but also water tends to leave the microbial cells
by reverse osmosis to maintain equilibrium between the concentration of
solute outside and inside the cells. 
2. Hydrophilic colloids (gels) make water unavailable. As little as 3 to 4
percent agar in a medium may prevent bacterial growth by leaving too
little available moisture. 
3. Water of crystallization or hydration is usually unavailable to
microorganisms. Water itself, when crystallized as ice, no longer can be
used by microbial cells. The aw of water-ice mixtures (vapour pressure of
ice divided by vapour pressure of water) decreases with a decrease in
temperature below 0o
C (Graph 2.2). 
In a food, as more and more ice is
formed, the concentration of solutes in the unfrozen water increases, thus
lowering available water and thereby it’s aw is reduced. 
The water activity aw varies with temperature; these variations are only slight
within the range of temperatures that permit microbial growth. Variations in
temperature increase in importance with increasing concentrations of solutes
and increasing effects on ionization of solutes.
Each microorganism has a maximal, optimal, and minimal aw for growth. 
As
the aw is reduced below the optimal level, there is a lengthening of the lag
period of growth, a decrease in the rate of growth and a decrease in the
amount of cell substance synthesized, changes that vary with the organism and
with the solute employed to reduce aw. This range depends on a number of
factors which are mentioned below.

Factors Affecting Water Requirement

Factors that may affect aw requirements of microorganisms include: 
1. Kind of solute employed to reduce the aw: For some organisms, like molds,
the lowest aw for growth is independent of the kind of solute used. For
other organisms, however, lower limiting aw values differ from solute to
solute. 
For example potassium chloride usually is less toxic than sodium
chloride, and it in turn is less inhibitory than sodium sulphate. Thus,
sodium sulphate at a lower concentration may be as effective in reducing
aw as potassium chloride at a higher concentration.
2. Nutritive value of the culture medium: In general, the better the medium
for growth, the lower the limiting aw permitting growth of microorganism. 
3. Temperature: Most organisms have the greatest tolerance to low aw at
about optimal temperatures.
4. Oxygen supply: Growth of aerobes takes place at a lower aw in the
presence of air than in its absence, and the reverse is true of anaerobes. 
5. pH: Most organisms are more tolerant of low aw at pH values near
neutrality than in acid or alkaline media. 
6. Inhibitors: The presence of inhibitors narrows the range of aw for growth
of microorganism.
Each organism has its own characteristic optimal aw and its own range of aw
for growth in a given set of environmental conditions. This range of aw
permitting growth is narrowed if any of the above mentioned environmental
factors are not optimal and is narrowed still more if two or more conditions
are not favourable. 
An unfavourable aw will result not only in a reduction in
the rate of growth but will also reduce the yield of cells. The delay (lag) in
initiation of growth or germination of spores will be more under more
unfavourable aw of the substrate. It is known that growth of at least some cells may occur in high numbers at reduced aw values, but the production of certain
extracellular products may be limited or these may not be produced at all. 
For
example, reduced aw results in the cessation of enterotoxin B production by
Staphylococcus aureus even though high numbers of cells are produced at the
same time This often is as important in food preservation as reduction in the
rate of growth of the organism. 
Microorganisms that can grow in high
concentrations of solutes, e.g., sugar and salt, obviously have a low minimal
aw. Halophilic bacteria require a certain minimal concentration of dissolved
sodium chloride for growth. Osmophilic yeasts grow best in high concentrations of sugar.

Leave a Comment