Why is water activity important?
Water activity is a critical factor that determines shelf life.
While temperature, pH and several other factors can influence
if and how fast organisms will grow in a product, water activity
may be the most important factor in controlling spoilage.
Most bacteria, for example, do not grow at water activities
below 0.91, and most molds cease to grow at water activities
below 0.80. By measuring water activity, it is possible to predict
which microorganisms will and will not be potential sources of
spoilage. Water activity — not water content — determines the
lower limit of available water for microbial growth. In addition
to influencing microbial spoilage, water activity can play a
significant role in determining the activity of enzymes and
vitamins in foods and can have a major impact their color, taste,
and aroma. It can also significantly impact the potency and
consistency of pharmaceuticals.
Free water versus bound water.
Water activity describes the continuum of energy states of the
water in a system. The water in a sample appears to be “bound”
by forces to varying degrees. This is a continuum of energy states,
rather than a static “boundness.” Water activity is sometimes defined
as “free”, “bound”, or “available water” in a system. These terms are
easier to conceptualize, although they fail to adequately define all
aspects of the concept of water activity. Water activity instruments
measure the amount of free (sometimes referred to as unbound or
active) water present in the sample. A portion of the total water content
present in a product is strongly bound to specific sites on the chemicals
that comprise the product. These sites may include the hydroxyl
groups of polysaccharides, the carbonyl and amino groups of proteins,
and other polar sites. Water is held by hydrogen bonds, ion-dipole
bonds, and other strong chemical bonds. Some water is bound less
tightly, but is still not available (as a solvent for water-soluble
food components). Many preservation processes attempt to eliminate
spoilage by lowering the availability of water to microorganisms.
Reducing the amount of free — or unbound — water also minimizes
other undesirable chemical changes that occur during storage.
The processes used to reduce the amount of free water in consumer
products include techniques like concentration, dehydration and
freeze drying. Freezing is another common approach to controlling
spoilage. Water in frozen foods is in the form of ice crystals and
therefore unavailable to microorganisms for reactions with food
components. Because water is present in varying degrees of free
and bound states, analytical methods that attempt to measure
total moisture in a sample don’t always agree. Therefore, water
activity tells the real story.
Controlling non-enzymatic reactions.
Foods containing proteins and carbohydrates, for example, are
prone to non-enzymatic browning reactions, called Maillard reactions.
The likelihood of Maillard reactions browning a product increases as
the water activity increases, reaching a maximum at water activities
in the range of 0.6 to 0.7. In some cases, though, further increases
in water activity will hinder Maillard reactions. So, for some samples,
measuring and controlling water activity is a good way to control
Maillard browning problems.
Slowing down enzymatic reactions.
Enzyme and protein stability is influenced significantly by water
activity due to their relatively fragile nature. Most enzymes and
proteins must maintain conformation to remain active.
Maintaining critical water activity levels to prevent or entice
conformational changes is important to food quality.
Most enzymatic reactions are slowed down at water activities
below 0.8. But some of these reactions occur even at very low
water activity values. This type of spoilage can result in formation
of highly objectionable flavours and odors. Of course, for products
that are thermally treated during processing, enzymatic spoilage
is usually not a primary concern.
Measuring water activity.
There is no device that can be put into a product to directly
measure the water activity. However, the water activity of a product
can be determined from the relative humidity of the air surrounding
the sample when the air and the sample are at equilibrium.
Therefore, the sample must be in an enclosed space where this
equilibrium can take place. Once this occurs, the water activity of the
sample and the relative humidity of the air are equal. The measurement
taken at equilibrium is called an equilibrium relative humidity or ERH.
Choosing a measurement tool.
Two different types of water activity instruments are commercially
available. One uses chilled-mirror dewpoint technology while the
other measures relative humidity with sensors that change electrical
resistance or capacitance. Each has advantages and disadvantages.
The methods vary in accuracy, repeatability, speed of measurement,
stability in calibration, linearity, and convenience of use.
Which sensor works best for measuring the water activity of products?
The major advantages of the chilled-mirror dewpoint method are
accuracy, speed, ease of use and precision. The AquaLab’s range
isfrom 0.030 to 1.000aw, with a resolution of ±0.001aw and
accuracyof ±0.003aw. Measurement time is typically less than
five minutes.Capacitance sensors have the advantage of being
inexpensive, but arenot typically as accurate or as fast as the
chilled-mirror dewpoint method. Capacitive instruments
measure over the entire water activity range—0 to 1.00 aw,
with a resolution of ±0.005aw and accuracy of ±0.015aw.
Some commercial instruments can measure in five minutes while
other electronic capacitive sensors usually require 30 to 90
minutes to reach equilibrium relative humidity conditions.
In the AquaLab, a sample is equilibrated within the headspace
of a sealed chamber containing a mirror, an optical sensor,
an internal fan, and an infrared temperature sensor. At equilibrium,
the relative humidity of the air in the chamber is the same as the water
activity of the sample. A thermoelectric (Peltier) cooler precisely controls
the mirror temperature. An optical reflectance sensor detects the exact
point at which condensation first appears. A beam of infrared light
is directed onto the mirror and reflected back to a photodetector,
which detects the change in reflectance when condensation occurs on
the mirror. A thermocouple attached to the mirror accurately measures
the dewpoint temperature. The internal fan is for air circulation, which
reduces vapor equilibrium time and controls the boundary layer
conductance of the mirror surface. Additionally, a thermopile sensor
(infrared thermometer) measures the sample surface temperature.
Both the dewpoint and sample temperatures are then used to determine
the water activity.During a water activity measurement, the AquaLab
repeatedly determines the dewpoint temperature until vapor equilibrium
is reached. Since the measurement is based on temperature determination,
calibration is not necessary, but measuring a standard salt solution checks
proper functioning of the instrument. If there is a problem, the mirror is
easily accessible and can be cleaned in a few minutes.
Capacitive sensor theory.
Some aw instruments use capacitance sensors to measure
water activity. Such instruments use a sensor made from a hygroscopic
polymer and associated circuitry that gives a signal relative to the ERH.
The sensor measures the ERH of the air immediately around it.
This ERH is equal to sample water activity only as long as the temperatures
of the sample and the sensor are the same. Since these instruments relate
an electrical signal to relative humidity, the sensor must be calibrated
with known salt standards. In addition, the ERH is equal to the sample
water activity only as long as the sample and sensor temperatures are the
same. Some capacitive sensors need between 30 and 90 minutes to come
to temperature and vapor equilibrium. Accurate measurements with this
type of system require good temperature control.
When evaluating water activity measurements, precision and
accuracy are, of course, important considerations. But equally important
to consider is how susceptible the sensor is to contamination and how
frequently calibration is required. Also, when comparing water activity
instruments, be sure to evaluate precision and accuracy over the entire
range of water activities most commonly found in your specific products.
Water activity — accepted and approved.
For many products, water activity is an important property.
It predicts stability with respect to physical properties, rates of
deteriorative reactions, and microbial growth. The growing recognition
of measuring water activity in foods is illustrated by the U.S. Food and Drug
Administration’s incorporation of the water activity principle in the
definition of non-potentially hazardous foods (Potentially Hazardous
Foods means food with a finished equilibrium pH greater than 4.6 and
a water activity greater than 0.85). They use this and other criteria to
determine whether a scheduled process must be filed for the thermal
destruction of Clostridium botulinum (Botulism). In the past, measuring
water activity of foodstuffs was a frustrating experience. New instrument
technologies have vastly improved speed, accuracy and reliability of
measurements. AquaLab is definitely a tool not only for quality control labs,
but for new product design and development.
Is there “Bound Water” in Foods?
This question occupied a major part of the roundtable discussion
on the glassy state in foods at a recent ISOPOW meeting.
Questions about bound water are not unique to foods. The terms
“bound” and “free” water are also used to describe the state of water
in many porous substances which retain moisture. However, in porous
media physics, as in food science, it is generally not sufficient to just
give qualitative descriptions.
The need for a standardized definition.
It is important to quantify how bound or how free the water is.
Since Decagon serves both food scientists and porous media physicists,
we think it is useful to apply knowledge from both areas to the
quantification of the state of water in porous systems such as foods.
Two ways to bind water in porous systems.
Water in porous systems may be bound in two ways; by lowering the
energy state of the water in the system, and by reducing the rate of
movement of water to interfaces. To measure the energy state of water
we normally choose pure, free water as the reference state (zero energy).
Forces of adhesion and cohesion (van der Waal-London forces) lower
the energy state of adsorbed water compared to pure, free water.
Lower energy binds.
Solutes dilute the water, increasing its entropy and therefore
lowering its energy state. These two effects combine to lower the total
free energy of the water. The lower energy (compared to pure, free water)
of the water in the food binds it. In other words, work would need to be
done on the water to remove it from the food. The energy per unit mass
required to remove an infinitesimal quantity of water from the food and
transport it to the pure, free reference state is called the water potential.
Potential measures binding energy of water.
The water potential is therefore a quantitative measure of the
binding energy of water in the food. As the water content decreases,
the remaining water is more tightly bound, and the work required to
remove water increases. One could say that all of the water in food is
bound, since all of it is at water potentials below (more negative than)
pure free water. The important issue is not whether water is bound,
but how tightly it is bound.
The question of equilibrium.
Water potential describes the thermodynamic state of water
in foods and other porous media, and is an equilibrium measure.
A system is said to be in equilibrium when the water potential is
the same at every location in the system. Food and other porous
systems are often far from equilibrium, and this provides a
second sense in which water can be bound.
If the rate of movement of water in a system is so low that
equilibrium can not be achieved within the normal lifetime of
the food, then the water could be said to be bound. It is hypothesized
that when foods enter the glassy state the movement of water is so
slow that it is effectively bound.
Colligative properties of solutions.
Water activity is a direct measure of the energy state of the water
in food. A well-known equation from thermodynamics relates the
water activity, aw, and the partial specific Gibbs free energy or water
potential (Psi) of a system as follows:
where Mw is the molecular weight of water, R is the gas constant,
and T is the Kelvin temperature.
Freezing point depression.
Most people are familiar with the colligative properties of solutions.
One mole of an ideal solute in water lowers the freezing point of the
water by 1.86 degrees C, raises the boiling point 0.5 degrees C and
increases the osmotic pressure 22.4 atmospheres. People are often
not aware, however, that these properties also apply to the adsorption
of water in porous materials. These adsorptive forces are generally
much larger than solute effects in intermediate moisture foods and
other moderately dry porous media.
Freezing point depression.
If the water potential of a cellulose or protein matrix were -14 kJ/kg,
its water would not begin to freeze until its temperature reached about
-10°C. Water potential (whether from matric forces or from solutes)
can therefore be expressed in terms of freezing point depression.
An interesting result of this reduction in freezing point from the
binding of water is that the water in food and other porous media
does not all freeze at a single temperature like pure free water does.
Since the water potential of the water in the system ranges from its
value in the unfrozen state to very low values, it freezes over a range
of temperatures. An equilibrium exists between the frozen and
unfrozen water in a frozen porous system.
There is always some unfrozen water in the system, and the
unfrozen water content is determined by the temperature of
the system (which sets its water potential or water activity)
and the sorption isotherm of the matrix that holds the water.
Some confusion exists about temperature control during water
activity measurements. Most of this fuzziness is generated by
outdated technologies. Temperature control appears to adhere
to the calf-path metaphor by following where history has trod
on a bumpy path. For the majority of AquaLab users, temperature
control is not necessary or needed.
AquaLab displays water activity
When AquaLab finishes measuring a sample, two numbers are
displayed. The first number is the water activity and the second
is the temperature in degrees Celsius. The water activity is calculated
for the precise sample temperature. The temperature displayed is
the sample’s surface temperature at the time the final water activity
reading was taken.