Formula Foods

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 is from 0.030 to 1.000a_w, with a resolution of ±0.001a_w and accuracy of ±0.003a_w. Measurement time is typically less than five minutes. Capacitance sensors have the advantage of being inexpensive, but are not 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 a_w, with a resolution of ±0.005a_w and accuracy of ±0.015a_w. 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.

Chilled-mirror theory.

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 a_w 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.

Purchasing decisions.

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, a_w, 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.

Freezing systems.

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.

Sorption isotherms.

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 and temperature.

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.