Water activity and bacterial growth: How capacitive sensors and chilled- mirror dewpoint sensors compare
WATER ACTIVITY IS A CRITICAL FACTOR that determines the shelf life of food products. While temperature, pH and several other factors can influence whether an organism will grow in a food product and the rate at which it will grow, water activity may be the most important factor. 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 the water activity of foodstuffs, 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 on food color, taste and aroma. Controlling water activity is an important way to maintain the chemical stability of foods.
Maillard non-enzymatic reactions.
Foods containing proteins and carbohydrates, for example, are prone to non- nzymatic browning reactions, called Maillard reactions. The likelihood of Maillard reactions browning a product increases as the water activity of foods 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 foods, measuring and controlling water activity is a good way to control Maillard browning problems.
Slowing down enzymatic reactions.
The spontaneous autocatalytic breakdown of triglycerides (the major constituent of fats and oils) to produce free-fatty acids is strongly influenced by water activity. This type of food spoilage increases at high water activity values and results in formation of highly objectionable flavors and odors. 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. Of course, for foods that are thermally treated during processing, enzymatic spoilage is usually not a primary concern.
Free water vs. bound water.
Water activity instruments measure the amount of free, unbound water (sometimes referred to as "active" water) present in the food sample. A portion of the total water content present in foods is strongly bound to specific sites on the chemicals that comprise the foodstuff. 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. Additionally, some water in foods is bound less tightly but is still not available (as in a solvent for water-soluble food components). Many food 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 occurring in foods during storage. The processes used to reduce the amount of free water in foods include techniques like concentration, dehydration and freeze drying. Freezing is another common approach to controlling food 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 foods don't always agree. Water activity tells the real story.
Types of water activity sensors.
The water activity of a food sample can be determined from the relative humidity of the air surrounding the sample when the two are at equilibrium. The sample must be in an enclosed space. The sample and the water vapor in the air need time to come to equilibrium. Then, 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.
Capacitive sensor theory.
Many aw sensors use capacitance to measure water activity. They 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. Capacitive sensors need between 30 and 90 minutes to come to temperature and vapor equilibrium. Accurate measurements require good temperature control.
Chilled Mirror Theory
AquaLab uses a chilled-mirror sensor to measure dewpoint. The instrument's fan circulates air in the sensing chamber, speeding up vapor equilibrium. An infrared thermometer measures sample temperature independently. AquaLab can then compute aw from first principles without making assumptions about sample temperature. Independent sample temperature measurement also virtually eliminates the need for temperature control.
Choosing a measurement tool.
Which sensor works best for measuring the water activity of foods? The major advantage of the chilled mirror dewpoint method, which is a primary method approved by AOAC International, is speed. While electronic capacitive sensors usually require 30 to 90 minutes to reach equilibrium relative humidity conditions, chilled mirror instruments can make measurements in less than 5 minutes. AquaLab employs a fan to speed up equilibrium. For some applications, fast readings allow food processors to perform at-line monitoring of a food product's water activity. Processing changes can then be made during production. Chilled mirror instruments make readings over a wider water activity range (from 0.0300 to 1.000 aw) than capacitive water activity sensors. With chilled-mirror technology, temperature control is unnecessary for most applications. The CX-2 is ideal for the measurement of products at room temperature, (22-28°C).
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 foods, water activity is an important property. It predicts food 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 low-acid foods. They use this, and other criteria, to determine whether a scheduled process must be filed for the thermal destruction of Clostridium botulinum. ("Low-acid foods means food with a finished equilibrium pH greater than 4.6 and a water activity greater than 0.85.") 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 food product designers but for food quality control labs.