Water activity of foods is an important thermodynamic property affecting stability with respect to physical, chemical and microbiological changes. Water activity, aw, is the ratio of the vapor pressure of water in a system, (Pw)sy, to the vapor pressure of pure water, (Pv)w, at the same temperature. It is equal to the equilibrium relative humidity (ERH) established in the surrounding air. Thus:
Several instruments based on electronic sensors are commercially available for direct determination of water activity. This type of equipment is based on the measurement of the characteristic response of an electronic sensor in equilibrium with the air in a small chamber containing the sample in a plastic cup (usually with a volume of 15 ml). A schematic diagram of typical equipment is shown in Figure A2.5.1. Different types of sensor are commercially available (Rahman, 1995). Currently, most food laboratories are using sensor-type instruments, especially immobilized salt solution sensors.
In electrical resistance- or capacitance-type sensors, the conductivity or capacitance of a salt solution (usually lithium chloride) in equilibrium with the air is measured. The electrolytic sensor consists of a small hollow cylinder covered with a glass fiber tape impregnated with saturated lithium chloride solution. A spiral bifilar electrode is wound over the tape and a temperature sensor is mounted at the center of the cylinder. An alternating voltage is applied to the electrodes and a current is allowed to pass through the electrolyte. The resulting rise in temperature opposes the absorption of moisture by the lithium chloride, and the sensor rapidly reaches an equilibrium temperature at which the vapor pressure of the salt solution equals that of the air. Temperature is determined by a sensor at the core of the cylinder and a calibration chart is used to convert this to relative humidity. Hygroscopic organic polymer films are sometimes used instead of lithium chloride salt. Another type of sensor is the anodized aluminum sensor. This sensor
Figure A2.5.1 Schematic diagram of setup for water activity measurement by electronic sensor.
Vapor Pressure Measurements of Water
consists of an aluminum strip that is anodized by a process that forms a porous oxide layer. A very thin coating of gold is then evaporated over this structure. The aluminum base and the gold layer form the two electrodes of what is essentially an aluminum oxide capacitor (Smith, 1971; ASHRAE, 1993; Rahman, 1995).
A multilaboratory, multi-instrument study of the use of electronic sensors applied to a cross-section of commodities and reference standards demonstrated that measurements of water activity by the described method (with instruments using immobilized salt solution sensors) can be made with an accuracy and precision within ±0.01 water activity unit, provided there are no interactions between the instrument and the commodity under investigation (Stoloff, 1978). Commodity-instrument interactions did exist with a number of products, especially those containing highly volatile compounds.
10 g saturated salt solution (Table A2.3.1 and Table A2.3.2) or standard supplied with the equipment from the manufacturer 5 g sample from bulk food, retaining its original state (e.g., solid with original structure liquid, or ground sample)
Instrumentation for measuring water activity by electronic sensors (American
Instrument Company, Beckman Instruments, Nova Sina AG, and Rotronic AG) Plastic sample holder with lid (diameter ~4.0 cm; height ~1.0 cm; usually supplied with instrument)
Measurement of Water Activity by Electronic Sensors
1. Turn on the instrument and set it at a specified temperature (that at which water activity of sample needs to be measured), and then keep it at that temperature for at least 20 to 30 min to allow stabilization.
Commercial equipment is usually available for measurements between 25° and 80°C. Calibration standards and samples must be measured at the same known temperature.
2. To prepare a calibration curve, choose at least five different saturated salt solutions that have water activities spread over the range of 0.11 to 0.99 at the temperature chosen for the sample.
Calibration is performed by measuring the water activities of various saturated salt solutions and comparing them to values in the literature or values calculated from Table A2.3.2. Literature values at a range of temperatures are found in Table A2.3.1. Values at intermediate temperatures can be predicted from the equations in Table A2.3.2.
Saturated salt solutions should contain excess salt (i.e., a saturated salt slurry) to ensure that the solution is completely saturated.
3. Place 10 g of each saturated salt solution in a plastic container (usually supplied with the equipment).
4. Place the plastic containers with salt solution in the equipment chamber one at a time for equilibration. Record instrument response at t = 0.
Calibration should be done at the temperature selected in step 1.
5. Continue taking instrument readings of the samples, staggering them so that each is measured at 15-min intervals. Continue until equilibrium is reached (up to 120 min, if necessary).
Two consecutive readings at 15-min intervals that vary by <0.01 aw unit are evidence of adequate equilibration.
6. Record the water activity displayed by the equipment and calculate the correction factor by using the value given in Table A2.3.1 for the solution.
This is the correction factor for the specific instrument.
7. Prepare a calibration curve by plotting the instrument readouts for the five (or more) saturated salt solutions against their actual water activities given in Table A2.3.1.
It is good to calibrate the equipment frequently, for example, every week. If the test sample contains high volatile content, it is preferable that calibration be performed after each experiment. Make all measurements within the range of calibration points, and do not extrapolate the calibration line. Make all measurements in the same direction of change (i.e., from lower to higher water activity or higher to lower water activity).
Measure water activity
8. Place 5 g of sample in a plastic container and then place the plastic container with sample in the equipment chamber for equilibration.
9. Wait until the reading does not change by more than 0.01 aw unit.
Equilibration usually takes 15 to 30 min depending on the type of food and the measurement temperature.
10. Record the water activity reading. Remove the plastic container from the chamber and keep it sealed with its plastic lid until moisture content is measured.
11. Determine the moisture content by gravimetric or other methods (unitsai.i & A1.2).
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