SOP: Measuring Water Activity (a_w) Using the Novasina aw Center

SOP: Measuring Water Activity (a_w) Using the Novasina aw Center

GIFSQ — Global Institute for Food Safety & Quality

Health Canada, Compendium of Analytical Methods, Volume 3 — Laboratory Procedure MFLP-63, January 2006 (replaces MFLP-63, dated April 1999)

Principle

This method is used to determine the water activity (a_w) of foods, to establish compliance with the requirements of Sections 4 and 7 of the Food and Drugs Act. It replaces MFLP-63, dated April 1999.

The Novasina aw Center consists of a measuring station and a temperature-controlled a_w-box with three sensors. Studies have shown that the Novasina aw Center gives satisfactory results for measuring water activity in foods.

Water activity plays an important role in the preservation of food and in the control of microbial growth, especially pathogens. The Novasina aw Center measures the relative equilibrium humidity in % rh, which is directly correlated with the water activity according to the formula a_w = ERH/100.

Definitions used in this method:

• Water activity (a_w): the ratio of the water vapour pressure in any food system to the water vapour pressure of pure water at the same temperature.
• Equilibration: a change in the a_w reading of ± 0.001 a_w in a 10-minute period.
• Humidity standard: a saturated solution of a salt of known % relative humidity at a designated temperature.

Sample handling

• Keep samples sealed during transport to prevent loss or gain of moisture.
• In the laboratory before analysis, except for shelf-stable foods, keep sample units refrigerated or frozen, depending on the nature of the product.
• Thaw frozen samples in a refrigerator, or under time and temperature conditions which prevent loss of moisture.
• Analyse sample units as soon as possible after their receipt in the laboratory.

Apparatus

Equipment:

• Novasina aw Center (available from Novasina AG, a division of Axair Ltd., or JEB Controls Inc.)

Materials:

• Novasina humidity control standards (available from Novasina AG, a division of Axair Ltd., or JEB Controls Inc.)
• a_w disposable bowls (available from Novasina AG, a division of Axair Ltd., or JEB Controls Inc.)
• Pre-filters (available from Novasina AG, a division of Axair Ltd., or JEB Controls Inc.)

Nominal a_w values of the SAL humidity standards, as printed in the appendices:

• SAL-11: 0.113
• SAL-53: 0.529
• SAL-75: 0.753
• SAL-84: 0.843
• SAL-90: 0.901 (the calibration record prints 0.901; the regression-worksheet example prints 0.902 — each as printed)
• SAL-98: 0.980

a_w of reference salt slushes at 25 °C (the method’s Table I):

Procedure

Calibration of the instrument

Turn the power switch on and leave the instrument for a minimum of 2 hours
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Set the temperature dial to 190 (25 °C) and make sure the temperature is holding
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Set the RTD-24 switches to “S”
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Remove the pre-filter from each sensor (see Maintenance of pre-filters)
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To decide which humidity standard to use for the upper and lower points, consider the anticipated range of the sample and choose a humidity standard range which will include this value. Only samples within the range of the upper and lower points can be reported as a numerical value rather than > the upper point or < the lower point.
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For routine analysis of food, use SAL-75 as the lower point and SAL-98 as the upper point.
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If using more than one humidity standard with the same water activity, dedicate one humidity standard to one sensor and label it.
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Calibration can proceed two ways: by using the built-in dummy values (see the Novasina Operation Manual) or by using the D-Pot procedure (below). With only one D-Pot cable there will be some connecting and disconnecting to calibrate all three sensors at the same time.
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Adjustment of the low range is optional and should be done after the medium and high range adjustments are made. Do this using SAL-53.
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To check the linearity of the calibration, measure the a_w of 4 humidity standards with each sensor and calculate a regression line (see Calculation). The salts used should include the upper and lower points plus two in between (see the salt table above for a selection of salts). Dedicate one humidity standard to one sensor if you have more than one humidity standard with the same water activity.
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Use the Calibration Record form in the method’s first appendix to record the calibration and regression line data.
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Install fresh pre-filters in front of each sensor.

D-Pot calibration procedure

Perform calibration by inserting the D-pot into the junction of each wire between the conditioning chamber and the control apparatus
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Install SAL-75 and leave it for about 2 hours to reach equilibrium
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Set the D-Pot selector button on “S” or “sensor”, depending on the unit, and the channel selector button to the channel to be calibrated
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When equilibrium has been reached, if there is a difference between the SAL-75 value (0.753 at 25 °C) and the digital readout on the a_w center, adjust the value by turning the screw of trimmer potentiometer “M”
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Set the D-pot selector button on “D” or “POT”, depending on the unit, and adjust the D-pot setting using the thumb wheels until the digital readout on the a_w center is 0.753. Record the D-pot value on the calibration sheet.
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Set the D-pot selector on “S” or “sensor”
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Record the time at which the final readings were taken, the temperature, the channel number and the humidity source
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Replace SAL-75 with SAL-98, marking the time at which the salt was placed under the sensor
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Wait about 2 hours for equilibrium to be reached
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Place the selector button on the channel to be calibrated
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When equilibrium has been reached, if there is a difference between the SAL-98 value (0.980 at 25 °C) and the digital readout on the a_w center, adjust the value by turning the screw of trimmer potentiometer “H”
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Place the D-pot selector button on “D” or “POT” and adjust the D-pot setting using the thumb wheels until the digital readout on the a_w center is 0.980. Record the D-pot value on the calibration sheet.
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Adjust the thumb wheels of the D-pot to the value recorded while adjusting “M” and turn the “M” trimmer potentiometer screw so that the digital readout on the a_w center is 0.753
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Adjust the thumb wheels of the D-pot to the value recorded while adjusting “H” and turn the “H” trimmer potentiometer screw so that the digital readout on the a_w center is 0.980
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Repeat the last two steps until the 0.753 and 0.980 values appear on the digital readout.

Moving the “M” and “H” trimmer potentiometers results in a reciprocal value change. Through a series of adjustments, these values become increasingly accurate. The apparatus is now calibrated.

Validation of the humidity standards

To determine the reproducibility or variation of the humidity standards, take 20 daily a_w measurements of each humidity standard for each sensor, using the standard dedicated to that sensor, and record each on a Novasina Control Chart. For Novasina-supplied humidity standards, the upper and lower limit should not exceed the labelled value ± 0.005, which is the Novasina stated claim.
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To determine the accuracy of uncertified humidity standard material, use a method that is independent of the calibration methodology, in order to have a meaningful check on the standard material.
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There are several options:

• Send blind samples to another CFIA facility to be checked on the same type of a_w apparatus.
• Test blind samples on a type of apparatus different than the one you are using.
• Send blind samples to an external calibration laboratory to certify the standards.

Certified humidity standards are best if available.

Preparation of control charts

Prepare quality control charts using graph paper or a software package. Determine the acceptable limits of the humidity standards (see Validation of the humidity standards above).
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Determine the upper and lower limits of the humidity standards to be used to determine readiness (usually SAL-90 and SAL-98), using the data generated in the validation step. After 20 readings have been taken, calculate the mean and standard deviation for each sensor and each humidity standard.
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Use this data to determine the upper and lower limits, where the upper and lower warning limits equal 1.5 standard deviations and the upper and lower control limits equal 2 standard deviations.

Determining the readiness of the a_w Center

The humidity standards used to determine whether the a_w center is ready to take sample readings must not be the same standards used to calibrate the unit.
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Before taking sample readings, check the validity of the calibration by taking a reading of the humidity standard nearest to the anticipated water activity of the sample(s), usually SAL-90 or SAL-98 for most foodstuffs.
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Correct the reading using the regression line or regression formula of the last valid calibration and plot this value on the appropriate control chart for the sensor and the standard. Proceed with the sample measurement if the value falls between the upper and lower limits determined in the control-chart step.
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If the humidity standard reading is outside the upper or lower limits, repeat the reading 3 times, allowing the sensor to return to ambient humidity between readings.
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If the 3 readings fall within the upper and lower control limits, then no adjustment is necessary — continue with the sample readings. If the 3 readings are still abnormal, then recalibrate.
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If the problem persists, do a 4- or 5-point calibration check. Results which are in a different range than previous calibration checks but are linear indicate an equipment malfunction. Results which are not linear indicate a problem with one or more humidity standards, which will need to be replaced.

Daily preparation of the instrument

Turn the power switch on and leave the instrument for a minimum of 2 hours
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Set the temperature dial to 190 (25 °C)
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Set the RTD-24 switches to “S”
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Turn the printer on and select “1” to record only one channel or “6” to record all three channels (optional)
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Determine the readiness of the instrument by measuring humidity standards as described under Determining the readiness of the a_w Center
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Install fresh, appropriate pre-filters
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Make sure the a_w calibration range includes the anticipated reading of the sample material.

Sample measurement

Samples should be analysed in triplicate.
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Prewarm refrigerated and frozen samples to 25 °C before opening, to prevent condensation on the sample and to reduce the equilibration time. Care should be taken to avoid evaporation of water from the sample.
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Crush or cut up the sample material to homogenize. Fill the plastic sample bowls to the rim, but the material must not protrude beyond the rim. Take care not to directly handle the sample material.
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Samples which are not analysed right away can be stored by sealing the filled plastic sample bowls in plastic bags and storing them at an appropriate temperature. Again, prewarm refrigerated and frozen samples to 25 °C to prevent condensation on the sample.
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Place the plastic sample bowls into the stainless steel measuring bowls by pulling up on the appropriate sensor knob and using the tongs to install the stainless steel measuring bowls under the water activity sensors, making sure that the fit is proper and the seal is snug.
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Close the chamber door securely.
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When equilibrium has been reached, note the water activity value to three places after the decimal and the temperature reading for the appropriate channel.
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Correct the a_w values with the latest calibration regression line for each channel. Report sample a_w values by first determining the mean of multiple readings and then reporting 2 places after the decimal, rounded as necessary. Record the sample a_w reading on the appropriate lab sheet.
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After every reading, dry the sensors by removing the bowls and allowing the sensors to return to ambient humidity. Clean and dry the stainless steel bowls and filter as required and discard the pre-filter.

Maintenance of the humidity standards

Humidity standards with a labelled water activity should be purchased from a reputable source. Certified humidity standards would be ideal.
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For humidity standards above room humidity, if they show signs of drying out, rejuvenate them with distilled water so that the water is just visible on the surface. To store these humidity standards, seal them in their containers with tape and store in an airtight container when not in use.
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For humidity standards below room humidity, if they fill up with water, use an absorbent tissue to remove the water until it returns to the normal level. These standards should be stored in sealed containers in a dry area such as a desiccator.
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The humidity standards should be replaced if they become contaminated or if all of the salt has dissolved. Replacing the humidity standards occasionally is recommended.

Maintenance of the pre-filters

Over time, exposure of the sensors to fatty, acidic and other foods can lead to sensor failure. As a preventative measure, pre-filters should be used for every sample measurement.
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A selection of pre-filters is available from the Novasina dealer. Except in cases where special pre-filters are required, chemically-neutral, fast-flowing paper filters can be used.
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To install or remove the pre-filters:

• Lift the gold-coloured retaining ring and, while lifting the sensor with the knob, grasp the sensor and pull down firmly to disengage the sensor.
• Turn the sensor upward, remove the C-clip, and turn the sensor downward to drop the metal screen out.
• DO NOT TOUCH THE BUTTON SENSOR.
• Remove or replace the filter, replace the metal screen and C-clip, reengage the sensor, and replace the retaining ring.

Sensor care

• Do not submerge sensors in water or any other liquid.
• Do not leave sensors in draft.
• Do not spray with liquid or blow on the sensor.
• Do not subject the sensor to changes in temperature which allow condensation to form.
• Avoid physical shocks.
• Protect sensors from strong substances such as acid or alkaline vapours.
• Do not cool or heat sensors beyond specified limits.
• Do not place sensors in a humid environment.
• For long-term storage, place sensors in a sealed container.
• Use the proper filters.
• Avoid direct contact between the sample and the sensor.
• Clean the stainless steel filter frequently.
• Protect the sensor from contamination by dust or particles.
• Allow sensors to return to ambient humidity after each use.

Calculation

Water activity from the measured equilibrium relative humidity:

a_w = ERH / 100

Where,

• a_w = water activity
• ERH = equilibrium relative humidity (in % rh)

Correction of a_w values with the latest calibration regression line:

y = a + b × x

Where,

• y = corrected a_w value
• a = y-intercept
• b = slope
• x = reading

The slope and intercept are calculated from the standard readings as follows:

b = [Σxy − (Σx)(Σy)/n] ÷ [Σx² − ((Σx)²/n)]

a = (Σy − b(Σx)) ÷ n

Precautions

• The Laboratory Supervisor must ensure that the analysis described in this method is carried out in accordance with the International Standard referred to as “ISO/IEC 17025:2005 (or latest version): General Requirements for the Competence of Testing and Calibration Laboratories”.

References

• Health Products and Food Branch. Compendium of Analytical Methods, Volume 3, Laboratory Procedure MFLP-63, January 2006.
• Gaudreault, France. 1994. Determination of Water Activity of Foods Using the NOVASINA. Procedure # MM-INS13-001. HPB, Quebec.
• Kiltic, D., Favetto, G., Chirife, J., and Resnik, S. 1986. Measurement of water activity in the intermediate moisture range with the Novasina Thermoconstanter humidity meter. Lebensmittel Wissenschaft und-Technologie 19: 297-301.
• Novasina AG. 1988. Novasina aw center Operation Manual. Pfäffikon, Switzerland.
• Stoloff, L. 1978. Calibration of Water Activity Measuring Instruments and Devices: Collaborative Study. JAOAC 61: 1166-1178.
• Stroup, W.H. and Peeler, J.T. 1987. Evaluation of Precision Estimates for Fiber-Dimensional and Electrical Hygrometers for Water Activity Determinations. JAOAC 70: 955-957.
• Troller, J.A. 1977. Statistical Analysis of aw Measurements Obtained with the Sina Scope. Journal of Food Science 42: 86-90.
• Association of Official Analytical Chemists. 1990. Water Activity of Canned Vegetables. In: Official Methods of Analysis – AOAC – 15th Edition. pp. 987-988.