Testing water activity in cottage foods

The Elitech GSP-6 meter measures relative humidity. Relative humidity in a closed container is the same as water activity. In expensive water activity meters, a food product is placed into a sealed container. Once the moisture in the food equilibrates with the air in the container, the relative humidity (vapor pressure) is measured. Water activity is measured by dividing the vapor pressure in the container by the vapor pressure of pure water (which is 1.0 a_w).

For a sample to have an a_w measurement of 0.85 or less, the relative humidity of a sealed container needs to be 85% or less. The Elitech meter shows the relative humidity as a percentage. To convert the percentage to the a_w value, move the decimal point two spaces to the left. For example, a measurement of 76.8% on the Elitech meter equals a_w of 0.768. This is the number you would enter in the Excel Correction Tool, explained in detail below.

The Minnesota Farmers’ Market Association (MFMA) assembles and ships complete water activity (a_w) testing kits, at cost, to registered cottage food producers. Kits can be ordered on the MFMA website.

Water activity (a_w) kit contents

• Elitech GSP-6 meter and the humidity and temperature probes included with the unit
• 5 – 2-ounce sample jars and lids; one lid is adapted for securely inserting the humidity and temperature probes
• 1 set of a_w standard solutions (0.25 a_w, 0.50 a_w, 0.76 a_w and 0.92 a_w).
• Access to the a_w Excel Correction Tool
• A 6-quart tote for storage when the system is not in use.

When your a_w kit arrives

Open the package and check that all items have arrived in good condition, with no obvious signs of damage. The temperature and humidity probes should be smooth and clean. If probes are damaged or contaminated, readings will not be accurate. If you have any issues with the kit, contact info@mfma.org.

Before you use your kit

Read the  Elitech manual to familiarize yourself with the unit’s functions, such as battery type requirements and button controls. Then you can begin the calibration process and taking measurements according to the instructions in the following sections of this page. If you have any questions, contact the UMN Extension Food Safety educators at exfdsafe@umn.edu.

Note: The Elitech GSP-6 was not developed for testing the water activity of food. It was developed as a tool to accurately monitor the temperature and relative humidity inside trucks and storage spaces for the transport and storage of vaccines which requires precise and reliable measures over time. This unit comes with software and a programmable setting that is used for that purpose. Cottage Food producers using the Elitech unit for testing the a_w of food do not need to use this software or programmable settings.

What is the calibration process?

The calibration process checks the accuracy of your meter and allows you to make adjustments to ensure the most accurate reading possible. Because this meter cannot be calibrated directly like most pH meters, you must use the Excel Correction Tool to calibrate your meter.

The calibration process takes 30 hours (5 readings per standard x 4 standards x 1.5 hours each). Readings can be completed over several days. 

This process should be repeated annually, using a fresh set of a_w standards purchased from the MFMA. 

Calibration steps

Elitech GSP-6 meter with a sample jar containing the 0.76 standard solution. The red arrow indicates the power and on/off button. The red box shows the relative humidity measurement (with the % symbol). 

The number above relative humidity is temperature. Temperature does not need to be recorded, but ensure that all readings are taken at room temperature, since temperature affects readings. 

The bottom reading is used for Elitech software not used for this process. It can be ignored.

1. Open the appropriate vial and squeeze the solution out of the tube into the matching 2-inch. jar, ensuring the lids are tightened snuggly.
2. Place the lid with the two inserted probes onto the red 0.25 solution jar and tighten securely. 
   1. Start your timer for 1.5 hours. 
   2. At the end of 1.5 hours, press the on/off button on the GSP-6 meter and record the middle number on the meter (relative humidity indicated with a % symbol). Note: You will need to convert the measurement shown on the meter from percent relative humidity to a decimal value that indicates water activity, by moving the decimal point two places to the left. For example: 84.3% becomes 0.843. Record the dates that you calibrated your meter. 
3. Move the lid with the probes from the red 0.25 jar to the green 0.50 jar and tighten securely.
   1. Start your timer for 1.5 hours.
   2. Replace the 0.25 (red) lid on the 0.25 solution jar and tighten snuggly so the solution doesn’t evaporate.
   3. At the end of 1.5 hours, press the on/off button on the GSP-6 meter and record the reading as you did in step 2.2 above (as a decimal water activity rather than as % relative humidity).
4. Repeat steps 2 and 3, moving the lid with the probes from standards 0.25 red to 0.5 green to 0.76 blue to 0.92 purple. Repeat until you have recorded five of the 1.5-hour readings from the GSP-6, for each of the four solutions. 

Recording five measurements for each standard will capture the natural variability of your meter. You will see that despite being in a known solution, for example, 0.25, the meter will not always report that value. It might display 0.248 for one reading, 0.250 for another, or 0.249 for another. These values will be used to calculate a correction factor for measurements of food items to give you the most accurate reading possible.

It is okay if this calibration process happens over a day or so. The tightened lids on the jars will maintain the integrity of the solutions for several days.

You can discard the solutions into your garbage or pour them down the drain once you’ve completed all the measurements. The standards will not maintain integrity for another year. You can order the four solutions again from MFMA to recalibrate in a year.

The a_w Excel Correction Tool

The accuracy of your meter may change over time, as is the case with all electronic measuring devices. It is important to calibrate your meter to known standard solutions regularly to ensure accuracy. 

The Excel tool will allow you to correct the measurements recorded on your Elitech meter based on the known error rate of your specific unit. The calibration you conduct using your unit will not necessarily have the same correction factor needed for another Elitech unit. Each unit needs to be calibrated individually. 

Downloading the a_w Excel Correction Tool

When you order your water activity kit from the MFMA, they will email you the Excel spreadsheet tool. Download it to your computer and open it. 

The file works best on a computer. It will open on a phone, but some functions do not work on a mobile device. This is a brand-new resource and is still in development. 

If you have your kit but have not yet received the Excel tool, contact info@mfma.org.

Note: The team at UMN Food Science and Nutrition and UMN Extension will update the Excel Correction Tool as needed. As we make improvements you will be notified via email. If you have any questions about the tool, contact the UMN Extension Food Safety educators at exfdsafe@umn.edu. 

Entering your measurements into the Excel tool

1. Open the “aw Excel Correction Tool” and type the measurement made for each of your standards into the yellow cells. Note that all the other cells in the Excel tool are locked so you can’t accidentally delete or change anything.
    

   

2. Be sure to save the “aw Excel Correction Tool” with all of your measurements, so it’s ready when you need to test food. Tip: You could add the date to the file name as a reminder to calibrate your GSP-6 in one year. For example: “aw Excel Correction Tool 5-01-2024”

Taking care of the probes

Do not allow food or liquids to touch the probe or get inside the slot of the relative humidity probe. If the sensor is contaminated or damaged, it may be permanently damaged and give inaccurate measurements.

Making an airtight seal

Ensure your lids have an airtight seal to get the most accurate reading. The inside of the lid has a liner to help create an airtight seal when hand tightening. You will move the lid with the probes and insert it from jar to jar to test different standards or food samples. Be sure to take care when handling the lid so the probes do not move around which could cause them to begin leaking air. Also, ensure the humidity probe is correctly positioned so air cannot enter.

Preparing your food sample

For the most accurate results, take a sample of the food item to be tested that is:

• as large as possible, while still fitting in the 2-ounce “Food Sample” jar provided without touching the probes in the lid.
• from the most moist part of the food. For example, if testing bread, take the sample from the middle of the loaf, not the crust.
• at room temperature.

Process to test foods for water activity (a_w)

1. Clean, rinse and dry the “Food Sample” jar. To ensure that the jar doesn’t add moisture to the sample chamber, towel dry it and allow it to air dry until you feel confident there is no residual moisture in the jar that could increase your water activity measurement. 
2. Place a sample of your food into the bottom of the jar. Make the sample as large as possible while ensuring no food touches the probes. For example, 2 tablespoons of jam or jelly or a slice of cake/bread cut to fill the bottom of the jar without touching the probes.
3. Tighten the lid with the probes onto the “Food Sample” jar and start your timer for 2 hours. This is the minimum amount of time it takes for any free moisture in your food sample to equilibrate with the air space in the jar.
4. At the end of 2 hours, record the first measurement of your food product. Press the on/off button on the Elitech GSP-6 meter. The middle number (relative humidity) will appear as a percentage. You will need to convert the measurement shown on the meter from percent relative humidity to a decimal value that indicates water activity, by moving the decimal point 2 places to the left. For example: 84.3% becomes 0.843.
5. Repeat steps 1 through 4 for a minimum of 3 measurements (5 is better) for your food product.*
6. Calculate the average water activity test results for your food product.** The average = (sum of all the measurements) ÷ (the number of measurements).

*Why a minimum of three?

To get an average of anything you need at least three measurements. The “average” of any two values is the mid-point between the two values and may not have any real meaning. The more repeated samples collected, the better the average indicates an estimate of the real average. Looking at the table, the first and the fourth measurements were over 0.85, which is higher than the allowed limit for Minnesota cottage foods. But three measurements are under 0.85. The average of all five measurements gives us the best-estimated average for our food sample.

**Why do we want to use an average instead of just one measurement?

When measuring almost anything, the measuring tool or meter will not always provide the same measurement each time. For example, if you step on a scale, it might read 170 lbs. If you step off and back on again, it might read 170.5 lbs. Repeat the process once more and it might read 170.3 lbs. This all depends on the accuracy of your scale. This kind of natural variability of a measurement tool is normal. We take multiple measurements and then calculate an average to get the most accurate average possible. In this example, your average weight would be 170.266. The average of your weight might not be important to you, but in some cases, calculating an accurate average is very important — like in the case of the water activity of cottage foods. This is especially important if the water activity of a food product is close to the 0.85 limit. The more accurate your readings, the more confident you can be that your measurements accurately represent the food product.

Checking your average a_w measurement in the Excel Correction Tool

Open your “Excel Correction Tool” spreadsheet and enter the average water activity of your food product in the yellow box in the grid on the right side of the “Excel Correction Tool.” 

Note that in the example image:

• The variability of the meter is relatively high (the difference between 0.917 to 0.847 is 0.07). 
• The meter tends to underestimate the actual water activity with a measured water activity of 0.820 a_w but an estimated actual value of 0.881 a_w. 

This illustrates the importance of using the Excel Correction Tool to correct the measurements taken with the meter. If you had just taken the reading from the meter, and assumed it was accurate, you would have been selling an unsafe, and illegal, product. 

Each meter will have a different level of natural variability. Once you enter the measurements of the standard solutions into your Excel Correction Tool, the resulting correction applies only to that meter. 

What does the green button mean?

If the “Upper limit” number is below 0.85 and the cell shows a green button, then the water activity is low enough to make your food eligible to be sold as cottage food under the Minnesota Cottage Food law. Keep records in case a food inspector questions the a_w value of your food. Your records should include the date, water activity values, and the specific recipe tested, so you can recreate it in the future.

What does a yellow or red button mean?

If either the yellow or the red button appears in the “Upper limit” cell, you should reformulate your recipe. It is currently not within the range of a food product that would be considered food safe, or legal for sale as cottage food in Minnesota.

The process of using the Elitech meter and the Excel Correction Tool to test the water activity of foods is new and still being tested and improved. Use this tool when developing recipes that might fit the water activity criteria for cottage foods. If you have a recipe that reliably results in water activity under 0.85, we recommend sending it to a professional lab for testing and confirmation. 

It should also be stressed that any change in a tested and approved recipe, ingredients, or processing time and temperatures can impact the water activity. Food must be tested and confirmed again if any of these elements change.

Food testing labs

You may choose a commercial testing lab that fits your needs. Pricing varies but expect to pay about $30 for a water activity test per product. 

• Market Fresh Food Testing Laboratory
  (612) 331-4050, Minneapolis
• Minnesota Valley Testing Lab
  (507) 354-8517, New Ulm
  Sample Sheet Form
• Medallion Labs
  1-800-245-5615 or (763)764-4453, Minneapolis

MFMA is offering at-cost pricing to registered cottage food producers for the following items to use the Elitech GSP-6 correctly. 

The following supplies can be ordered on the MFMA website:

1. Complete kit(includes Elitech GSP-6 meter, jars for calibration and calibration standards)
2. Partial kit (includes jars for calibration and calibration standards)
3. Calibration standards(solutions needed for annual calibration of meter)

• After completing device calibration and using your Excel Correction Tool to test your food products, print hard copies of your results and save the paper version in your water activity kit container as a backup to your electronic files.

• Use this Cottage Foods Testing Log or something like it to record your food samples as you test them. 

• Have a record-keeping system for your cottage food business. A best practice is to follow FDA record-keeping standards and retain your records for at least two years.