Speed up testing, and reduce variability by creating simple, consistent SOPs for preparing and testing your samples. Here are 6 best practices to consider as you develop your own procedures.
Keep it representative
Always test products in their final state. If the product isn’t chopped, ground, or sliced before it reaches consumers, avoid doing this to the product before measuring its water activity. Some slicing or cutting may be necessary for the sample to fit inside the sample cup, but keep this to a minimum.
When measuring multicomponent foods, place a representative amount of each component into the sample cup. If measuring a product such as filling sandwiched between two cookies, put both parts (cookie and filling) into the sample cup.
Perform different preparation tests on samples, and measure the change in water activity that results. Use this data to determine the method which produces the most stable results.
Eliminate unnecessary steps
Use testing data as a guideline to determine what kind of preparation procedure is right for each product. Eliminate any unnecessary steps.
Once you’ve determined the best method, establish a sampling procedure with set guidelines. For example, specify how many seconds the samples should be crushed, how they should be handled, and the temperature at which measurements should be made.
Keep it covered
Exposed products with a water activity higher than room humidity will lose moisture, and those with a water activity below room humidity will gain moisture. To avoid moisture gain or loss, minimize exposure time. Sample lids are excellent for this purpose. For storage beyond an hour, parafilm the lids on, or store samples in hermetically sealed containers.
Keep it clean
Place the instrument in a clean location. Wash your hands, or wear lab gloves. Wipe the bottom, the edges, and the rim of the sample cup with Kimwipes before setting it into the chamber. This prevents contaminants from spreading into the chamber. Fill the sample cup only half full.
Types of products that may need special consideration
The following products may need special sampling practices in order to obtain an accurate reading.
Products with a moisture barrier
Products that have a coating, like chocolate-covered nuts or coated candies, may be slow to come to equilibrium. They can be cut or crushed to expose interior, but do not grind. Grinding causes moisture loss and is rarely necessary.
Products high in fat
In fatty meat products, oils, or fatty desserts, the oils and fats become a moisture barrier, and samples are slow to emit moisture. Quick water activity testing methods will give inaccurate results. These samples must equilibrate in the chamber for a longer period of time. You can also use low-emitting mode for faster readings. Please contact support for more information.
Products with extremely low water activity
These products may be slow to come to equilibrium, so expect longer read times.
Multiple component products
Cakes with filling, cheese and crackers, or cereal with fruit are all multiple component products. For an accurate reading, one option is to check components individually. You can also use a representative sample if you are able to keep the ratio of the components right. Watch for moisture migration, and test larger products in several places to avoid microbial growth hotspots.
Sample Prep FAQs
Will pet food coated with fat have an impact on water activity measurements?
All coatings impact water activity readings because they act as barriers to water molecules moving out of a product matrix and into the vapor phase. For any type of material with a coating around it, it’s important to break the sample apart.
How long can extruded pet food samples be stored in Whirl-pak bags before water activity is affected?
Pet food can be stored in whole kibble form in a Whirl-pak bag for half a day without a significant impact on water activity. However, grinding a sample may cause substantial changes in water activity within five to ten minutes. For longer term storage, to store ground pet food, or to send a sample to another lab, use a parafilmed sample cup with a screw top cap or a foil pouch.
What would be the best way to prepare an oily meat product for testing?
An oily product requires longer read times. Breaking a product into smaller pieces will help, but don’t grind it. Grinding will tear apart the tissue structure and affect water activity. Slice it or pull off pieces to put into a sample cup.
Does temperature change the water activity of a material?
There’s no good way to predict which types of products are sensitive to temperature, but reading a product at a series of different temperatures will show which products are sensitive. Meat products tend to be fairly stable, however some powdered compounds and sugars are more affected by temperature changes.
Would freezing a product increase its water activity?
It depends on the freezing process. If the product is not frozen correctly, and cells begin to rupture, the water activity may change. Remember to thaw a frozen sample before testing, as water activity can’t be read in a frozen state.
How do you read water activity in large, dense products, such as brazil nuts?
Break large products into pieces so the read times aren’t excessively long. This will also ensure product testors capture the water activity at the center of the product and not just the exterior.
When testing finished pharmaceutical products such as tablets, should you test the entire tablet? Or should the tablets be crushed or ground?
If the tablet has no coating, there won’t be a significant difference in water activity whether crushing it or reading it whole. If the tablet is coated, crush it to expose the interior (no grinding). Otherwise, the water activity meter is only testing how effective the coating material is.
Would you recommend breaking up a tablet with a mortar and pestle?
A mortar and pestle is a valuable tool for sample preparation because it breaks things apart without being too excessive. Achieve the same effect with a coffee grinder or food processor, but use only a couple of quick pulses–enough to break the sample apart without grinding it.
Is the whole tablet measurement of value?
It depends if the tablet is non-coated or coated. If the coating material can absorb water, there is a critical water activity where the coating material will begin to break down. Thus, if monitoring that aspect of the product, then reading the water activity of a whole tablet would be important.
How can I minimize moisture migration in hygroscopic products during sampling?
The best approach is to put the water activity instrument and the sample into controlled humidity conditions. Use a glove box, or get the sample inside the chamber as quickly as possible to prevent water uptake from the environment. With an instrument that maintains the seal in the chamber, it’s possible to perform consecutive readings. But if the instrument requires pulling out the sample to take a new reading, the water activity will continually change as the sample takes up more and more moisture.
What percent propylene glycol or ethanol is allowable in a formulation before it causes problems in dew point (chilled mirror) instruments?
Volatile components in different sample matrices are dependent upon the sample. If using propylene glycol in one product, there may be a noticeable impact on chilled mirror readings at 0.5%. In a completely different product, it might be 2-3%. Generally, propylene glycol will be the most problematic component. For concerns about whether or not a concentration in a product is enough to be a problem, feel free to contact our support department. Our testing lab can also perform tests that show whether the volatiles in a product are impacting the chilled mirror method. If so, using the TDL sensor (which is unaffected by volatiles) should solve the problem.
What is the accuracy of readings in the low emitter sensor setting?
An instrument with a low emitting mode allows the user to manually adjust to a desired accuracy. To investigate the low emitting mode, take a reading of the product in the standard read mode, and record the water activity value and read time. Then, in the low emitting mode, take a reading of the sample at the best accuracy setting and record the read time. Continue adjusting the accuracy level and recording readings and times. The goal is to remain close to the original water activity value, while saving read time. This will determine what accuracy setting is best for a particular product.