Volatile ingredients & water activity: How to choose the right aw sensor
Volatile ingredients & water activity: How to choose the right aw sensor
When measuring the water activity (also known as
aw, RH, ERH, or vapor pressure) of a product, certain ingredients – volatile compounds – can cause problems. Some water activity sensors are better protected against volatiles than others. So how much protection do you need?
There’s no strict scientific definition of a volatile compound, but generally speaking, it’s any substance that readily evaporates and becomes a gas at room temperature or below. Volatiles occur naturally in spices and foods and in many of the substances used in food and pharmaceutical processing.
There are four generally accepted types of water activity sensors. Each has a unique way of measuring water activity, and each varies in its ability to accurately measure in the presence of interfering volatile compounds (VCs).
- Dew point sensors are generally the fastest and most accurate, but they can struggle in the presence of volatile compounds due to VCs’ co-condensation on the chilled mirror.
- Capacitance sensors are able to overcome the condensation issue, but generally delivers lower precision data with longer data acquisition times, and can be poisoned by certain compounds (some alcohols have been shown to interfere).
- Resistive electrolytic sensors require filters to protect them from volatile compounds and improve the readings. Different filters are needed to protect against different volatile compounds, and they often significantly slow read times.
- The tunable diode laser (TDL) sensor was specifically designed to measure water activity in the presence of volatile compounds. It is the most reliable, most accurate instrument for this purpose.
To help you choose the best instrument for your situation, the METER Food R&D Lab tested 20+ frequently used ingredients containing various concentrations of volatile compounds. All of the ingredients were tested with the sensors listed above.
We did our best to use standard, widely used ingredients for these tests, but it is worth noting that sensor performance can vary depending on how ingredients are manufactured and/or processed. This report will present best and worst case scenarios when applicable. Instrument recommendations are based on typical use cases.
How we performed these tests
Prior to running any samples, all instruments were calibrated and verified against a set of known aw standards. A representative sample of each ingredient was analyzed in a minimum of three individual instruments per sensor. Each sample was run until the instrument gave three readings within the sensor specification (Δaw accuracy: dew point ±0.003, capacitance ±0.015, and TDL ±0.005).
The spices used in this study were taken from a quickly rotating bulk foods section at a grocery store. The essential oils were sourced from a USDA organic supplier and marked for internal use. The other food additives used were all ACS reagent grade.
All spices and essential oils were measured undiluted directly from their packaging. The other food additives were diluted with water to test a range of mass concentrations. No substrates were used - generally speaking additional substrates may lessen sample volatility.
The data collected was reviewed to determine whether or not volatile compounds were interfering with the water activity measurements.
Given the TDL sensor’s performance in the presence of volatile compounds, it was taken as the most accurate and set as the standard for comparing the other instruments. Using our sensor specifications, the additive Δaw was taken as the acceptable error. The stated sensor specification for the TDL is Δaw ± 0.005 and therefore the maximum acceptable error between two TDL sensors is the sum of the errors, or Δaw ± 0.01.
Generally speaking, a Δaw < 0.01 corresponds to a difference between instruments of 1-2% and is within the normal instrumental error. An error between Δaw ± 0.01 and 0.02 (3 and 5%) suggests some interference may be present, and above 0.02 (5%) the readings are significantly different and above twice the acceptable error, likely due to interfering volatile compounds.
The findings – and how we interpreted them
These data points were collected from samples that typical consumers are likely to encounter. Fresh spices and herbs, perhaps directly from a producer, may have higher concentrations of interfering volatile compounds and may not produce the same results. Additionally, the volatile compound content of herbs and spices can vary widely depending on species, handling and processing, and storage and age.
Sensor recommendations in the table below are divided into three categories:
- Dew point sensor – If an ingredient can be read accurately at all concentrations, a dew point sensor is the best choice.
- Capacitance sensor – If the sample is sufficiently volatile that it can cause issues with the dew point sensor at high concentrations but fares well at lower concentrations, a dual block instrument or capacitance sensor is recommended.
- Tunable diode laser (TDL) – If the ingredient causes inaccurate readings even at low concentrations, a TDL sensor is recommended.
Dried Spices & Herbs
| Ingredient | Test results† and volatility profile | Sensor recommendation |
|---|---|---|
| Allspice, ground | Dew sensor reads on average 0.014 (3%) higher than TDL. This is outside the range of acceptable accuracy.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of allspice are used occasionally then a capacitance sensor may help. If fresh allspice is a major component then a TDL should be considered. |
| Basil | Dew sensor reads on average 0.025 (6%) higher than TDL. This is far outside the range of acceptable accuracy.
| TDL TDL is best for samples with substantial amounts of basil. |
| Black pepper, ground | Dew sensor gives contaminated mirror error.
| TDL TDL is best for samples with substantial amounts of fresh ground black pepper. |
| Cayenne, ground | Dew sensor reads 0.007 (2%) higher than TDL. Difference is below the combined error threshold. | Dew Point Dew point sensor gives good accuracy. |
| Cinnamon, ground | Dew sensor reads 0.01 (2%) higher than TDL. Difference is at the combined error threshold.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of cinnamon are used occasionally then a capacitance sensor may help. |
| Cloves, whole | Dew sensor reads 0.03 (8%) higher than TDL. This is far outside the range of acceptable accuracy.
| TDL TDL is best for samples with substantial amounts of clove. |
| Coriander seeds, ground | Dew sensor reads 0.02 (5%) higher than TDL. This is far outside the range of acceptable accuracy.
| TDL TDL is best for samples with substantial amounts of ground coriander. |
| Cumin, ground | Dew sensor reads 0.01 (2%) higher than TDL. Difference is at the combined error threshold.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of cumin are used occasionally then a capacitance sensor may help. |
| Garlic, powder | Dew sensor reads 0.01 (3%) higher than TDL. Difference is at the combined error threshold. | Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of garlic powder are used occasionally then a capacitance sensor may help. |
| Ginger, powder | Dew sensor reads 0.01 (3%) higher than TDL. Difference is at the combined error threshold.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of ginger powder are used occasionally then a capacitance sensor may help. |
| Medium chili, powder | Dew sensor reads 0.005 (1%) higher than TDL. This is below the combined error threshold. | Dew Point Dew point sensor gives good accuracy. |
| Nutmeg, ground | Dew sensor reads 0.014 (3%) higher than TDL. Difference is above the combined error threshold.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of nutmeg are used occasionally then a capacitance sensor may help. TDL is best for samples with substantial amounts of ground nutmeg. |
| Onion, powder | Dew sensor reads 0.003 (1%) higher than TDL. This is below the combined error threshold. | Dew Point Dew point sensor gives good accuracy. |
| Oregano | Dew sensor reads 0.01 (3%) higher than TDL. Difference is at the combined error threshold.
| Capacitance In low concentrations a dew point sensor should be sufficiently accurate. If higher levels of oregano are used occasionally then a capacitance sensor may help. |
| Paprika, ground | Dew sensor reads 0.008 (2%) higher than TDL. Difference is below the combined error threshold. | Dew Point Dew point sensor gives good accuracy. If higher levels of paprika are used occasionally then a capacitance sensor may help. |
| Sage | Dew sensor reads 0.02 (5%) higher than TDL. This is far outside the range of acceptable accuracy.
| TDL TDL is best for samples with substantial amounts of sage. |
| Smoked paprika, ground | Dew sensor reads 0.008 (2%) higher than TDL. Difference is below the combined error threshold. | Dew Point Dew point sensor gives good accuracy. If higher levels of smoked paprika are used occasionally then a capacitance sensor may help. |
| White pepper, ground | Dew sensor reads 0.005 (1%) higher than TDL. Difference is below the combined error threshold. White peppercorns differ from black peppercorns in that they are soaked in water and their skins removed, meaning many of the volatile compounds are lost. | Dew Point Dew point sensor gives good accuracy. If higher levels of white pepper are used occasionally then a capacitance sensor may help. |
Other Food Additives
| Ingredient | Test results† and volatility profile | Sensor recommendation |
|---|---|---|
| Ascorbic acid | At all concentrations ascorbic acid can be accurately read by a dew point sensor. M.P. 190-192 °C, no appreciable vapor pressure at 25 °C | Dew Point Dew point sensor gives good accuracy.
|
| Acetic acid | At 1% mass or below acetic acid nears the combined error threshold. At 3% mass acetic acid exceeds the error threshold. 100% acetic acid (glacial) is not recommended to be put into an instrument and can cause permanent damage to the sensors and surfaces. A thorough cleaning is recommended after running samples containing any concentration of acetic acid. B.P. 118 °C | Concentration dependent Dew point sensor gives good accuracy if acetic acid is kept at <1% by mass. If the mass percent acetic acid occasionally reaches several percent then a capacitance sensor may help. TDL is the only recommendation for nearly pure volatile compounds.
|
| Citric acid | Citric acid does not interfere with dew point sensors at concentrations up to 100%. M.P. 156 °C | Dew Point Dew point sensor gives good accuracy.
|
| Ethanol | At 1% mass or below ethanol nears the combined error threshold. At 3% mass ethanol exceeds the error threshold. B.P. 78 °C | Concentration dependent Dew point sensor gives good accuracy if ethanol is kept at <1% by mass. At concentrations of 1-5%, a capacitance sensor can be used occasionally. Prolonged exposure to ethanol risks poisoning the capacitance sensor causing irreversible drift, necessitating replacement of the sensor. At concentrations greater than 5% ethanol, a TDL is the only recommendation. |
| Glycerin | Glycerin does not interfere with dew point sensors at concentrations up to 100%. Vapor pressure of 0.003 mmHg at 50 ºC | Dew point |
| Isopropanol | At 1% mass or below isopropanol nears the combined error threshold. At 3% mass isopropanol exceeds the error threshold. B.P 83 °C | Concentration dependent Dew point sensor gives good accuracy if isopropanol is kept at <1% by mass. If the mass percent isopropanol occasionally reaches several percent then a capacitance sensor may help. TDL is the only recommendation for nearly pure volatile compounds. |
| Malic acid | Malic acid does not interfere with dew point sensors at concentrations up to 100%. M.P. 130 ºC | Dew point |
| Propylene glycol | Only above concentrations of 5% by mass does propylene glycol begin to interfere with a dew point sensor. B.P. 188 °C | Concentration dependent Dew point sensor gives good accuracy if propylene is kept at <3% by mass. If the mass percent propylene glycol occasionally reaches several percent then a capacitance sensor may help. TDL is the only recommendation for nearly pure volatile compounds. |
| Lactic acid | At concentrations up to 42.5% by mass lactic acid can be accurately read by a dew point sensor. 85% lactic acid is not recommended to be put into an instrument and can cause permanent damage to the sensors and surfaces. B.P. 122 °C | Dew Point Dew point sensor gives good accuracy up to concentrations of 42.5% by mass. |
Essential Oils
| Ingredient | Test results† and volatility profile | Sensor recommendation |
|---|---|---|
| Cinnamon oil, steam distilled cinnamomum zeylanicum | Dew point sensor reads 0.12 higher than TDL on average, well outside the acceptable range of accuracy.
| TDL TDL is the only recommendation for nearly pure volatile compounds. |
| Clove oil, steam distilled eugenia caryophyllata | Contamination of dew point sensor.
| TDL TDL is the only recommendation for nearly pure volatile compounds. |
| Lemon oil, cold pressed | Contamination of dew point sensor.
| TDL TDL is the only recommendation for nearly pure volatile compounds. |
| Peppermint oil, steam distilled mentha piperita | Contamination of dew point sensor.
| TDL TDL is the only recommendation for nearly pure volatile compounds. |
| Rosemary oil, steam distilled rosmarinus officinalis | Contamination of dew point sensor.
| TDL TDL is the only recommendation for nearly pure volatile compounds. |
Coffee, Teas, Etc
| Ingredient | Test results† and volatility profile | Sensor recommendation |
|---|---|---|
| Coffee | Depends highly on freshness of sample. Dew point sensor often reads consistently, but due to variability in samples may not always be reliable.
| Highly variable If you will be consistently measuring a wide variety of fresh coffee samples it is likely that a TDL will offer the greatest accuracy. If occasionally measuring coffee then a dew point sensor may suffice, but an instrument with a capacitance sensor will be more reliable. |
| Spiced Black Tea | Spiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor. | Variable If you know you will be consistently measuring a wide variety of spiced tea samples it is likely that a TDL will offer the greatest accuracy. If occasionally measuring spiced teas then a dew point sensor may suffice, but an instrument with a capacitance sensor will be more reliable. |
| Black Tea | Tea leaves by themselves are able to be accurately measured with a dew point sensor. | Dew point |
| Green Tea | Tea leaves by themselves are able to be accurately measured with a dew point sensor. | Dew point |
| Lemon Black Tea | Spiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor. | Variable |
| Mint Black Tea | Spiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor. | Variable |
| Earl Grey | Spiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor. | Variable |
| Chicory Drink Mix | Chicory root extracts are known to contain many aroma compounds and components with high volatility.16 | TDL |
† The full data set is available on request. Please contact us to discuss.
Summary and conclusions based on ingredient category
Dried spices and herbs
The herbs and spices presented in this report fall into three categories regarding their performance:
- Non-interfering: cayenne, medium chili, onion. A dew point sensor is best.
- Concentration dependent: allspice, cinnamon, cumin, garlic, ginger, nutmeg, oregano, paprika, smoked paprika, white pepper. If using undiluted, a TDL is best, but at low concentrations a dew point sensor will work. A capacitance sensor is a good compromise.
- Highly interfering: basil, black pepper, cloves, coriander, sage. A TDL is the best way to reliably measure these herbs and spices unless they are significantly diluted.
The spices and herbs used in this report were not diluted or adulterated – it’s up to the end user to determine whether or not this represents their own samples.
Essential oils
Clove oil, peppermint oil, lemon oil, rosemary oil all had dew point sensors fail due to mirror contamination. Cinnamon oil was able to be measured in a dew point sensor, but it gave a reading significantly higher than the average TDL reading.
If you are looking to determine aw of essential oils, a TDL is the only reliable sensor given the low water content and high volatility of these mixtures.
Other food additives
The compounds presented in this report fall into two categories:
- Non-interfering: Ascorbic acid, citric acid, lactic acid, malic acid, glycerin. A dew point sensor will work well for typical concentrations in food.
- Concentration dependent: Acetic acid, ethanol, isopropanol, propylene glycol. A TDL is the best instrument, but a capacitance sensor instrument may also be sufficient.
There are numerous use cases for each of these components, and it’s up to the end user to determine what instrument best suits their needs.
For example, distilled white vinegar is typically 5% acetic acid and if you need to regularly determine the aw for this type of sample, then you may need to consider a TDL. If you use samples containing diluted vinegar then a capacitance sensor may work well. If you know you have vinegar, but it’s a small percentage of your sample, then a dew point sensor may be sufficient.
Coffee, tea, etc.
Coffee is often problematic for a dew point sensor, but it is highly variable. There is variation from roast to roast and among varieties, but also between freshly ground vs. pre-packaged samples. If you are a coffee producer or roaster and are interested in determining the aw of your product then the best choice is an instrument with a TDL sensor.
Teas can be accurately measured with a dew point sensor so long as they aren’t flavored with highly volatile compounds, such as in some spiced teas. If you are a producer of spiced teas a TDL is necessary to accurately determine aw in the presence of essential oils.
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