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 a_w 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 (Δa_w 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 Δa_w was taken as the acceptable error. The stated sensor specification for the TDL is Δa_w ± 0.005 and therefore the maximum acceptable error between two TDL sensors is the sum of the errors, or Δa_w ± 0.01.

Generally speaking, a Δa_w < 0.01 corresponds to a difference between instruments of 1-2% and is within the normal instrumental error. An error between Δa_w ± 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.

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