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Volatile Ingredients & Water Activity

Volatile Ingredients & Water Activity

Introduction

Meter Group produces instruments with three types of water activity sensors: a dew point sensor, a capacitance sensor, and a tunable diode laser (TDL) sensor. Each sensor has a unique way of measuring water activity (aw) and each sensor varies in its ability to accurately measure in the presence of interfering volatile compounds (VCs). Traditionally the dew point sensor is the fastest and most accurate sensor, but it can struggle in the presence of VCs due to their co-condensation onto the chilled mirror. The capacitance sensor is able to overcome the condensation issue, but this type of sensor generally has a lower precision, a higher data acquisition time and can be poisoned by certain compounds (some alcohols have been shown to interfere). The TDL sensor was specifically designed to measure water activity in the presence of other volatile compounds and is the flagship instrument for this purpose. This guide will cover many common ingredients found in food products or other consumables that you may encounter while developing recipes or formulations. The goal of this guide is to aid the end user in choosing an instrument that will best fit their needs for determining water activity across a wide spectrum of use cases. While many of the samples presented in this report contain volatile compounds or are themselves volatile, instrument performance varies greatly depending on the processing of these compounds or ingredients. Best and worst case scenarios will be presented when applicable and an instrument recommendation will be made based on typical use cases.

Procedure

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.

Results

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.

Three levels of recommendations are given in the table below. If an ingredient gives good performance at all concentrations then a dew point sensor is the best choice (non-interfering). If the sample is sufficiently volatile that it can cause issues with the dew point sensor at high concentrations, but fairs well at lower concentrations then a dual block instrument is recommended (concentration dependent). If the ingredient causes inaccurate readings even at low concentrations then a TDL sensor is recommended (highly interfering).

These data points represent samples that a consumer would encounter. Fresh spices and herbs, perhaps directly from a producer, may have higher concentrations of interfering volatile compounds and won’t perform to the same level of accuracy as our samples. Furthermore, volatile compound content of herbs and spices can vary widely depending on plant species, handling and processing, and storage and age. A literature review of the potential volatile compounds is found in the volatility profile of spices.

Ingredient NameExperimental Results†
& Volatility Profile
Instrument recommendation
Dried Spices & Herbs
Allspice, groundDew sensor reads on average 0.014 (3%) higher than TDL. This is outside the range of acceptable accuracy.

Up to 6% mass is essential oil. The major component is eugenol.1
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.
BasilDew sensor reads on average 0.025 (6%) higher than TDL. This is far outside the range of acceptable accuracy.

Up to 2% mass is essential oil. The major component is linalool.2 Lots of variation among species.
TDL

TDL is best for samples with substantial amounts of basil.
Black pepper, groundDew sensor gives contaminated mirror error.

0.4%-7% of mass is essential oil. The major component is piperine.3
TDL

TDL is best for samples with substantial amounts of fresh ground black pepper.
Cayenne, groundDew sensor reads 0.007 (2%) higher than TDL. Difference is below the combined error threshold.Dew Point

Dew point sensor gives good accuracy.
Cinnamon, groundDew sensor reads 0.01 (2%) higher than TDL. Difference is at the combined error threshold.

Cinnamaldehyde is a major component of essential oil.4
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, wholeDew sensor reads 0.03 (8%) higher than TDL. This is far outside the range of acceptable accuracy.

45-90% of the essential oil is eugenol.5
TDL

TDL is best for samples with substantial amounts of clove.
Coriander seeds, groundDew sensor reads 0.02 (5%) higher than TDL. This is far outside the range of acceptable accuracy.

0.5-2.5% of mass is essential oil. The major component is linalool.6
TDL

TDL is best for samples with substantial amounts of ground coriander.
Cumin, groundDew sensor reads 0.01 (2%) higher than TDL. Difference is at the combined error threshold.

Up to 3% of mass is essential oil. The major component is cuminaldehyde.7
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, powderDew 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, powderDew sensor reads 0.01 (3%) higher than TDL. Difference is at the combined error threshold.

Up to 3% of the weight of fresh ginger is essential oil. These are primarily zingerone, shogaols, and gingerols.8 Lots of species variation, and the drying technique plays a large role in what compounds are present.
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.

Up to 1.4% of mass is essential oil, mainly myristicin.9
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, powderDew sensor reads 0.003 (1%) higher than TDL. This is below the combined error threshold.Dew Point

Dew point sensor gives good accuracy.
OreganoDew sensor reads 0.01 (3%) higher than TDL. Difference is at the combined error threshold.

Over 60 compounds, primarily carvacrol and thymol.10 Lots of species variation.
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, groundDew 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.
SageDew sensor reads 0.02 (5%) higher than TDL. This is far outside the range of acceptable accuracy.

Numerous compounds including α-humulene, β-pinene, eucalyptol, and camphor.11
TDL

TDL is best for samples with substantial amounts of sage.
Smoked paprika, groundDew 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, groundDew 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
Ascorbic acidAt 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 acidAt 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 acidCitric 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.
EthanolAt 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.
GlycerinGlycerin does not interfere with dew point sensors at concentrations up to 100%.

Vapor pressure of 0.003 mmHg at 50 ºC.
Dew point
IsopropanolAt 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 acidMalic acid does not interfere with dew point sensors at concentrations up to 100%.

M.P. 130 ºC
Dew Point
Propylene glycolOnly 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 acidAt 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
Cinnamon oil, steam distilled cinnamomum zeylanicumDew point sensor reads 0.12 higher than TDL on average, well outside the acceptable range of accuracy.

90% cinnamaldehyde, B.P. 248 °C.4
TDL

TDL is the only recommendation for nearly pure volatile compounds.
Clove oil, steam distilled eugenia caryophyllataContamination of dew point sensor.

45-90% of the essential oil is eugenol.5
TDL

TDL is the only recommendation for nearly pure volatile compounds.
Lemon oil, cold pressedContamination of dew point sensor.

Limonene (40%) and pinene (25%) dominant components.12
TDL

TDL is the only recommendation for nearly pure volatile compounds.
Peppermint oil, steam distilled mentha piperitaContamination of dew point sensor.

The main constituents are menthol (41%) and menthone (23%).13
TDL

TDL is the only recommendation for nearly pure volatile compounds.
Rosemary oil, steam distilled rosmarinus officinalisContamination of dew point sensor.

Major components are α-pinene and eucalyptol.14
TDL

TDL is the only recommendation for nearly pure volatile compounds.
Coffee, Teas, Etc.
CoffeeDepends highly on freshness of sample. Dew point sensor often reads consistently, but due to variability in samples may not always be reliable.

Coffee contains numerous volatile compounds that vary in concentration with variety, roast, and sample handling.15
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 TeaSpiced 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 TeaTea leaves by themselves are able to be accurately measured with a dew point sensor.Dew point
Green TeaTea leaves by themselves are able to be accurately measured with a dew point sensor.Dew point
Lemon Black TeaSpiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor.Variable
Mint Black TeaSpiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor.Variable
Earl GreySpiced teas often have essential oils added to the tea leaves which can cause interference with the dew point sensor.Variable
Chicory Drink MixChicory root extracts are known to contain many aroma compounds and components with high volatility.16TDL

Conclusions

Dried Spices & Herbs

The herbs and spices presented in this report fall into three categories regarding their performance: non-interfering, concentration dependent, and highly interfering. The spices and herbs measured for this report were not diluted in any way, and it’s up to the end user to determine whether or not this represents their own samples.

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.

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 regarding their performance: non-interfering, and concentration dependent. 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.

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. The compounds presented in this report fall into two categories regarding their performance: non-interfering, and concentration dependent. 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.

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.

Coffees, Teas, 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.

Supporting Information for Volatile Ingredients & Aw

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.

The data set 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 a first level – the acceptable error on one TDL is Δaw ± 0.005 and between two would be Δaw ± 0.01. Generally speaking a Δaw below 0.01 corresponds to a difference between instruments of 1-2%. 3-5% suggests some interference may be present, and above 5% the readings are significantly different and above twice the threshold (Δaw ± 0.02), likely due to interfering volatile compounds.

References

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  2. Janick, Jules. Basil: A Source of Aroma Compounds and a Popular Culinary and Ornamental Herb. Perspectives on New Crops and New Uses 1999, 499-505.
  3. Stojanovic-Radic, Zorica. Piperine – A Major Principle of Black Pepper: A Review of Its Bioactivity and Studies. Appl. Sci. 2019, 9, 4270.
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  9. Piras, Alessandra; Rosa, Antonella; Marongui, Bruno; Atzeri, Angela; Dessi, Assunta; Falconieri, Danilo; Porcedda, Silva. Extraction and Separation of Volatile and Fixed Oils from Seeds of Myristica fragrans by Supercritical CO2: Chemical Composition and Cytotoxic Activity on Caco‐2 Cancer Cells. J. Food Sci. 2012, 77, 448-453.
  10. Marques, Antonio. Chemical composition and bioactivity of different oregano (Origanum vulgare) extracts and essential oil. J. of the Sci. of Food and Ag. 2013, 93, 2707-2714.
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  12. Halima, Nihed. Citrus lemon essential oil: chemical composition, antioxidant and antimicrobial activities with its preservative effect against Listeria monocytogenes inoculated in minced beef meat. Lipids in Health and Disease 2017, 16, 146.
  13. Schmidt, Erich. Chemical Composition, Olfactory Evaluation and Antioxidant Effects of Essential Oil from Mentha x piperita. Nat. Prod. Comm. 2009, 4, 1107-1112.
  14. Carbonell-Barrachina, Angel. Composition of rosemary essential oil (Rosmarinus officinalis) as affected by drying method. J. Food Eng. 2010, 97, 253-260.
  15. Ochiai N, Tsunokawa J, Sasamoto K, Hoffmann A. Multi-volatile method for aroma analysis using sequential dynamic headspace sampling with an application to brewed coffee. J Chromatogr A. 2014, 1371, 65-73.
  16. Cadwallader, Keith; Wu, Tiandan. Identification of Characterizing Aroma Components of Roaster Chicory “Coffee” Brews. J. of Ag. and Food Chemistry. 2019, 67 (50), 13848-13859.