Unlocking The Secrets Of Key Aroma Compounds

Characterizing key aroma compounds involves identifying the compounds, understanding their impact on sensory perception, and determining their odor activity values. This process typically combines gas chromatography-mass spectrometry (GC-MS) with orthonasal threshold testing and descriptive sensory analysis. The GC-MS technique separates and identifies volatile compounds, while orthonasal threshold testing determines the concentration at which a compound becomes detectable by smell. Descriptive sensory analysis, performed by a trained panel, evaluates the aroma profile and intensity of the compounds. Odor activity values (OAVs) are then calculated by dividing the concentration of the compound in the food matrix by its orthonasal threshold, providing insight into the compound's contribution to the overall aroma. This comprehensive approach ensures an accurate characterization of key aroma compounds and their role in the sensory experience.

Gas chromatography-mass spectrometry-olfactometry (GC-MS/O)

GC-MS/O is an essential technique in aroma analysis. It is often performed in parallel with GC detection, for example, using a mass spectrometer. The eluate is split so that the analytes reach both detectors simultaneously. The retention times of the analytes might differ for the two detectors due to the mass spectrometer working under vacuum conditions and the olfactometric detector working under atmospheric pressure conditions. This can be overcome by installing a restrictor before the mass spectrometer.

The design of all commercially available olfactometric ports is very similar. The eluate reaches the port through an uncoated transfer line and is sniffed in a glass or polytetrafluoroethylene (PTFE) conical port, fitted to the shape of a nose. The transfer line is heated to prevent the condensation of semi-volatile analytes on the walls of the capillary. Auxiliary gas (moist air) is added to prevent the drying of the assessors' nose mucous membranes. The transfer line length can vary but must be long enough to ensure a comfortable sitting position and to avoid discomfort due to the vicinity of hot chromatograph components during detection.

GC-MS/O can be used to identify and quantify aroma-active compounds. It is particularly useful when many volatile chemicals are often present at concentrations lower than the instrumental detection limit, and when information about human perception is required.

Odor activity values (OAVs)

OAVs are used to determine the contribution of different compounds to the overall aroma of a substance. Compounds with larger OAVs are believed to contribute more to the overall odor properties of a substance.

OAVs are calculated by dividing the concentration of an aroma compound by its odor threshold in water or air.

OAVs are used to rank odorants in foods, with those with an OAV ≥ 1 considered to be potent odorants that contribute significantly to the overall aroma.

OAVs can be used to identify key odorants in a substance, with the compounds with the highest OAVs typically having the greatest contribution to the aroma.

For example, in a study on the key aroma compounds in the fruit of Litsea pungens Hemsl. (LPH), D-Limonene and 3,7-dimethyl-2,6-octadienal had the highest OAVs (OAV = 9803 and 8399, respectively), and were identified as key odorants contributing to the lemon attribute of LPH.

OAVs can also be used in combination with other techniques, such as sensory evaluation, to confirm the contribution of specific compounds to the overall aroma of a substance.

Sensory evaluation

Sensory analysis can be used in conjunction with instrumental analysis, which helps to identify flavour compounds. The combination of the two provides a platform for revealing key flavour compounds associated with consumer liking.

• Develop a lexicon for a product
• Assess the impact of ingredients on flavour
• Evaluate the impact of manufacturing, storage, or shelf-life on flavour
• Assess the impact of consumption temperature on sensory properties
• Assess the impact of different matrices on the perception of volatiles
• Assess the impact of physicochemical interactions on flavour release and perception
• Assess the impact of aroma compounds on consumer acceptance

Liquid–liquid extraction (LLE)

Liquid-liquid extraction (LLE) is a technique used to investigate the volatile flavor components of Chinese Zhiduwugu light-flavor (ZD) Baijiu. LLE is combined with headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry-olfactometry (GC-MS-O) to identify the aroma compounds in ZD Baijiu.

The process of LLE involves the following steps:

• The ZD Baijiu sample is diluted with boiled ultrapure water to achieve a 10% ethanol solution.
• Sodium chloride is added to the diluted solution until it reaches saturation.
• The solution is then extracted multiple times with dichloromethane, a solvent, using an ultrasonic process at a specific temperature and power.
• The organic phase is collected after filtration, and the solvents are combined and subjected to further distillation and concentration.

Through this LLE process, 40 aroma compounds were selected by aroma extract dilution analysis (AEDA). Among these, isobutyric acid, ethyl laurate, guaiacol, and 2-phenylethanol exhibited the highest flavor dilution (FD) factors.

Further identification and quantification of the aroma compounds were performed using mass spectrometry (MS) analysis, retention index (RI) comparison, and reference to aroma characteristics and standards. A total of 35 aroma-active compounds were identified, and 30 of these were quantitatively determined using internal standards.

The odor activity values (OAVs) were calculated by dividing the concentration of the aroma compounds by their odor thresholds. Compounds with OAVs greater than 1 were considered significant contributors to the aroma of ZD Baijiu. Based on the FD factors and OAVs, isoamyl acetate, ethyl 3-phenylpropionate, guaiacol, isobutyric acid, ethyl laurate, 3-(methylthio)-1-propanol, ethyl acetate, 2-phenylethanol, and 3-methyl-1-butanol were identified as the top contributors to the aroma of ZD Baijiu.

Gas chromatography-olfactometry (GC-O)

GC-O is used to establish the correlation between specific chemicals and the odour concentration. The sample gas is passed through an olfactometer to be 'sniffed' by the panelist, and is also analysed to identify the chemical compounds present. When the panelist senses an odour, they signal it and the concentrations of compounds present are compared.

GC-O utilises a GC-MS system equipped with an olfactory detection port: at the outlet of the GC there is a sniffer mask, where a trained panelist can smell the gas and provide information about the presence of odour. At the end of the GC column, after the separation of the chemical compounds in the gas mixture, the sample is divided and an equal flux of it reaches the MS detector and the panelist.

The panelist sniffs the gas, and every time they sniff an odorous substance, they provide a sensorial response, in terms of presence and type of odour. Every time the panelist perceives an odour, they push a button and describe the odour. This way, an olfactogram is obtained, which allows the correlation of the chemical information supplied by the chromatogram and the sensorial perceptions of the panelist.

GC-O is affected by the subjectivity and inattention of the panelist. Panelist distraction can provoke important errors, in particular when the odorous stimulus is short or weak.

Frequently asked questions