Food aroma compounds are arguably the most important consituent of a food, as consumers often base their choice of food on flavour as well as appearance. Two main approaches to food flavour analysis exist: the use of so called ‘electronic noses’ and the use of techniques involving chromatography. This article takes a closer look at the two approaches, including developments in sample preparation, and the direction that food flavour analysis is taking in order to benefit the food industry.
By Professor Henryk H. Jeleń and Dr Malgorzata Majcher
The nature of food odourants
Flavour is a characteristic feature of a particular product and an indicator of its freshness. Depending on the product, only a few compounds may be responsible for its flavour (e.g., cucumber, cinnamon). However, more often than not, food aroma is a sum of tens or hundreds of compounds (as in wine, or foods where flavour is generated in thermal reactions, such as in coffee). In these examples, the compounds are present in concentrations varying from ng/kg to mg/kg.
In most cases, food is a complicated matrix: its constituents react with flavour molecules, influencing their binding and release. During the analysis of food flavour compounds an important feature has to be considered: odour thresholds. These are the lowest concentrations of a vapour in air, which can be detected by smell. Often, strong odourants are present in very small amounts (ng/kg or ng/L range) [Table 1] and, as a result, create a challenge for instrumental analysis. The methods used for flavour analysis should be characterised by limits of detection comparable to those achieved by the human nose.
Two main approaches to food flavour analysis exist, one based on ‘electronic noses’ (that are designed to mimic the human sense of olfaction), and one that uses chromatographical methods. However, only the latter approach provides answers about the identity of key odourants and allows their quantification. Electronic noses (e noses) were designed as a tools to measure ‘whole’ aroma, usually by transferring a headspace of the analysed product into a chamber equipped with an array of non-specific sensors. The response of the e nose to a specific flavour is to create a flavour ‘fingerprint’. On the contrary, chromatographic methods seperate the volatile compounds responsible for a characteristic flavour, then individually identify them using various detectors and the human nose.
Developments in extraction technology
To achieve the sensitivity required for the analysis of food odourants, appropriate sample preparation and pre-concentration steps are required. As the food flavours perceived by our noses are volatile compounds, methods based on distillation, solvent extraction and headspace analysis are used. Headspace methods (that measure the components in the gas present in the space above the sample) comprise ‘static headspace’, developed in the early 60s, ‘dynamic headspace’ (purge and trap), developed in early 70s, and ‘solid phase microextraction’ (SPME) developed in the late 80s. Dynamic headspace and SPME methods provide lower detection limits as a result of their pre-concentration and enrichment steps. These are the milestones in solventless extraction technology that have influenced most volatile compound analysis. Other sample preparation methods, such as solid phase extraction (SPE), liquid/liquid extraction (L/LE) and simultaneous distillation – extraction (SDE) are used for flavour compound isolation and their use is determined by the type of compounds to be isolated. SDE is a method of choice for many food products because of its versatility and ease-of-use.
Elevated temperatures applied during distillation may lead to artefact formation, however. In particular, when dealing with food samples that are rich in free amino acids and sugars, the compounds can interact in a course of Maillard reactions to form artifacts. One method, which provides unrivalled data for subsequent flavour compound analysis is vacuum distillation. This method allows the low temperature separation of flavour compounds from different matrices. It also does not alter either the aroma of the extract or the thermally labile compounds. SAFE (Solvent Assisted Flavour Evaporation) is a relatively easy to perform and robust method that uses a vacuum in order to isolate flavour compounds from the matrix. This technique is especially ideal for gas chromatography – olfactometry [1].
Developments in the separation and detection of flavour compounds
Developments in the analysis of flavour compounds have been associated with developments in gas chromatography (GC) and the combination of gas chromatography with mass spectrometry (GC/MS). In particular, the invention of capillary columns and their routine use has increased the number of compounds identified in the last 30 years. After the invention of capillary columns for gas chromatography, the next milestone was the development of multidimensional chromatography. This technique allows the transfer of a particular fraction from one column into another column, with the new column possessing a different mechanism of separation and equipped with another detector.
However, the development of comprehensive gas chromatography, especially in connection with high speed time-of-flight mass spectrometers (GCxGC-ToF-MS), has opened new horizons in compound separation and identification. With a GCxGC system it is possible to resolve the most complicated mixtures [Figure 1], based on simultaneous separation by orthogonal separation mechanisms (usually a pair of nonpolar-polar columns). as in. Mass spectrometers have become routine detectors for the identification of flavour compounds. With improvements in spectra acquisition speed and deconvolution algorithms, they provide abundant and relatively reliable information on compound identity. For the analysis of flavour compounds, gas chromatography takes advantage of a detector that is crucial in this type of research – the human nose. Use of the human nose as a detector in chromatography has become a basic tool for sensory-oriented flavour analysis (gas chromatography-olfactometry, GC-O), especially after the invention of quantitative methods such as the aroma extract dilution analysis (AEDA), Charm or OSME methods [2].
Main directions in food flavour analysis
Research into food flavours encompasses several important areas of study: i) sensory-oriented analysis of key odourants, ii) measurement of flavour compounds during food mastication (in vivo) and release of flavour compounds from food, iii) use of flavour/volatile compounds for the detection of changes during storage and processing and, iv) volatiles for food authenticity testing.
Sensory-oriented analysis of key odourants
The determination of key odourants responsible for the aroma of a particular food or food product uses all the techniques described above to isolate, separate and identify key odourants. Of particular importance is gas chromatography – olfactometry, which allows the sensory identification of main volatile fractions. The AEDA technique calculates the dilution factor (FD) of each compound as a last dilution in which the compound can still been sensed and an aromagram can be produced [3] [Figure 2]. The higher the FD factor, the higher the contribution of the compound to the flavour of the food product. In order to determine the importance of a particular compound on the overall profile of a product, Odour Aroma Values are often used (these are the ratios of the compound concentration to its odour threshold). Flavour reconstitution experiments allow researchers to check out the results obtained using GC-O [4]. The techniques described here are used for the determination of key aroma compounds, but can also be used in the detection of compounds responsible for product taint or off-odour.
Measurement of flavour compounds during food mastication
The measurement of compounds released in vivo during chewing requires special tools guaranteeing instant measurement and a good sensitivity. Methods that do not require compound separation are preferred and special techniques have been used for this purpose, such as atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) and selected ion flow tube mass spectrometry (SIFT-MS).
Detection of changes during storage and processing
The analysis of food aroma or volatile compounds is also used for monitoring chemical, enzymatic or microbial changes in food production and storage. In this area of research, selected compounds can be use as indicators of unwanted processes or a fingerprint of volatile compounds can provide information on changes in food products. For this purpose GC data peaks are treated by multivariate statistical methods and sample results are presented usually in the form of poly-component analysis (PCA) graphs. Another approach uses the electronic nose philosophy - a fingerprint of total volatiles forming in the headspace is subjected to statistical evaluation. In this field of research, a combination of SPME with mass spectrometry, which works as an electronic nose, has shown promise. This approach is used for food authenticity testing, detection of microbial spoilage and detection of chemical changes during processing and storage [5] [Figure 3].
The analysis of food aroma compounds is made more complicated by the rich matrix in which odourants are present. In addition to this, low concentrations and odour thresholds make analysis difficult (often due to the instability and reactivity of flavour compounds). The sample preparation and separation techniques that have been developed in recent years, provide limits of detection comparable to that of the human nose. Yet despite the increased efficiency of chromatographical systems, and the devlopment of multivariate statistical methods to aid sample comparison, the analysis of food flavours remains an interesting challenge for the analytical chemist.
References
1. Engel W, Bahr W, Schieberle P. Solvent Assisted Flavour Evaporation – a New Versatile Technique for the Careful and Direct Isolation of Aroma Compounds from Complex Food Matrices. Eur Food Res Technol 1999; 209: 237-241.
2. Grosch W. Detection of potent odourants in foods by aroma extract dilution analysis. Trends in Food Science & Technol 1993; 4: 68-73.
3. Majcher A M, Jeleń H H. Identification of potent odourants formed during the preparation of extruded potato snacks: J Agric Food Chem 2005; 53: 6432-6437.
4. Sollner K, Schieberle P. Decoding the key aroma compounds of Hungarian type salami by molecular sensory science approach. J Agric Food Chem 2009; 57: 4319-4327.
5. Jeleń H H, Mildner-Szkudlarz S, Jasińska I, Wasowicz E. A headspace-SPME-MS method for monitoring rapeseed oil autooxidation. J Amer Oil Chem Soc 2007; 84: 509-517.
The authors
Professor Henryk Jeleń and Dr. Małgorzata Majcher
Fats & Flavour research group
Faculty of Food Science and Nutrition
Poznań University of Life Sciences
Poznań, Poland
www.up.poznan.pl/zks
Email: henrykj@up.poznan.pl
