By Gas chromatography–ion mobility spectrometry (GC–IMS) is a recent technique applied to food flavor analysis – including the evaluation of food freshness, processing of food products, and changes in aroma during storage.
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Wang, Chen and Sun from the Beijing Advanced Innovation Centre for Food Nutrition and Human Health summarise recent studies demonstrating the application of the technique in different food flavour analysis contexts. The trio concludes by evaluating the future of GC-IMS in food flavour analysis.
Owing to the complexity of food matrices, two-dimensional gas chromatography is a more powerful analytical tool to study food samples as a result of its increased ability to separate compounds. An alternative analytical tool is gas chromatography-olfactometry-mass spectrometry (GC-O-MS) which effectively selects aromatic compounds from complex foodstuffs.
However, the technique is time-intensive, involving aroma extract dilution analysis (AEDA), to identify aromatic constituents present in food. Consequently, it cannot be used to characterise the volatile constituents of food rapidly.
To identify volatile organic compounds, other techniques have been developed. These include proton transfer reaction mass spectrometry (PTR-MS), atmospheric pressure chemical ionization mass spectrometry (APCI-MS), and selected ion flow tube mass spectrometry (SIFT-MS).
These techniques are all limited by their ability to separate compounds which are especially problematic in the context of complex food matrices. As a result, GC-IMS represents a new gas-phase separation detection methodology which harnesses the high separation ability of gas chromatography, together with the fast response of IMS.
GC-IMS Analysis
The general principle of IMS involves a drift tube, comprised of an ionization region on the separation region. Samples undergo vaporization, followed by transfer to the ionization area. These neutral sample molecules, which are carried by carrier gas molecules, both undergo ionization.
The resultant ionised species formed vary and have different drift times when they encounter the separation region. The time at which they are detected reflects information about the ionised analyte type and concentration. The drift time oven ion is related to the mobility of the ion.
IMS represents a suitable analytical tool; however, it is limited when applied to food flavour analysis. this is because moisture can cause samples to be obscured by other matrix constituents, which prevents accurate detection. However, by coupling chromatography to IMS, both the separation power and information gathering potential has been increased. This places it as a prime analytical tool for food flavour analysis.
GC is used to separate complex compounds into individual constituents before undergoing IMS. Each of these neutral compounds moves to the ionization area of the IMS at different times, which improves separation.
The data obtained by GC-IMS has an additional dimension, so the data includes retention and drift time (hence referred to as two-dimensional analysis). GC-IMS is a rapid technique, operating at low cost and does not require pre-treatment. Its high sensitivity and variable volume injection make it highly applicable to food research.
GC-IMS has been used to differentiate different grades of olive oil, enabling the identification of desirable volatile compounds for inclusion in a database. Aside from this, the technique can determine sources of olive oil as well as others such as canola oil and crude palm oil.
In the context of agriculture, GC-IMS has been applied to determine food alteration. For example, the Iberian dry-cured product differs substantially in price and quality.
By applying GC IMS analysis, the foodstuff with which the pigs that provide the source of Iberian product can be distinguished (feed-fed and acorn-fed Iberian hams).
Similarly, the origins of different honey can be determined using the technique; GC-IMS represents a more robust in and rapid method of origin detection in comparison to NMR-based techniques. The results obtained from studies using GC-IMS demonstrate their potential in preventing fraudulent honey trading.
Detecting Aromas
The key aroma components of food can also be determined by GC-IMS. The levels of polyamines and monoamines can be Quantified using the technique – these serve as an indicator of food freshness as these products are formed by the degradation of amino acids.
With regards to off flavour, GC IMS can be used cat to quantify product of lipid oxidation which produce undesirable flavours in food (and undetectable by human smell). In the context of fermentation, the technique can quantify characteristic flavours to enable fermentation to be terminated at the correct time point; this is especially useful in the production of beer, as well as cheese ripening.
Volatile metabolites such as those produced by lactic acid bacteria can be identified by GC-IMS. As a result, the technique can be used to select strains of bacteria the influence cheese flavour.
Conclusions
The authors of the review conclude that GC-IMS represents an optimal tool for food flavour analysis because of its rapid detection, easy operation and portable nature.
This makes it amenable to broad applications in food classification – from the classification of foodstuff’s, identification of freshness and characterisation of off-flavour and aromatic compounds in food. Since GC-IMS has been developed, the technique has been expanded to included different applications.
The most notable characteristic of the technique is high separation ability, followed by reduced time to obtain analytical results. The 3D spectrum that is produced by the technique is accurate as it includes the drift time, retention times, and signal intensity.
GC-IMS also operates at atmospheric pressure, which renders it capable of being portable, capable of high throughput, easy operation and lower cost. Although the promise offered by GC-IMS is evident, GC-MS remains the gold-standard analysis tool for food flavour analysis.
This is because IMS has limited ability to precisely quantify, in addition to the lack of a spectral reference library. It is hoped that as the technology evolves, a full database of GC-IMS will have complied, facilitating sensitive, fast, and automated food component characterisation.
Source
Wang, S., Chen, H.C. & Sun, B. (2020) Recent progress in food flavour analysis using gas chromatography–ion mobility spectrometry (GC–IMS) Food Chemistry. Doi: https://doi.org/10.1016/j.foodchem.2019.126158