Rapid Aroma Profiling of Cheese Using a Micro-Chamber/Thermal Extractor with Thermal Desorption–GC–MS Analysis

This application note describes the benefits of using Markes’ Micro-Chamber/Thermal Extractor in conjunction with thermal desorption (TD) and gas chromatography–mass spectrometry (GC–MS) to analyze the aroma profiles of cheese. Various cheeses are examined and compared, and it is demonstrated how this ‘multi-hyphenated’ technique allows rapid yet powerful assessment of the volatile organic compounds (VOCs) released.

The Importance of Cheese Aroma
The annual value of cheese to the global economy runs into many billions of dollars, and manufacturers expend a great deal of effort in ensuring their product is of a consistently high quality. Rigorous attention is paid to the ingredients, and the appearance, texture, taste, and aroma of the final product.

The aroma profile is an important part of the consumer experience of cheese, with a range of compounds responsible for the wide variation of cheese odours. This presents analysts with a substantial challenge when wishing to identify key aroma components, many of which are present at trace levels and have low odour thresholds relative to more abundant components such as fatty acids.

Here we show how Markes’ technology can be used to characterize the aroma profile from a range of cheese samples, with analysis both of high- and low-concentration components.

Sampling Methodology
The sampling methodology used for the analysis of the cheese samples described here combines three powerful techniques:

• Dynamic headspace sampling flushes the organic vapours from the cheese onto a sorbent-packed tube, followed by thermal desorption to concentrate these vapours into a narrow band of carrier gas, suitable for introduction into a GC–MS system. Here, we use Markes’ Micro-Chamber/Thermal Extractor™, which is a stand-alone sampling accessory for dynamic headspace sampling of organic vapours from a wide variety of materials, including foodstuffs.

• Thermal desorption (TD) is a versatile “front-end” technology for GC that is applicable to the analysis of volatiles in a wide range of gaseous, liquid, and solid samples. Here, we use Markes’ fully-automated 100-tube TD-100™ thermal desorber, which allows full automation of both sample desorption and re-collection.

• Time-of-flight mass spectrometry is a powerful alternative to the quadrupole technologies often employed with GC. In this work, Markes’ BenchTOF-HD™ instrument is used, which provides the sensitivity otherwise only available using a quadrupole instrument in SIM mode, but across the full spectral range. Importantly, it also generates spectra that match those present in reference (typically quadrupole‑acquired) libraries. Post-run data processing used Markes’ TargetView™ software to remove baseline interferences, deconvolve overlapping peaks, and match spectra against those in the NIST database.

Experimental Procedure
A variety of cheeses (grated, 5 g per sample) were sampled using the Micro-Chamber/Thermal Extractor (Figure 1) for 20 min under a flow of dry nitrogen, with a chamber temperature of 40 °C used to generate a full VOC profile that reflects conditions produced in the mouth. Samples were collected on to sorbent tubes packed with quartz wool–Tenax® TA–Carbograph™ 5TD, which is a sorbent combination that can handle the full range of analytes expected to be present in the VOC profile of cheese.

The use of multiple sorbents in this way is only possible because Markes’ TD systems are designed so that analytes enter and leave the tube (or trap) at the end with the weakest sorbents. This ensures that low-volatility “sticky” analytes are retained on the weakest possible sorbent, so that when the gas flow is reversed, they desorb easily. The tubes were analyzed using an overall TD split of 51:1 (high split), 6:1 (low split).

Gas chromatography used a 30 m × 0.25 mm HP-Innowax column to best handle the polar compounds expected, with a temperature ramp from 40 °C to 260 °C, and an overall run time of 36.0 min. Mass spectra were acquired over the range m/z 33–350, with a data rate of 2 Hz (with 5000 spectra per data point).

Figure 2: Parallel analysis of the VOC profiles of four cheeses under high-split conditions.

Comparison of Different Cheeses
One of the benefits of the Micro-Chamber/Thermal Extractor is that several samples can be run under identical conditions quickly and easily. Figure 2 shows the results of a comparison of four cheeses, collected simultaneously from adjacent micro-chambers, and run under identical TD–GC–MS conditions so that the peak sizes give a good approximation of the relative abundances of the individual components. Note the presence of the highly odorous component dimethyl disulphide in the Emmental, and of two branched-chain carboxylic acids in the Comté.

Figure 3: Analysis of the VOC profile of full-fat (extra-mature) Cheddar. (a) High-split (51:1) conditions provide an indication of the quantities of high-concentration components. (b) Low-split (6:1) conditions, to aid identification of trace-level components (see inset). Example peaks are highlighted; for a full peak listing, please see Markes Application Note 101.

Analyzing High- and Trace-Level Components in a Single Run
Markes’ TD splitting technologies offer a particular advantage for aroma profiling because of their ability to run a single sample twice, using different split ratios to accurately measure trace‑level and high-concentration compounds in the same sample (Figure 3). First, the sample is desorbed using a “high split”, with a small volume being sent to the GC. This allows the high-concentration components to be analyzed without overloading the analytical system. The remainder is re-collected onto a fresh sorbent tube, and then desorbed as before but using a “low split” - sending a higher proportion of the sample to the GC. This allows the trace‑level components to be quantified more accurately.

Conclusions
In this application note we have shown how the Micro-Chamber/Thermal Extractor allows rapid and straightforward sampling of volatiles from a range of cheese samples. In conjunction with TD–GC–MS, a wealth of information is provided that allows users to identify key components and compare samples side-by-side.
The flexibility of this approach makes it suitable for a wide range of sampling situations - from initial screening of “unknown” samples to in-depth analysis of samples for quantitation of trace‑level components.

For more experimental results and details of the conditions used, please refer to Markes Application Note 101, available at www.markes.com.

Markes International
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