Analyzing Carotenoid-Derived Aroma Compounds Using Gas

Chapter 5

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: November 21, 2001 | doi: 10.1021/bk-2002-0802.ch005

Analyzing Carotenoid-Derived Aroma Compounds Using Gas Chromatography-Olfactometry Jane E. Friedrich and Terry E. Acree Department of Food Science and Technology, Cornell University, New York State Agricultural Experiment Station, Geneva, NY 14456

Although the terpenes and terpenoids contained in natural products are mixtures of many chemicals only a very small number have olfactory and taste properties at their natural concentrations. Therefore, analyzing natural products for the flavor-active chemicals requires methods that are capable of distinguishing the small fraction of odor-active volatiles from the much larger number of odorless components. Gas chromatography - olfactometry (GC/O) is ideal for this purpose even though it has been underutilized for the analysis of terpenoids. In this paper we will discuss the benefits of GC/O for the analysis of aroma compounds in samples.

The study of aroma is often approached via two different methods, analytical chemistry methods and sensory science methods. Analytical chemists approach the study of aroma by measuring all volatile chemicals present whereas sensory scientists attempt to correlate sensory data with analytical data. The technique of gas chromatography - olfactometry (GC/O) brings these two groups together by providing sensory responses to chromatographically separated chemicals. The sensory scientist finds the human responses useful and convincing, while the analytical chemist finds the retention times at which the responses were made useful for chemical identification (1).

© 2002 American Chemical Society

In Carotenoid-Derived Aroma Compounds; Winterhalter, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Methods Two methods are commonly used to analyze aroma compounds in natural products, gas chromatography - mass spectrometry (GC/MS) and gas chromatography - olfactometry (GC/O). Gas chromatography - mass spectrometry (GC/MS) is a powerful tool for the separation and characterization of chemicals whether they are odor active or not. In the analysis of aroma, G C / M S can selectively focus on the odor-active compounds once their spectral and chromatographic properties are known. However, the task of determining which compounds in a sample are odor active requires a bioassay. In other words, we must first determine "which constituent or constituents is/are contributing to the characteristic sensory properties of the food product being investigated" (2). G C / O is a bioassay that reveals odorants in terms of their pattern of smell-activity thus eliminating odorless compounds from consideration. It has been shown that the human detector is much more sensitive than the chemical detector. One reflection of this sensitivity is seen in the detection of Bdamascenone in natural products. B-Damaseenone has been found to be one of the most potent odorants in many food products. Its characteristic floral odor contributes to the odor character of apples (3), various grape varieties, beer, coffee oil, buchu leaf oil, Satsuma mandarin (4), rhambutan (5), lychee (6), textured soy protein (7), soymilk (8), tobacco, tea, raspberry oil (9), and cane molasses (10). However, β-damascenone is often not detected by G C / M S analysis. This is due to the low odor threshold of β-damascenone, 2 - 2 0 pg/g in water (11), which means that trace amounts of β-damascenone are often detectable by smell but not by chemical detectors. Other compounds for which this is often the case are β-ionone, 2-acetyl-1 -pyrroline, geosmin, and 2-secbutyl-3-methoxypyrazine. Figure 1 shows a Venn diagram that is representative of the current situation in most aroma research. There is a degree of overlap between the compounds detectable by GC/O and G C / M S analysis. However, G C / M S analysis alone will not detect some of the odor active compounds in a natural product, often missing the most odor potent chemicals in a sample. The percentage of odorants missed varies, however, it can be stated that just using G C / M S over emphasizes the importance of some compounds well under emphasizing the importance of others. It is important to note that G C / M S fails to indicate which compounds are odor active in a sample as well as how odor potent the compounds are. This is an important issue because G C / M S analysis often detects "odorants" although they may not be above their odor. For example Ong et al. identified 51



Figure 1. Venn diagram showing the situation in aroma research.


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70 compounds via G C / M S analysis that did not possess odor activity in rhambutan fruit (5) even though they have been shown to have odor activity in other natural products. For example, hexanal has been shown to be a lipid oxidation product and one of the most odor potent compounds in soy milk as well as one of the major contributors of the beany odor of soymilk (8). Therefore, the use of G C / M S to analyze the odorants in a sample is seldom representative of the odor profile of that sample although it is a measure of the volatile components of that sample. Table 1 below shows two specific examples that incorporated both G C / M S and G C / O to analyze the aroma active compounds in two natural products, bell pepper and rhambutan fruit. Seventy-four total compounds were identified in bell pepper, of these 14% were detected only by G C / O analysis. If GC/O analysis had not been used one third of the odor active compounds in the bell pepper sample would have not been detected. In the rhambutan study 112 total compounds were detected by the combination of GC/O and G C / M S analyses, GC/O helped to identify 11% of the total compounds detected. This accounts for 20% of the odor active compounds in rhambutan fruit.

Table 1. Number of compounds detected by two analytical methods of aroma analysis.

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Luning et al., 1994(74). *Ong et al., 1998 (5,6). When analyzing carotenoid derived aroma compounds for odor activity, a bioassay must be performed to determine i f the compounds possess odor in the product they are extracted from. Table 2 lists some common carotenoid derived aroma compounds and their corresponding odor descriptor. These compounds are often found to be one of the most odor potent compounds in a sample. However, they are also found in products where they do not contribute to the odor. For example, Ong et al. (5,6) found β-ionone present in rhambutan fruit using G C / M S analysis. However β-ionone was not found to be odor active in the fruit because β-ionone was not above its odor threshold. Another common carotenoid derived aroma compound, β-damascenone, however, was found to be the most potent odorant in rhambutan.


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Table 2. Odor descriptors of common carotenoid-derived aroma compounds.

Figure 2 shows a comparison of the GC/FID chromatogram and an odor spectrum value (OSV) chromatogram of rhambutan fruit. Plotting odor spectrum values versus retention index produces an O S V chromatogram. Odor spectrum values are normalized flavor dilution values or Charm values and modeled on Steven's law, Ψ=1

Analyzing Carotenoid-Derived Aroma Compounds Using Gas

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