Sensory Relevance

In many earher studies on the composition of aromas, each volatile compound was regarded as an aroma substance. Although fists with hundreds of compounds were obtained for many foods, it was still unclear which of the volatiles were really significant as odorants and to what extent important odorants occurring in very low concentrations were detected. 

The studies meanwhile concentrate on those compounds which significantly contribute to aroma. The positions of these compounds in the gas chromatogram are detected with the help of dilution analyses. Here, both of the following methods based on the aroma value concept (cf. 5.1.4) find application. 

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Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

Trace compounds without any sensorial relevance (odour activity value much less than 1) should not be evaluated in the sense of authenticity assessment, as the fraudulent addition of a sensorially ineffective compound makes no sense. 

As perception of most aroma depends on a subtle balance of different odorants, understanding complex aroma at the molecular level means focusing on sensorially relevant odorants. 

The following review is limited to the discussion of those studies which have been carried out to find sensorially relevant volatile compounds. 

The first comprehensive investigation of the volatiles of wheat bread was carried out by Mulders et al. (11. 16-18). After a qualitative analysis (16-18) which led to the identification of 90 compounds, the authors attempted to get an insight into the sensory relevance of the major components which were found in the headspace extract of white bread. A mixture of the main compounds identified was prepared in water to match the gas chromatogram obtained from the bread sample (11,). The odor of the synthetic mixture resembled that of the fermented dough but not that of wheat bread. 

Blank, L, Sensory relevance of volatile organic sulfur compounds in food. In Heteroatomic Aroma Compounds, ACS Symposium Series 826 (Reineccius, G.A., Reineccius, T.A., eds.), American Chemical Society, Washington, DC, pp. 25-53, 2002... 

The substance selected as an i-IST should be a characteristic genuine compound of less importance in view of sensorial relevance. 

Identification of sensory relevant compounds can be achieved by applying a... 

On the basis of the odor activity value (OAV) concept, only a limited number of volatile components are sensorially relevant in a given foodstuff (Grosch, 1994). Teranishi et al. (1996) also presented a computer-assisted correlation of chromatographic and sensory data in food aromas. In a bar graph were shown the logarithms of the concentrations and of the odor units of the individual constituents of the aroma. The representation shows clearly the constituents present above the threshold values and therefore the most useful for the aroma. 

Figure 5. Sensory relevant compounds idemilied in Ihe aroma uxlrau of tcmiviufk The numbers correspond lo those in Table I...

Banana flavor Of the 350 flavor compounds identified to date in B. f. 3-methylbutyl acetate as well as 3-methylbutyl butanoate and 3-methylbutyl 3-methyl-butanoate (see fruit esters) are of particular sensory relevance. Fatty-fruity and exotic-fruity notes result mainly from aroma substances with the (Z)-4-config-uration, e.g., Z)-4-hepten-2-one (C7H12O, Mr 112.17, CAS [90605-45-1 ]) and (Z)-4-hepten-2-ol (C7H14O, Mr 114.19, CAS [34146-55-9]), as well as their acetates and butanoates. Eugenol, elemicin (see safrole), and 0-methyleugenol are responsible for the spicy aroma. Bilberry flavor The aroma of the European bilberry (Vaccinium myrtillus) is mainly due to ( )-2- hexenal, ethyl 2- and 3-methylbutanoates (see fruit esters) as well as ethyl 3-hydroxy-3-methylbutanoate (C7H,403, Mr 146.19, CAS [18267-36-2]). 

Melon flavor The typical M. f. of the various sorts are due to secondary flavor compounds from the degradation of linolic and linolenic acids, e.g., hexenals, (Z)-6-nonenal (see alkenals), (Z,Z)-3,6-nonadien-I-ol (C9H16O, Mr 140.23, CAS [53046-97-2], water melon odor), (Z,Z)-3,6-nonadienyl acetate (C, H,g02, Mr 182.26, CAS [130049-88-2]), and (Z)-l,5-octadien-3-one (see tea flavor). Sweet melon (muskmelon Cu-cumis melo) also contains fruit esters, especially 2-methylbutanoates, alkanolides, anisaldehyde, eugenol, and some (methylthio)carboxylates with sensory relevance. ... 

There is no relationship between the sensory perception of quality deterioration and the levels of FFA in fats which contain low-molecular acyl residues (e. g., milk, coconut, and palm kernel fats) because among the free fatty acids, the sensory-relevant compounds (C number <14)... 

Table 20.20. Esters in wine with sensory relevance...

Progress in instrumental analysis has led to long lists of volatiles (1). Unfortunately, the sensory relevance of these volatile compounds has not been as extensively evaluated, although the use of the human nose as a sensitive detector in gas chromatography (GC) was proposed by Fuller and coworkers as early as 1964 (2). In the meantime, much has been published on food aroma, often without identifying the impact compounds. Therefore, one of the major problems in aroma research is to select those compounds that signiflcantly contribute to the aroma of a food. 

In general, the aroma of a food consists of many volatile compounds, only a few of which are sensorially relevant. A first essential step in aroma analysis is the distinction of the more potent odorants from volatiles having low or no aroma activity. In 1963, Rothe and Thomas calculated the ratio of the concentration of an odorant to its odor threshold and denoted it aroma value (3). This approach was the first attempt to estimate the sensory contribution of single odorants to the overall aroma of a food. Since that time, similar methods have been developed odor unit (4) based on nasal odor thresholds, flavor unit (5) using... 

GC in combination with olfactometric techniques (GC-0) is a valuable method for the selection of aroma-active components from a complex mixture (7). Experiments based on human subjects sniffing GC effluents are described as GC-0. This technique helps to detect potent odorants, without knowing their chemical structures, which might be overlooked by the OAV concept (ratio of concentration to threshold) if the sensory aspect is not considered from the very beginning of the analysis. Experience shows that many key aroma compounds occur at very low concentrations their sensory relevance is due to low odor thresholds. Thus, the peak profile obtained by GC does not necessarily reflect the aroma profile of the food. 

In general, it is very difficult to judge the sensory relevance of volatiles from a single GC-0 run. Several techniques have been developed to objectify GC-O data and to estimate the sensory contribution of single aroma components. This issue seems to be of great concern, as a considerable part of the 7th Weurman Symposium was dedicated to this topic (8). Dilution techniques and time-intensity measurements are the two main GC-0 methods. 

The sensory relevance of individual odorants can be estimated by injecting various headspace volumes. This is equivalent to AEDA of liquid samples. In contrast to AEDA, where aroma compounds are separated from the food matrix, static headspace GC-O provides data about the aroma above the food. This technique is suitable for studying the effect of the food matrix on the aroma profile. Therefore, AEDA and static headspace GC-O result in complementary data. 

GC-O is the method of choice for selecting those components that are responsible for aroma deviation in food, i.e., an off-flavor. In general, it can be applied to both foodbome off-flavor formation and off-flavor problems related to contamination. The latter is caused by odorants that normally do not belong to the overall aroma of the product, i.e., external contaminants (e.g., packaging) or compounds formed upon processing and storage (e.g., microbial spoilage). In both cases, the comparison of the off-flavor of the contaminated food with the reference product usually results in a limited number of sensory-relevant compounds, which reflect the difference in aroma profiles. Identification work can then be focused on these odorants. 

As mentioned earlier (Sec. II.C), headspace GC analysis yields additional data about very volatile compounds, which are usually lost during conventional sample preparation. The sensory relevance of odorants present in the headspace above a food can be evaluated by combining AEDA with the static headspace technique (18). This new approach, called static headspace GC-O, was applied to the food flavoring discussed above. 

The information about odor thresholds determined by GC-0 can be of great help in identifying sensory-relevant compounds of both positive and off-flavors,... 

Differing intensity functions for volatiles account for a well-known phenomenon in GC-O some intensely smelling compounds disappear after a few dilution steps (e.g., vanillin), whereas others with a lower aroma intensity in the original extract have the highest FD factors. The sensory relevance of the latter is overestimated. ( )-P-Damascenone is a typical representative for such compounds, which are characterized by a relatively flat dose/intensity function. This is most likely why ( )-P-damascenone does not play a major role in the aroma of coffee, despite low threshold values (Table 12), i.e., high FD factors and OAVs. In other words, threshold concentration does not necessarily correlate with aroma potency. 

Certainly, GC-O is a first essential step for distinguishing odor-active compounds from volatiles without odor impact. This screening procedure is the basis for identification experiments. GC-O also provides a first indication about the odor potency of volatile compounds, i.e., to what extent they individually contribute to the overall aroma. However, in most cases, final conclusions about then-sensory relevance cannot be drawn and further work is necessary (see Sec. VI). 

Analytical and sensorial data cannot be presented with the same precision. While R1 values and mass spectra can be precisely determined, GC-O data lack comparable accuracy and reproducibility. FD factors and Charm values are approximations of the sensory relevance of an odorant. In fact, a 256-fold dilution is a rough estimation, depending on extraction yields and the assessor, and could also be 128 or 512. Such exact values are rather misleading GC-O techniques are not this accurate. The use of 2", where n is the number of dilution steps, may help to avoid overinterpretation of GC-O data (76). Though 256 and 2 represent the same value, the latter gives a more realistic idea of the odor potency of a compound. It should be mentioned, however, that FD factors do not normally differ by more than two dilution steps. 

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